Freshwater Life - A field guide to the plants and animals of southern Africa

Southern Africa has a remarkably rich and diverse fauna and flora, much of which is endemic. There is also a vibrant natural history publication industry in the region, and this has resulted in the publication of numerous well-illustrated and accessible regional field guides for a range of different plant and animal groups. Several are comprehensive all-in-one guides that attempt to describe all major elements of the fauna and flora of a habitat or region within one volume. These include a number of guides to the terrestrial wildlife of southern Africa as a whole, and to those of specific regions, such as the Kruger Park, the Okavango wetlands, or the Cape Peninsula.

To date, however, there has been no single illustrated book on freshwater life in southern Africa that gives readers access to a broad range of taxa within a single volume. Several excellent semipopular guides offer comprehensive coverage of specific animal groups, notably freshwater fishes, frogs, dragonflies and damselflies, and, to a lesser extent, molluscs (see Further reading p.354). There is also a nine-volume series of more detailed guides to invertebrate groups, but these are too technical and too detailed for most readers.

The aim of this book is to provide readers with a reasonably comprehensive, well-illustrated and easy-to-use guide to the most frequently encountered freshwater animals, flowering plants and algae of southern Africa, all in one accessible volume. We have attempted to make it sufficiently comprehensive to be useful to secondary- and tertiary-level students, conservationists and water managers, but also accessible enough for members of the public to use, if they want to learn about life in their local rivers, ponds or dams.

Books of this nature are largely collations of existing knowledge, which is constantly growing and evolving. We have thus had to depend on available information as to the distribution patterns of species, some of which are poorly known. Similarly, the availability of suitable images has in many cases constrained the selection of species we could feature.

HOW TO USE THIS BOOK

Using this book should enable you to identify and learn about the ecology of common freshwater organisms across a range of animal, plant and unicellular groups. We also include a brief overview of the ecology of freshwater ecosystems in the region and of the threats these face, as well as simple guidelines for collecting and studying freshwater organisms and undertaking freshwater-related projects, such as conserving water, building a wetland or pond and photographing freshwater organisms.

The area of coverage is the wider southern African region, defined here as Africa south of approximately 17ºS and thus including South Africa, Namibia, Botswana, Zimbabwe and southern Mozambique. Many of the taxa featured extend much further north, however, so this guide will be useful to readers throughout sub-Saharan Africa.

Many more freshwater species occur in the region than can be accommodated in a book of this size. We have selected those that are featured on the basis of their size, importance, abundance and distributional range: essentially, by how likely they are to be encountered by readers. Thus, coverage of large conspicuous taxa, such as the vertebrates, is relatively comprehensive and usually pitched at species level, while that of highly diverse, small to microscopic taxa is more superficial and often pitched at family or group level. Each species or group description is illustrated with one or more colour photographs.

Key to numbers on example spread (p.7)

Group heading – Entries are arranged in chapters that cover major taxonomic groups at the phylum, class or order level (see p.39 for the taxonomy of the groups included in this book), depending on how well the group is known. The text below the heading describes features common to all members of the group and gives general information about their biology, as well as approximate numbers of species globally and/or in the region.

Family – Describes features common to all genera and species that fall within the family. This text should thus be read in combination with the genus or species entries below it. For some lesser-known groups only family-level entries are provided.

Genus or species – These entries give notes on the identification features, size and biology of the organism depicted. Both common and scientific (Latin) names of the organisms are given. Size is the average adult length, unless otherwise indicated. For some groups, such as insects, size is relatively constant within a species and is a good identification feature. In others groups, such as crustaceans and fish, animals are born small and grow to a much larger adult size, so individuals seen or collected may be immature and thus much smaller than the adult size indicated here.

Related and/or Similar species – Where there are other easily confused or related species that could not be accommodated as separate entries we include a short section on how to distinguish these and where they occur.

Maps – These depict the known distribution of the species or group featured. The distributions of many aquatic species are poorly documented, so it is quite possible that readers may find species outside of the depicted ranges (the authors would be keen to be informed of such records). For some groups, distributions are so poorly known that they cannot be plotted with any confidence, and here we have simply shaded the whole region with a light colour wash.

Photographs – Wherever possible, these are taken in the wild, or in conditions that simulate the natural habitat of the species. Note that each species is scaled to appear more or less the same size on the page, so small species are greatly magnified. Since size can be an important identification feature, it is thus critical to check the actual size of the species as given in the text entry.

THE LIFE-GIVING PROPERTIES OF WATER

Water is without doubt the most valuable of all natural resources; indeed, it forms the very basis of life on Earth. All plant and animal life relies on water, which is also the largest constituent of most living organisms, making up about 60% of the human body and 80–90% of the body mass of most aquatic organisms. Water is also an enormously abundant substance; in fact more than 70% of the planet’s surface is covered by sea water, much of it several kilometres deep.

Only about 2.5% of the world’s water is fresh, however, and 96% of that is tied up in polar icecaps and glaciers, or is under the ground.

Just 0.3% of the world’s fresh water is held in wetlands, lakes and rivers, and these are very unevenly distributed. Indeed, more than a third of the land surface can be classified as desert or semidesert, meaning that life here is seriously constrained by a lack of water.

Despite being common, colourless and odourless, water is a truly remarkable substance with several unique properties, many of which derive from its unusual molecular structure: it consists of a single oxygen atom to which two hydrogen atoms are bonded, facing each other at an angle of about 105º. This gives the molecule a negative charge at the oxygen end and a positive charge at the hydrogen ends, which causes water molecules to cling to one another like tiny magnets. This feature is responsible for many special properties of water that are critical to life, the most important of which are the following:

The climate of southern Africa ranges from semiarid to hyper-arid: rainfall greatly exceeds 500mm per year in only a few regions and, where it does, rain is almost always strongly seasonal. Generally, mean annual rainfall decreases, while the rate of evaporation increases westwards and northwards across the subcontinent, far outstripping precipitation in most of the region.

In the lower Orange River Valley, for instance, in most years more than 10 times as much water evaporates than falls as rain. There is simply no such thing as a water surplus anywhere in southern Africa. Rainfall in the subcontinent is highly seasonal, being produced by different weather systems in different regions at different times of year.

In winter, the prevailing northwesterly winds hit the southwestern part of the country first and drop much of their rain there, leaving the southern interior, including the Karoo, in an arid rain shadow. In summer, rain normally falls in the north and east, while dry high-pressure air masses may persist for long periods in the south and west, preventing rain from falling. Thus southern Africa experiences a wide range of climates: winter rains and hot windy summers in the southwestern Cape, erratic non-seasonal rainfall and extremes of temperatures in the arid west, hot summers with rain and bitterly cold winters on the highveld, and subtropical, usually mesic, conditions in the east. Furthermore, rainfall varies erratically from year to year, resulting in unpredictable periods of drought and flood.

Rivers and wetlands

Rivers and wetlands reflect the topography and climate of their basins. The western parts of the subcontinent are arid and rather flat, so rivers are few – most are seasonal or ephemeral (short-lived), running only after episodic rainfall. In this region most wetlands, too, are seasonal or ephemeral. When it rains, it usually rains heavily, so dry flats become huge, shallow, ephemeral wetlands, such as Etosha Pan in Namibia and Makgadikgadi Pan in Botswana.

The Okavango Delta is an exception. Its existence is a result of tectonic faulting, and it receives its water mostly from the perennial Kavango River, which is fed by summer rains in the much wetter southern parts of Angola, where it rises.

The western part of the subcontinent is drained mostly by the Orange River, which, although large by South African and Namibian standards, is a mere trickle when compared to the world’s biggest rivers. The Zambezi, in contrast, is up there with the big ones. It arises in the eastern highlands of Angola and drains most of Zambia, much of Zimbabwe, all of Malawi and much of Mozambique. Further south, rivers tend to be much shorter, cutting through the steep escarpment and draining the central inland basins. Wetlands in the east are mostly perennial and are often associated with rivers. Here the Mfolozi, Mkuzi and Pongola rivers all have extensive floodplains with pans that fill seasonally and act as nursery areas for the rich freshwater fauna of the area.

Distribution patterns

Across the globe, each species of plant and animal is distributed according to its habitat requirements, its tolerance of physical and chemical conditions (including the availability of water), and its evolutionary history. The Palaeotropical Kingdom covers all of Africa apart from the extreme southwest, where the plants form a separate and unique assemblage known as the Cape Floral Kingdom or Cape Floristic Region (CFR). The distribution of animals follows much the same pattern, with many species being confined to either the Palaeotropical or the Cape region. The CFR is only about 90,000km2 in area but contains more terrestrial plant species than the whole of Europe or North America. The plants of the CFR are adapted to unusual physical conditions that, in turn, impart unusual chemical features (like acidity and dark brown colour) to the waters of the rivers and wetlands in the region.

Few species from outside the area are able to survive in these unusual conditions. As a result, a high proportion of aquatic species found in the CFR are narrowly endemic, in that they are found nowhere else on Earth. Examples are the notonemourid stoneflies, the paramelitid amphipods and the blepharicerid midges, whose closest relatives occur on other southern continents. These regions constitute the ancient supercontinent of Gondwana, which began to break apart some 145 million years ago through the process of continental drift.

Less distinctive patterns are shown within the aquatic fauna of the southern part of the Palaeotropical Kingdom. Invertebrates, for instance, are generally widely distributed throughout sub-Saharan Africa, although some are confined to the arid west, some to highland areas and some to the tropics. These ancient distribution patterns are disrupted by habitat alteration, pollution and invading alien species, sometimes causing the extinction of species such as the redfin minnows of the CFR.

TYPES OF RIVERS AND WETLANDS

The biggest division within inland aquatic ecosystems is between standing (lentic) waters, such as lakes, ponds, reservoirs and still-water wetlands, and running (lotic) waters – rivers and their estuaries. This distinction is also useful biologically, because different suites of organisms inhabit still and running waters.

Lakes and reservoirs

Amazingly, South Africa has only one natural water body – Lake Fundudzi in Limpopo – that truly fits the definition of a lake: a permanent body of water with areas deep enough that rooted plants do not occur.

Obviously, a lake or a pond can exist only if there is a suitable depression that can retain water, and enough water to keep it inundated. The northern hemisphere is rich in lakes, because glaciers ripped up the land during past ice ages, gouging out depressions that are now filled with water. The southern African landscape has not been glaciated for hundreds of millions of years, however, so the surface is generally flat and unsuitable for the formation of lakes. Lake Fundudzi is the result of a natural rock fall damming a river, but it is also in the only part of the country where mean annual rainfall exceeds mean annual evaporation. In contrast, Barberspan (North West) and Lake Chrissie (Mpumalanga), which are sometimes said to be lakes, dry out on occasion, because they are situated in areas where evaporation exceeds rainfall.

While the water in lakes and reservoirs ('man-made' lakes) may mix vertically as the currents circulate, a feature known as 'stratification' often occurs in these systems. As temperatures increase in spring, the upper waters of a lake heat up, becoming less dense than the deeper waters, which remain cool. Eventually a sharp temperature difference (thermocline) develops between the upper (warmer) and lower (cooler) water masses, effectively separating them from each other. You may well have felt this thermocline while swimming in a farm dam in summer – your lower limbs encountering water that is surprisingly colder than that near the surface. The upper layers are relatively warm, oxygen-rich and nutrient-poor. The lower layers, conversely, are cooler, poor in oxygen and rich in nutrients: they are effectively aquatic deserts in which few organisms thrive. Stratification is maintained until the upper waters cool, usually in autumn, and the waters mix again, resulting in a uniform temperature throughout the water column in winter.

Most lakes are no more than a few thousand years old. They fill with sediments fairly quickly and therefore disappear. Just a few lakes are more than a million years old, and two of them are in central Africa. They are lakes Nyasa (Malawi) and Tanganyika, which are older (about 2 million years), deeper (Tanganyika 1,470m) and longer (Tanganyika 676km) than almost all other lakes on Earth. They are located in the African Great Rift Valley, where tectonic forces are pulling the continent apart faster than the lakes are filling with sediment. These two African lakes support astonishing biodiversity, especially of fishes.

In southern Africa humans have made up for the lack of natural lakes by damming rivers to form artificial lakes for water storage and the production of hydropower. These lakes are commonly called ‘dams’ in the region, although the dam is actually the wall and the correct term for these water bodies is reservoir or impoundment. In South Africa alone the Department of Water Affairs has built more than 500 dams, of which the Gariep Dam on the Orange River is the largest, with a wall 88m high and a full supply capacity of 5,674,000m3.

The largest dams in the region are Kariba (wall 128m high, 98,000,000m3 at full supply capacity) and Cahora Bassa (wall 171m high, 52,000,000m3 at full supply capacity) on the lower Zambezi River. The largest water supply and hydropower scheme in the region is the Lesotho Highlands Water Project (LHWP), designed to bring water to Gauteng, the economic heartland of South Africa. This complex project has cost $3.9 billion and so far includes the Katse and Mohale dams on the Senqunyane River in Lesotho.

The quantities of water involved are relatively small: full supply capacity for Katse is only 1,950,000m3, about a fiftieth that of Kariba, yet the value of that water to both South Africa and Lesotho is enormous. South Africa now has water for the mining and industrial centres of the country, and the purchase cost of water doubled the gross domestic product of Lesotho when the first dam, Katse, was built in 1996. Completion of the final phase of the project, which began in 2013, is expected to cost another $4 billion or more. These costs are indicative of the extreme importance of water to developing economies.

With so few natural standing waters in the region, reservoirs have added a new dimension to the aquatic landscape, providing new and perennial standing-water habitat for fishes, waterbirds and plankton. At the same time, rivers are greatly altered when dams are built on them. The dam acts as a barrier to the up- and downstream migration of fishes and turns the river into a lake, sometimes a very big one, with altered temperatures and flows. Lake Kariba turned the Zambezi River into a lake 220km long, with a maximum depth of 95m and a width of 40km. Completed in 1958, Kariba Dam is, by volume, still the largest man-made lake on Earth.

The effects, mostly negative, of dams on rivers are discussed briefly on pp.27–28.

Wetlands

Wetlands can be large or small, perennial or temporary. The Okavango Delta (1) consists of a mosaic of islands and small waterways between reed beds. Coastal lakes, such as Soetendalsvlei (2) near Cape Agulhas, are shallow lakes fringed by reeds; although of estuarine origin, their water is usually fresh. Lake Otjikoto (3), in northern Namibia, is a sinkhole lake resulting from the collapse of the limestone roof of a subterranean cave. Wide, shallow valleys like this one (4) in the Cederberg in the Western Cape sometimes form perennial wetlands. In arid areas such as the Northern Cape, ephemeral wetlands (5) may appear only after unusually heavy rain. In contrast, every deep tyre track (6) may form a temporary pool after seasonal rain. Ephemeral wetlands (5, 6) support thriving populations of fairy shrimps.

Wetlands include vleis, pans, floodplains, ponds, bogs, mires, marshes and swamps. In fact the Ramsar Convention (an international convention for the protection of wetlands important for birds) uses the term ‘wetland’ to refer to all inland water bodies, including rivers, as well as shallow seas. In this book, however, we use the definition in the South African National Water Act (Act No. 36 of 1998), which defines a wetland as ‘land which is transitional between terrestrial and aquatic systems, where the water table is usually at or near the surface, or the land is periodically covered with shallow water and which land in normal circumstances supports, or would support, vegetation adapted to life in saturated soil’. In South Africa we also have the very useful word ‘vlei’, which derives from Dutch and refers to a wetland or coastal lake. In the Western Cape, a vlei refers to almost any kind of wetland, while further north a vlei fairly specifically refers to a reed bed in a dambo or valley-bottom wetland.

A classification system is available for South African wetlands, but for most purposes wetlands can be divided into those that are permanent and those that are temporary, and those that are associated with rivers, and those that are not. Some different kinds of wetland are illustrated below.

Wetlands of whatever type exist, because they are places where water can collect; characteristically they retain water in the landscape, either permanently or temporarily. Generally, wetlands are rather short-lived features, seldom more than a few hundred or thousand years old, disappearing as they silt up over time. Most of them support dense stands of plants adapted to living under, or with their roots in, water. When the plants die, or when they shed dead material such as leaves and flowers, decomposer bacteria break down the plant material, using up oxygen as they do so. The underwater and soil environment in wetlands is therefore characteristically anoxic (lacks oxygen), and wetland organisms must be able to tolerate these conditions. Depressions in seasonally or permanently arid areas may fill up with water, forming temporary wetlands. These systems range from small ponds on the Cape Flats that fill with water every winter to the massive vloere or temporary flats in the Kalahari Desert. The remarkable adaptations of invertebrates living in these systems are briefly discussed on p.18.

Wetlands are some of the most threatened and fastest disappearing inland waters in South Africa. Considering that they provide ecological benefits (‘ecosystem services’) to humans on a massive scale (see p.36), though, we ought to preserve and manage them better than we do.

Coastal lakes

The south and east coasts of southern Africa are dotted with shallow lakes or vleis, some of which contain fresh or almost fresh water, while others are brackish. It is likely that most or all of them originated as estuarine lagoons and, indeed, some, such as the interlinked Wilderness Lakes along the southern Cape coast and the Kosi system in northern KwaZulu-Natal, are normally open to the sea. Other coastal lakes, such as Lake Sibaya (northern KwaZulu-Natal), are now quite fresh, but their estuarine past can be inferred from their relict faunas, which include crabs, fishes and mussels whose closest relatives are marine. Many of these coastal systems are of recreational importance and are particularly sensitive to disturbance.

Rivers

Rivers are the ‘drains’ of the landscape, moving water and sediments to the sea. They are fed by rainfall, but also by ground water (which is why they flow even when it is not raining). The eastern and extreme southern parts of southern Africa are relatively well watered, while the western parts are arid: with few exceptions, rivers in the west are intermittent, flowing seasonally in wetter areas, or every few years in the driest areas.

The biggest river south of the Zambezi is the Orange, which drains a vast area, but, as previously mentioned, is not large by world standards. The remaining perennial rivers of southern Africa are much shorter than the Orange, although some are probably older; the courses of the Berg and Breede rivers can be traced back for at least 80 million years.

Along its length, a river changes considerably in appearance, reflecting changes in the physical conditions, the chemical content of the water, and the organisms that live in the river.

During floods, rivers may burst their banks, fan out, and drop eroded material as alluvium, forming a type of wetland known as a floodplain. The major floodplains of southern Africa are those of the Okavango, Chobe, Zambezi and Kafue rivers to the north, the Limpopo, Luvuvhu, Pongola and Mkuzi rivers to the northeast, the relatively small Sundays, Swartkops and Gourits rivers to the southeast, and the Berg River in the southwest. Floodplains are usually inundated with floodwaters carrying nutrient-rich silts and organic matter during the rainy season, so these can be very rich environments. The floodplains of KwaZulu-Natal, for example, fill a series of floodplain pans that support large numbers of fish, which are crucial to the diets of the local people.

Ground water

About two-thirds of the Earth’s fresh water is not found in rivers or wetlands, but lies underground in fractured rocks and in pore spaces in sandy substrates. It is no wonder, then, that billions of people throughout the world rely almost entirely on ground water for domestic and industrial use and for irrigation. A geological formation that holds a significant amount of ground water is known as an ‘aquifer’. While some water collects in aquifers as a result of the infiltration of rainwater, the rest may have accumulated over vast periods, in which case it is usually referred to as ‘fossil water’ and may remain undisturbed below the ground in deep aquifers for millennia. The size of an aquifer and the amount of water it contains are often unrelated to the amount of surface water in a region and, indeed, some of the largest aquifers occur in deserts or semidesert areas. Saudi Arabia provides a good example.

In the 1970s the Saudis started to grow and export grain from fields irrigated with water from an enormous aquifer. Having depleted the aquifer to the point that it was no longer useful, by 2009 they were again entirely dependent on imported grain. Given the current rainfall in the region, the aquifer is unlikely to be significantly recharged for a few more millennia.

A great deal of ground water is already being used in South Africa, as is evidenced by the large number of windmills (‘wind pumps’ is the technical term) seen in farming areas throughout the country.

Ground water currently supplies about 13% of South Africa’s water requirements, over about two-thirds of the country’s surface area. Although irrigation is the largest user, ground water also supplies over 300 towns and smaller settlements, mostly in the Orange River basin. It is estimated that South Africa currently uses a quarter to a third of the ground water that can reasonably be exploited; more can be exploited, but this would come at a significant economic and environmental cost. In addition, a very large and deep aquifer is now known to exist in the fractured rocks of the Table Mountain Series, stretching roughly from the west coast to Port Elizabeth and inland for 200km or so. The total amount of water in the aquifer seems to be immense, but we do not yet know how much of it is fossil water (and therefore not to be ‘mined’). Investigations are currently under way to see how much of this water can be exploited annually without seriously depleting the aquifer.

The upper water surface of an aquifer, known as the water table, may ‘daylight’ (reach the surface) if the aquifer is filled to capacity. The extensive temporary winter ponds on the Cape Flats near Cape Town are examples of this phenomenon.

Some remarkable invertebrates live in ground water. In southern Africa, most are very tiny and poorly known, but some, such as amphipods of the genus Sternophysinx (p.248), are very large for ground water-dwelling animals.

Substances filtering down from the surface easily pollute aquifers. These may come from landfills, storm-water drains, septic tanks and cultivated fields or, near the sea, from sea water. A polluted aquifer may become unusable indefinitely, because there is no way of removing the pollutants once they are below the ground. Acid mine drainage (see p.29) is an interesting case of ground water becoming polluted in situ when it comes into contact with acid-producing rocks.

ADAPTATIONS TO LIFE IN FRESH WATER

An ‘ecosystem’ is composed of all the living and non-living entities that interact in a single place. It can be as small as a jam jar of pond water or as large as the Amazon River. A ‘habitat’, by contrast, is the area within an ecosystem where a particular species lives. The habitat of a redfin minnow is a mountain stream, whereas the habitat of a leopard toad is a small wetland. Within habitats, uniform regions are sometimes known as ‘biotopes’. So, for example, riffles (shallow areas of high turbulence) and backwaters are different biotopes within the stream habitat.

The organisms living in fresh waters are adapted to the habitats in which they live. Bottom dwellers – the ‘benthos’ – live in or on the bottom of a river, lake or wetland. Some benthic animals in rivers prefer to live in fast-flowing, rocky stretches known as ‘riffles’, while others prefer to live in ‘runs’, where water flow is smooth. Those living in the water column can be divided into active swimmers – the ‘nekton’, such as fish – and those that drift with the currents – the ‘plankton’. ‘Phytoplankton’ consists of planktonic organisms that photosynthesise, while ‘zooplankton’ is animal plankton. The suite of minute organisms, including algae, bacteria and blue-greens, coating the surfaces of stones is usually called the ‘periphyton’.

As a result of natural selection over the ages, plants and animals have become adapted to living in particular places and under particular physical and chemical conditions. Below we discuss some of the adaptations of aquatic animals to their habitats, including adaptations for feeding, respiration, reproduction and survival in temporary wetlands. For instance, animals living in fast-flowing mountain streams have a variety of ways of attaching to the substratum so that they are not washed away, and some insects cement their eggs to the substratum for the same reason.

Food and feeding

The whole complex of ‘who eats whom’ is termed a ‘food web’. Plants form the basis of the food web in virtually all ecosystems, but are especially important in standing waters since, generally speaking, little food material enters these systems from outside. Thus, the food available in a wetland is largely produced within its own boundaries and consists of large plants, phytoplankton and the slimy layer of periphyton that coats hard surfaces, such as rocks, stems and leaves. Periphyton, which consists of countless million single-celled algae, protozoans and fungi, forms the base of river food webs too, although in the case of rivers, substantial quantities of food also enter from upstream and from the bank.

All of this food can be fed upon by small grazing animals, such as insects and snails, while decaying material, including animal faeces, is decomposed by microorganisms, particularly bacteria and fungi. The rotting particles, with the bacteria, fungi and algae adhering to them, are eaten by animals that either sieve food particles out of the water, or feed on particles of the oozy mud itself, digesting the finely divided detritus and the bodies of the microorganisms contained in it. Other animals feed on larger chunks of decaying tissue and its associated aggregates of microorganisms. Small predatory invertebrates feed upon the grazers and detritivores and are, in turn, fed upon by other larger invertebrates, fish and birds.

Plankton and fish form a second food web consisting of free-drifting organisms. Phytoplankton is fed upon by planktonic animals (the zooplankton), which in turn are food for filter-feeding invertebrates and other larger organisms, such as fish. The two food webs are interlinked, because part of the food supply of the benthos comes from a rain of particles from the open water above, the domain of the plankton and fish, while some fish also feed on the benthic organisms living on the bottom. Ultimately, plants and animals die and their bodies, too, become food for others, mostly the microscopic decomposers: the bacteria and fungi. These decomposers break down organic matter and turn it back into its constituent parts – carbon dioxide and water, and nutrients such as compounds of nitrogen and phosphorus. The carbon dioxide and water are released back into the atmosphere to start the cycle again, while other living organisms such as plants take up nutrients, and the cycle starts again.

Respiration and gas exchange

Most organisms, both plants and animals, require oxygen for respiration. Animals living in mountain streams tend to have uncomplicated gills, as there is little difficulty in extracting oxygen from the ample supply in the water. Those that live in less oxygen-rich waters have a number of adaptations that facilitate respiration. Some, like certain mayfly nymphs, have large and complex gills; dragonfly nymphs pump water in and out of their gill-lined rectums; some beetles store a bubble of air under the wings or in pads of respiratory hairs in different parts of the body. The posterior ends of rat-tailed maggots, which live in foul anoxic conditions, are hugely extendible and can be stretched to the surface of the water in order to take in air. A few species, such as tubifex worms and some chironomid larvae, are able to survive in near-anoxic conditions, because they contain haemoglobin, which picks up even the slightest traces of oxygen in the water.

Reproduction and life cycles

In the sea, invertebrates most commonly reproduce by ‘broadcast spawning’, a process whereby males and females shed their sperm or eggs simultaneously into the sea, fertilisation taking place in the open water, away from the parents. The fertilised eggs develop into minute planktonic larvae, which grow and eventually metamorphose into small adults that settle down on the sea bed and continue the life cycle. This process is effective where plenty of food is available for the parents, and where the environment is relatively safe and predictable.Where food is limited or the environment is harsh, this simple way of reproducing is less advantageous, however, because not all eggs are fertilised, the developing larvae are unprotected, and many are lost in the current or preyed upon by plankton feeders.

For a number of reasons invertebrates in fresh waters almost never reproduce by broadcast spawning. Firstly, food resources for the parents are less readily available than in the sea. Secondly, eggs and sperm cannot survive the osmotic shock as water is drawn suddenly out of the body’s cells when exposed to fresh water. Thirdly, minute plankton-feeding larvae cannot maintain their positions in running waters; and lastly, the amount of food suitable for planktonic larvae is restricted. Freshwater invertebrates, and fishes, therefore generally tend to produce fewer, but larger, eggs than their marine relatives do.

Because large eggs are a good food source for predators, the eggs of many species, such as branchiopods, have a tough eggshell, which has to be added to the egg after fertilisation. This means that copulation must occur so that the eggs can be fertilised within the mother’s body before the shell is added. This increase in egg size has interesting domino effects: copulation necessitates specialisation of both the male and female reproductive systems and particular behavioural adaptations to allow male and female to recognise each other. The ultimate extension of these adaptations is ‘viviparity’, the retention of babies within the mother’s body until they are ready to be born as miniature adults. This extreme solution seldom occurs in fresh waters, however, but parental care of eggs does occur in groups as different as water bugs, cichlid fishes and snails.

Not all animals reproduce sexually. Some, such as sponges, hydras and many oligochaetes, generally reproduce asexually, by ‘budding’, as do certain stages in the life cycles of parasites such as the bilharzia fluke (Schistosoma). Another form of asexual reproduction is known as ‘parthenogenesis’, or so-called virgin birth, in which the eggs develop without being fertilised. Parthenogenesis is known in many freshwater groups, including some rotifers, ostracods and cladocerans. Bdelloid rotifers and darwinuloid ostracods are said to be ‘ancient asexuals’, because no males have ever been found, even in fossil forms. Parthenogenesis occurs in cladocerans like Daphnia only when conditions are favourable. During spring and summer, each female produces large batches of ‘summer’ eggs that develop and hatch without being fertilised. These eggs all hatch as females. When conditions begin to deteriorate in autumn (lower temperatures, less food available), the females begin to produce eggs that hatch into either males or females. When these offspring mature they copulate, the resulting fertilised ‘winter’ or ‘resting’ eggs being rather different from the ‘summer’ eggs. Each female produces only one or two ‘winter’ eggs, and these are housed in a thick-walled protective coat called an ‘ephippium’. They are able to survive harsh winter conditions, hatching and starting the parthenogenetic cycle again in spring. Sexual reproduction is important, because it produces genetic diversity, which is advantageous in harsh and unpredictable environments.

Surviving in temporary wetlands

All inhabitants of temporary ponds, especially those in arid areas, have to overcome many obstacles to staying alive and leaving viable offspring. Firstly, they must grow rapidly enough to be able to reproduce before the pond dries. Secondly, these inhabitants must be able to survive in the deteriorating conditions as the pond shrinks. Thirdly, their eggs must hatch at an appropriate time.

Tadpole shrimps in the genus Triops are a good example of the difficulties faced. A female tadpole shrimp lays her eggs on the mud at the bottom of a pond. The pond dries up, so the female dehydrates and dies, leaving her eggs unprotected on the drying mud until the rains arrive some months or years later. There is no guarantee that the next rainfall will be sufficient to form a pond that lasts long enough for the eggs to hatch and grow into adults that can themselves lay eggs; on the other hand, the next rainfall could equally well be the best in years, creating ideal conditions. For this reason, there needs to be some way of preventing all the eggs from hatching at once – otherwise they could all die before reproducing, and the species would run the risk of becoming extinct – but at least some of the eggs ought to hatch after every soaking to take advantage if the conditions should be good. How is this achieved? When deposited by a female, the fertilised egg begins to develop into a young tadpole shrimp larva within the eggshell. Then all development stops. The egg inside the cyst dries out and the outer spongy coat becomes rock hard and resistant to mechanical abrasion, attack by bacteria or fungi, and degradation by the intense sunlight and ultraviolet radiation to which it is subjected during the years of lying inert on the floor of the dry pond. The larva inside is in a state of ‘anabiosis’: there is no sign of life whatsoever until the cyst gets wet again, whereupon it may possibly hatch.

We do not really understand why it is that only some cysts hatch when re-wetted. The eggs of some species of fairy shrimp will not hatch if they are kept in darkness or in water low in oxygen, so eggs that are buried in mud, even just a couple of millimetres deep, are unlikely to hatch. We also know that the cysts of certain fairy shrimps from the Namib Desert, even from a single batch laid by a single female, and kept under identical conditions, do not all hatch at the first wetting. Some hatch ‘first off’, some after only two, three, or even five or six wettings. The explanation for this versatility must be genetic variability or some difference in the way that the eggs are treated within the mother’s body. With the exception of brine shrimps, all the large branchiopods of southern Africa occur in temporary waters only, and their eggs have to dry out before they are able to hatch.

Many aquatic or semiaquatic flies, particularly midges, mosquitoes and certain biting flies, may raise their young in temporary waters. The famous chironomid midge Polypedilum vanderplanki, from West Africa, can breed in even the most ephemeral waters, because the larvae can be completely dehydrated and still slowly ‘come back to life’ when they are re-wetted.

In southern Africa, only two genera of fish are confined to temporary biotopes, although individuals of other genera may be stranded and survive in pools in drying riverbeds. The African lungfish and various species of Nothobranchius, the annual killifishes, are normally confined to temporary pans, and they or their eggs typically withstand desiccation while buried in mud during the dry season.

The tadpoles of many species of frog are known to use temporary waters at least occasionally, and several species in southern Africa apparently breed only in temporary wetlands. One remarkable species, the Pygmy toad (Poyntonophrynus vertebralis), lives deep in the Namib Desert, the adults hiding under sheets of fractured rocks during dry periods and finding temporary pools for their tadpoles after occasional desert rainstorms.

PHYSICAL AND CHEMICAL CHARACTERISTICS OF FRESH WATERS

Individuals of each species are able to live only within certain limited environmental conditions. Some species of fish, for instance, require the very pure waters of mountain streams, while others are pollution-tolerant or can live only in estuaries or in the sea. Here we discuss some of the major physical and chemical properties of water that affect the distribution and abundance of aquatic organisms.

Water availability

By definition, aquatic organisms need water, but the availability of water varies seasonally, except in really wet areas. Typically, wet and dry seasons result in rainy periods, in which rivers flow strongly and wetlands are deep, interspersed with drier periods when rivers may stop flowing entirely, and wetlands dry up.

So both the amount and the timing of water provision are important features to which aquatic animals have become adapted over time. The amount of water will obviously determine how deep the water is, how fast a river will flow, how much dilution of pollutants there might be, and for how long a wetland will be able to retain water at the end of the rainy season. This amount is mostly determined by rainfall, but in some areas snowmelt is the main source of water. Timing is also important. Often a period of rising (or falling) water is the cue for fishes and invertebrates to begin breeding. If this cue is received at the wrong time of year, reproductive cycles may become desynchronised with food availability, for instance, resulting in breeding failure. While the speed of the current is arguably the most important variable in rivers, in wetlands it is the length of the period of inundation (the ‘hydroperiod’). The edges of wetlands usually consist of zones of different wetland plants, from those near the water’s edge that require water all year round to dryland plants at the very edges of the moist soil zone.

Modification of the hydroperiod results in a change in the environmental conditions around the wetland, often preventing certain species of plant from living there. If these plants are important for flood control, for instance, then a change in hydroperiod can compromise the extent to which the wetland can hold back floodwaters.

Physico-chemical conditions in the water

Natural waters differ from each other in a number of important physical properties, such as temperature, and in the concentrations of numerous substances, such as common salt. Organisms of any species can survive in a particular river or wetland only within certain ranges of these physical attributes and between certain concentrations of chemical substances. The term ‘water quality’ is often used in this context but really refers to the suitability of physical and chemical conditions in the water relative to the requirements of the users – usually humans.

Temperature

The range of temperatures experienced in water is much narrower than that in the air because of the high specific heat of water (see ‘The life-giving properties of water’, p.7), and because water freezes at 0ºC. In contrast, the land gets much colder than this. Even relatively small changes in temperature may be felt acutely by aquatic animals, since they can neither escape into the shade nor use evaporative cooling to reduce their temperature as many land animals do under excessively hot conditions. The rate of chemical reactions, including photosynthesis and respiration, increases with increasing temperature, typically doubling with each 10ºC rise in temperature. This means that the higher the temperature, the faster the rate at which organisms live, the more energy they use and the more oxygen they require. Lowered temperatures result in lower metabolic rates, which, in turn, reduce the speed at which animals can move, and increase the length of time taken to reach maturity, the rate at which eggs can be produced, and so on.

Most species that breed seasonally use changes in particular environmental factors as cues to start (and sometimes to stop) breeding. Day length is one such cue and temperature is another. Increased temperatures resulting from global warming are already altering the time of year when many terrestrial birds start to breed; there is no doubt that the same is true for aquatic invertebrates and fish, although evidence is not yet available, because changes going on under the surface of the water are seldom obvious.

Oxygen

Oxygen is essential for most organisms, both plants and animals, but there is less oxygen in water than in air, and its concentration in the water depends on temperature, air pressure and salinity. At a temperature of 15ºC, fresh water fully saturated with oxygen contains only about 10mg/l oxygen, while sea water contains about 7.9mg/l oxygen. Furthermore, less oxygen can dissolve in warm water than in cold water. Raising the temperature of water from 5ºC to 40ºC, for instance, will halve the concentration of oxygen from 12.8 to 6.4mg/l, while boiling completely removes oxygen from water. Thus another important feature of temperature in water is that it controls the amount of oxygen available for biochemical reactions, including photosynthesis and respiration.

Decomposer bacteria are mostly responsible for the breakdown of organic material into its constituent parts – carbon dioxide, water and nutrients. Many of these bacteria require oxygen, so where a lot of organic material is being broken down, oxygen is very rapidly used up. Under these circumstances there is seldom any photosynthetic activity to replace the oxygen, so anoxic (oxygen-deficient) conditions develop. Many fish die-offs, mistakenly attributed in the local press to mysterious toxins, are in fact the result of anoxia: organically rich sewage or dying blooms of phytoplankton are decomposed by bacteria, oxygen is used up, and the fish die of asphyxia.

Salinity, conductivity and dissolved solids

Salinity, which refers to the saltiness of water, affects living cells by modifying the movement of water and salts across cell membranes. Most organisms have relatively narrow tolerance limits for saltiness and are confined to fresh water, brackish water, sea water or the extra-salty water found in salt pans.

Those species that are able to tolerate wide ranges of salinity are mostly found in estuaries, where the salinity is very low when the tide is going out and very high when the tide is coming in, bringing sea water into the river mouth. Non-estuarine species are seldom able to cope with big swings in the salt content of the water they live in, because this upsets the water and salt balance in their bodies. The salinity of the blood and other body fluids of most animals approximates that of sea water, probably because their distant ancestors evolved in the sea. Bony fishes are an exception, however. They have very dilute blood, which seems to be because the ancestors of bony fishes evolved in fresh waters.

To measure salinity, a meter that measures electrical conductivity (EC) is used. EC is a measure of the ability of water to conduct an electric current, which, in turn, is proportional to the number of ions in solution. The unit of measurement is Siemens per unit distance.

Salinisation is a process whereby fresh waters become saline over time. It usually results from human activities, although naturally saline soils or ground water are common in certain areas (e.g. in parts of the Berg River in the Western Cape). For instance, purified sewage return flows can cause increasing salinisation, because the sewage effluents are strongly aerated during treatment, allowing evaporation of a significant proportion of the water. In parts of South Africa, particularly the Berg and Breede river valleys in the Western Cape, and the Sundays River in the Eastern Cape, salinisation results from spray irrigation: when irrigation water is sprayed, some of the water itself evaporates, leaving slightly saltier water to fall onto the crops and the surrounding ground. Over the course of a hot windy summer season, salt accumulates on the ground and in the upper layers of the soil. This salt is washed into the ground water during the first rains of winter, and eventually bleeds back into the river water, which can become so salty from this process that it cannot be used to irrigate crops.

Light

Light is crucial for life on Earth, because it is needed for photosynthesis, and photosynthesis is the source of almost all living and once-living material (e.g. coal and oil) on Earth. The depth to which light penetrates water determines the depth to which plants can survive, and is affected by the colour and turbidity of the water. Even in the clearest waters, the depth at which plants can grow is seldom as much as 200m, and in turbid waters it is far less. It is only in the shallowest waters that rooted plants and phytoplankton can grow, producing all of the food for the entire ecosystem, other than that which falls in from outside. In stratified lakes (p.12) the lack of nutrients in the upper warmest layer reduces photosynthetic potential still further.

Particulate material and turbidity

Although some natural waters are very clear, many are quite murky, because they carry particles in suspension.

These particles include both finely divided inorganic materials, such as silt and clay; organic particles, such as plant and animal debris; and even living plankton. The presence of these particles in the water makes it ‘milky’ or murky, a phenomenon technically known as ‘turbidity’ (not to be confused with ‘turbulence’, which is the chaotic movement of water over rocks). The ability of visual predators to see their prey, and of prey to detect their predators, is greatly reduced in highly turbid waters. Interestingly, many African waters are very turbid and members of an African-endemic family of fishes, the Mormyridae, are able to navigate and locate prey in these murky waters by generating a weak electric field. The particles that make water turbid also have a direct effect on organisms living in turbid environments. They can clog gills, interfering with respiration, and some particles adsorb or release nutrients and toxins from their surfaces. In all, life in turbid waters is rather different from that in clear waters.

Colour

While turbidity derives from particles suspended in water, colour comes from substances dissolved in water. Like black tea or iron salts, these dissolved substances impart colour but not murkiness to the water.

Turbid water, in contrast, is like milky tea or coffee. At high concentrations, salts of heavy metals impart colour to water. Examples are the reddish-brown colour of some salts of iron (which are what stains white walls in contact with borehole water) or the blue tinge of copper sulphate in school laboratory experiments. Naturally coloured waters are often stained clear brown, reddish or yellow by complex organics contained in peat, which is undecayed plant material.

Acidity (pH)

pH is a measure of acidity, which is caused by hydrogen ions in water. The lower the pH, the greater the concentration of hydrogen ions, and the more acidic the water. The pH scale ranges from 0 (highly acidic) through 7 (neutral) to 14 (highly alkaline). Natural systems in most of southern Africa are slightly alkaline (pH 7–8), but the dark peat-stained waters in the Cape Floristic Region (CFR) are much more acidic (see box, below). Very high levels of photosynthesis, as seen at Zeekoevlei and the Hartbeespoort Dam, for instance, may drive the pH as high as 10 or so, while acid water draining from mines may fall below pH 1. Most organisms are unable to survive at pH values outside of the neutral range, which is about 6.5–8.5. The reason is partly that metabolic processes are slowed down, because the acidity or alkalinity has to be neutralised for cells to work effectively, but more importantly, it has to do with the effects of low pH on metals in solution. The best example is aluminium (Al), but the same applies to metals such as copper and nickel: at low pH, Al in water becomes very toxic. It interferes with ionic and osmotic balance, it thickens mucus (on the gills of fishes, for instance, resulting in impeded gas exchange); it also interferes with processes such as muscle contraction and the transmission of nerve impulses. Lakes polluted by acid rain, for instance, may be entirely sterile because of the effects of Al on the fauna.

WATER IN THE CAPE FLORISTIC REGION

The waters of most rivers and wetlands in the Cape Floristic Region (CFR) of the southern and southwestern Cape are characteristically dark in colour and exceptionally acidic.

The reason is that these rivers drain fynbos-covered slopes, whose soils are very low in nutrients. Rain falls almost entirely in winter, so the area experiences long periods of summer drought, when wildfires are common. Loss of leaves to herbivores is particularly serious for plants living under conditions where there is a poor nutrient supply and frequent fires. Thus, many fynbos plants produce a suite of organic chemicals known as ‘secondary plant compounds’ that seem to deter herbivores.

These substances, which are chemically complex, include tannins, flavonoids and humic acids. Because they are rich in a chemical group known as phenols, they are also sometimes called ‘polyphenols’. When the plants die and decay, the polyphenols are released into the soil, where they undergo transformation into a complex of chemicals together known as ‘humic substances’. Because humic substances are organic acids, they impart acidity when dissolved in water (i.e. they reduce pH). Some of them are coloured, and that is why peat-stained waters are dark in colour. (Tea is dark in colour because tea leaves contain high levels of these polyphenols.) Many of the invertebrates living in these black waters are endemic, adapted specifically to the unusual conditions, and occur nowhere else, being dependent on the unusual but natural chemistry of the water. Even slight increases in pH, for example from concrete in building materials, can raise the pH sufficiently to eliminate some of these extraordinarily adapted organisms.

The black-water Dawidskraal River in Betty’s Bay in spate; when this photograph was taken, the pH of the water in the river was 3.5, the lowest ever recorded in a pristine river in the CFR.

HUMAN ACTIVITY IN AQUATIC ECOSYSTEMS

Freshwater ecosystems are some of the most endangered on Earth, as are many of their inhabitants, particularly fishes. In southern Africa, much of which is semi-arid to arid, we have rather few rivers and wetlands, so the effects of human activities are particularly devastating. As the human population increases, so more and more water is needed for domestic purposes, for irrigation to supply food, and for industry to satisfy our constant demand for more goods. At the same time, many of our waste products end up in our rivers and wetlands, polluting them. Pollutants reduce the ability of aquatic ecosystems to cleanse themselves and to provide the goods that we expect from them: fish, rice, building materials and so on, as well as clean water. Rivers have traditionally been seen as cleansing systems in the landscape, unwanted human waste from faeces to sump oil being thrown into them and disappearing from view downstream.

Although rivers do dilute and remove a fair amount of unwanted material, their capacity to do so is limited, especially when water is also being removed (‘abstracted’) for human use. Furthermore, what is ‘downstream’ to one onlooker is ‘upstream’ to another: discarded waste moves downstream, being joined by the waste generated locally, so that the river becomes more and more polluted the further downstream one looks. Less obviously, whatever is happening in the catchment area can have an effect on its rivers. Poor ploughing practices cause erosion – and the soil lands up in the river. Excess fertilisers on fields filter down to the ground water and from there to the river itself, as do toxins from poorly managed landfills.

Lakes and wetlands are obviously also affected by human activities, sometimes with even greater consequences than for rivers. Wetlands occur where they do because drainage is reduced or non-existent: indeed, many wetlands have no outlet at all. Thus waste material from human activities will accumulate – it cannot be washed downstream.

Use of surface water

For as long as people have existed, they have been abstracting water from rivers and wetlands. Since water is one of the primary requirements for life, securing a reliable supply was one of humankind’s earliest technical triumphs. With the growth in human populations, however, it has become necessary to store more and more river water for industrial and domestic supply, as well as for irrigation. This is usually accomplished by constructing dams. In regions where rainfall is seasonal, dams are designed so that the volume of water retained will be sufficient to last through the dry season until the next rains come. In southern Africa, however, where rainfall is not only seasonal, but varies enormously from year to year, dam walls must be able to retain enough water to tide us over several dry years. This is why so many of southern Africa’s dams seem to be inordinately large: it is simply to ensure a reliable supply of water, even in droughts.

Damming a river essentially turns it into a lake. Plants and animals that live in still waters then replace riverine ones, and the entire ecosystem is transformed. Moreover, downstream of a dam less water is available for the river, so both width and depth tend to decrease and biotopes such as riffles may be lost. Naturally, organisms confined to such biotopes will not be able to survive, so biodiversity also tends to decrease. Fish are particularly likely to be affected by a loss of water in the river, and dams also act as barriers to fish migrating up- or downstream. Today, most designers of dams include ‘fish ladders’ that allow fish to move up and down the river.

Apart from causing an overall reduction in river flow, some dams, particularly those built to supply irrigation schemes, hold back the water during the wet season and release it during the dry season, when water is needed for irrigation. In this way the normal hydrological cycle is disrupted and may lead to a mismatch between cues for fish to breed and conditions suitable for them to do so. Furthermore, water released from thermally stratified reservoirs may be warmer than the river downstream if it is drawn from the upper layers of the reservoir, or colder if drawn from the lower layers, disrupting the normal thermal regime downstream.

Dams built to provide hydroelectric power have a different effect. Because electricity generation requires a constant flow of water through the turbines, discharge of water into the river below the dam is constant throughout the year. Since natural river flow is highly seasonal in most of southern Africa, the change from fluctuating to constant discharge results in hydrological conditions that some riverine animals are unable to cope with.

Use of ground water

At first glance, it would seem that abstraction of ground water would have few detrimental consequences, but pumping water from below the ground results in a lowering of the water table. Unless that water is entirely replenished by rainfall, the water table will progressively drop over time, drying out the plants whose roots tap into the aquifer and sometimes modifying the hydroperiod. Eventually the aquifer may become entirely depleted. Huge ground-water-fed agricultural developments in many parts of the world (e.g. in the Great Plains of the United States) are failing for this simple reason. Overall, excessive use of water is one of the most devastating consequences of human activities on this planet, and yet the most crucial for maintaining the human population.

Pollution

When human populations were small, and technologies were simple, pollutants were confined to human and animal wastes. The more people there are, though, and the more sophisticated their technology, the more waste is produced and the more toxic it is. Much of this waste lands up in rivers or wetlands. The term ‘water quality’ is usually used loosely to refer to the physical and chemical attributes of water, while ‘poor water quality’ is often taken to imply some kind of pollution. Pollution is just one factor that may be responsible for poor water quality, but it is one of the most obvious ones and the public is very much aware of phenomena such as sewage spills, fish kills, smelly green water and litter. A relatively simple definition of pollution is the action or process of making land, water, air, etc., dirty and not safe or suitable to use.

Pollutants can be divided into individual dissolved chemicals such as acids or pesticides, and particulate matter, which ranges from coal dust and soil in suspension to rubble, metals and plastics. Dissolved pollutants usually occur as mixtures in effluents discharged from pipes or storm-water drains into rivers, lakes or wetlands, or directly into the sea. These are said to be ‘point source’ discharges.

In contrast, sources of pollutants such as agricultural run-off and atmospheric pollution are diffuse and therefore very difficult both to quantify and to control. Pollutants entering water bodies in this way are said to be of ‘non-point-source’ origin.

Since living organisms thrive optimally in water with particular combinations of physical and chemical attributes, and since optimal conditions vary for each species, alterations in water quality will affect different species to a greater or lesser extent. Increasing changes in water quality may eliminate some species and allow others to invade, until not a single species of the original assemblage remains. The effects of altered water quality on aquatic communities include:

The subject of pollution is a vast one and only two examples are discussed here: persistent organic pollutants (POPs) and acid mine drainage (AMD).

Persistent organic pollutants are particularly toxic and, as the name suggests, take a very long time to degrade. They are of such concern that the international Stockholm Convention was set up in 2001 to regulate and discontinue their use. A document produced during the Convention notes that:

‘Exposure to POPs can lead to serious health effects including certain cancers, birth defects, dysfunctional immune and reproductive systems, greater susceptibility to disease and even diminished intelligence. Given their long-range transport, no one government acting alone can protect its citizens or its environment from POPs.’

The Convention ‘requires Parties to take measures to eliminate or reduce the release of POPs into the environment.’ POPs include pesticides like lindane, aldrin and DDT, industrial chemicals such as polychlorinated biphenyls (PCBs) and industrial by-products such as the dioxins.

Acid mine drainage is a topic of particular concern in the gold- and coal-mining regions of South Africa, although the same problems occur wherever subterranean rocks are exposed to the air. Most rocks naturally contain sulphide minerals such as iron pyrite (fool’s gold). In water, oxidation of the sulphide produces sulphuric acid, a process accelerated by iron-reducing bacteria, especially the archaean bacterium Acidithiobacillus ferroxidans. During the life of a mine, water is pumped to the surface and discharged, so acid does not build up. Water is not pumped from abandoned mines, though, so it remains in contact with the acid-generating rocks until the mineshafts fill with water that eventually spills over at the surface. This water is toxic, because it also contains other heavy metals; it is often radioactive if one of these metals is uranium; and it is corrosive, because it is strongly acidic. The problem is compounded in areas like Gauteng, much of which is underlain by limestone (calcium carbonate), which is soluble in acid. When the acidic water reaches the limestone rocks, these dissolve, resulting in sinkholes. The Western Basin is already overflowing near the Cradle of Humankind. Various solutions are possible, but all are extremely expensive. The first is to resume pumping (no doubt at the expense of the taxpayer). Another includes neutralising the acidic water.

Exploitation of living resources

The aquarium trade has exploited the colourful cichlid fishes of Lake Nyasa (Malawi), and commercial fishing boats catch large quantities of small fishes in Lake Kariba (Zimbabwe), but by and large southern Africa’s inland waters are exploited by local communities rather than by commercial enterprises. Fish are by far the most important living resource derived by subsistence farmers from inland waters over much of southern Africa, and many stocks are heavily utilised. Generally, the size of the fish being caught has grown smaller as the larger ones have been fished out.

In fact, in some rivers and wetlands in northern Namibia, for instance, the main fishing gear these days is mosquito netting, which allows even the smallest fish to be caught. Clearly this is detrimental to the health of the stock as a whole and is strongly discouraged by fisheries managers. When one is a poor subsistence farmer responsible for feeding one’s family, though, protecting the stock for the future is less important than catching enough food to feed the family for the day.

Wetlands and river banks also provide building materials in the form of wood and reeds, while some wild foods such as madumbe, or taro, are important items in the diet of many people living close to wetlands in the wetter parts of the region.

As a matter of interest, taro was one of the earliest food plants to be imported to Africa. It seems to have originated in southeast Asia but is known from remains in Ancient Egypt at least 5,000 years ago. Subsistence agriculture is practised in and around many wetlands in southern Africa: some wetland plants provide grazing for goats and cattle; some wetlands are partially drained to allow maize and other crops to be planted, while other areas are kept inundated to form rice paddies. Rice production is becoming more and more important in the northern parts of southern Africa, to the detriment of irrigation water for other purposes, and is transforming large areas of wetland.

Invasive and alien species

Many species of non-native plant and animal – known as alien species – have been transported by people from areas where they occur naturally to other parts of the world that they could never have reached without assistance. Such introductions can be deliberate, many plants being introduced for food or as ornamentals, for example, and many animal species have been introduced as pets, or for hunting, fishing and aquaculture. Some of these have subsequently escaped and established wild populations. A much larger group of introduced species has entered the region accidentally.

Because introduced species often lack the predators, competitors and parasites that kept their populations in check in their native habitats, some have become extremely abundant in their new homes, where they have greatly disrupted the ecosystem. Indeed, they have become ‘invasive’ and are now regarded as the second-largest threat to global biodiversity, after habitat destruction. In southern Africa, the invading species having the greatest effects on our inland waters include water hyacinth and Kariba weed, and fish such as bass and trout.

Introduced animals

The earliest freshwater introductions took place centuries ago, the first record being of the importation of carp into the Cape as a food delicacy in the 1700s. (Carp are bottom-feeders that stir up sediments and muddy the water, increasing water treatment costs, as well as affecting other fish that feed by sighting their prey). Early in the 20th century it became the official policy of some nature conservation authorities to introduce and propagate fish such as trout and bass to enhance recreational angling, without any consideration of their impact on native biodiversity; such policies were reversed only in the 1980s. Not all aliens come from other continents. Some, including species of yellowfish, kurper and catfish, have been translocated (moved from one area to another) within southern Africa, sometimes with detrimental effects on native fishes. Right now the voracious Sharptooth catfish is appearing in many systems throughout the region, presumably introduced illegally by misguided people for personal gain.

Currently there are more than 20 species of introduced or translocated freshwater animals in South Africa. By far the most important of these are fishes. It is well known that freshwater fishes are the most threatened single group of animals on the planet. The main reasons for this are pollution, abstraction of water, and physical degradation of habitat. In some places, like the CFR, however, these threats pale into insignificance in relation to the devastation caused by invasive alien fishes. Trout, particularly Rainbow trout, were introduced to the region in the late 19th century and were the first invaders to have significant effects on the distribution ranges of the highly endemic smaller fishes of the southwestern Cape. The species that has the greatest effect on native fishes throughout the region, though, is probably the Smallmouth bass, a voracious piscivore (fish-eater) from the United States. Several species of native redfin minnows are hanging on in a few small streams where they have not yet been consumed by the bass and trout that now rule these rivers. It is only a matter of time before these small natives will become extinct, however, since nature conservation authorities do not have the resources to eliminate the invaders.

Aquarists and aquaculturists have also imported various species of freshwater crayfishes into South Africa from different parts of the world. The American crayfish, which has been introduced into Mpumalanga, is a destructive and uncontrollable predator that has already been responsible for the elimination of invertebrate communities, and thus vital fisheries, in Lake Naivasha in Kenya. This crayfish burrows into mud banks, including the walls of small dams, causing these structures to fail, so affecting the natural environment as well as invertebrate biodiversity wherever it occurs. Another introduced crayfish, the Australian redclaw, has brought with it Diceratocephala boschmaia, a small ectoparasitic flatworm, which is now also parasitising native crabs. Various other fish parasites such as fish lice have also arrived in the region either inside, or attached to, their hosts when these hosts were imported from their native countries.

The native African fishes commonly known as tilapias or chambo illustrate a different problem. These fishes are crucially important sources of protein for local communities, so several species have been translocated from one water body to another, either to support fisheries, or for fish farming. They tend to interbreed quite easily, and in several large reservoirs or lakes where they have been introduced, the original species appear to have given way to hybrids. Concern has been voiced that the original species may soon disappear, to be replaced by just one or two hybrid ‘species’ throughout the continent – extinction by hybridisation. Without a clear understanding of the consequences, and specific permission from conservation authorities, translocation of fishes from one water body to another is irresponsible and should never be allowed to happen.

Invasive aquatic plants

A number of invasive aquatic plants, mostly waterweeds from South America, are major economic pests. They include Kariba weed, water hyacinth and the water fern Azolla. All of these, and some others, are ‘Category 1 declared weeds’ in South Africa. This means that they ’may no longer be planted or propagated, and all trade in their seeds, cuttings or other propagative material is prohibited. They may not be transported or be allowed to disperse’ (Conservation of Agricultural Resources Act No. 43 of 1983). These weeds are serious pests, because they all exhibit the potential for explosive growth and can cover and choke vast areas of standing and slow-running water.

These invasive alien plants are widespread throughout southern Africa and have already cost the region hundreds of millions of rands in attempts to eradicate them. Water hyacinth, for instance, prevents boating, angling and skiing, disrupts water flow in irrigation channels, and blocks sluices. It may also create the ideal conditions in which malarial mosquitoes and bilharzia-carrying snails breed. Although several different kinds of insect (including weevils, moth caterpillars and bugs), as well as a variety of rusts and other fungi, have been used as biological control agents, nothing seems to be able to eradicate the weed completely from the systems that it infests. For many years it covered huge areas of Lake Victoria. Biological control agents have hugely reduced its extent, but as soon as the plants are reduced in number, so are the insects that attack them, allowing the plant to expand once more. Currently the control agents are able to keep the hyacinth under control in most parts of the lake but are unable to eliminate it completely. Kariba weed is another major menace in southern Africa. At one stage it covered more than 2,200km2 of the surface of Lake Kariba with mats more than half a metre thick. A wide range of terrestrial invaders, such as the Black wattle, also thrive in relatively moist habitats along river banks. Their dense growth blocks channels and smothers natural vegetation. In addition, the shallow roots of some of these species are not tenacious enough to hold on during spates, so they do not promote bank stabilisation. Thus, erosion of river channels often accompanies their invasion. The greatest threat posed by acacias and other terrestrial plants is that they take up enormous quantities of water. Recognising this important threat, South Africa’s Working for Water programme clears large areas of plant invaders, on the understanding that clearing the aliens ultimately leads to improved stream flow – and thus more water for human use.

THE EFFECTS OF HUMAN ACTIVITIES ON OUR HEALTH

Degraded rivers and wetlands and new water-development projects have consequences for human health. New reservoirs, for instance, provide fresh environments for disease-transmitting organisms such as mosquitoes and bilharzia-carrying snails and may create favourable conditions for the spread of other human diseases. Such environmental changes are particularly problematic throughout Africa, because this continent is home to many water-associated diseases. Populations of disease-transmitting organisms, like mosquitoes and the snails that carry bilharzia parasites, may build up in new lakes, since both snails and the larvae of many malarial mosquitoes favour still backwaters, usually where there is some rooted vegetation. In the case of bilharzia, host snails may be free of the disease in a new lake behind a dam wall until just one person suffering from the disease infects them by defecating or urinating near, or into, the water. Thus, unhygienic habits and chronic lack of sanitation will inevitably lead to the infection of the snails, which then release bilharzia larvae, with the subsequent infection of local bathers.

Bilharzia and malaria are endemic in rural populations throughout most of Africa except for the southwestern tip. If global warming continues as expected, malarial vectors may become established in Cape Town and those parts of Europe and the United States where malaria was endemic until the 1960s. Wetlands are potential sources of both malarial mosquitoes and bilharzia snails, so wetland managers rehabilitating these systems should be aware of potential problems caused by restoring suitable habitat for these organisms.

HABITAT DESTRUCTION

While the major effects of humans on rivers are related to abstraction and storage of water – pumping and damming – and pollution, the major damage to wetlands is physical modification or complete annihilation of these systems. Agriculture for crops and livestock, and urban expansion, are probably the most significant ways in which wetlands are modified or lost.

Several decades ago, it was established that well over 60% of the wetlands in the Mfolozi catchment in KwaZulu-Natal had been entirely destroyed, and this is probably a fair reflection of the situation throughout South Africa. Additional pressures, like pollution and loss of water, place the remaining wetlands under great threat. It may be difficult to defend the protection of any individual wetland, but the combined loss of thousands of hectares of wetland can have profound effects on the hydrology of a catchment. The devastation caused by many floods is greatly increased where wetlands are no longer present to reduce run-off and retard floodwaters.

RESTORATION OF RIVERS AND WETLANDS

Two uniquely South African government initiatives are assisting in the rehabilitation or restoration of our rivers and wetlands.

‘Working for Water’ was established by the late Minister Kader Asmal to clear alien vegetation, which evapotranspires so much water that stream flow in invaded catchments can be hugely reduced. This programme has the additional benefit of creating thousands of jobs for unskilled people.

‘Working for Wetlands’ is a sister programme, developed later than Working for Water, which restores damaged wetlands, reducing loss of wetland function and preventing soil erosion. It, too, employs unskilled people to do much of the physical work. Both programmes have been carefully designed to provide skills to the labourers, several of whom are now managing restoration projects outside of government employ.

CONSERVATION OF BIODIVERSITY AND FRESHWATER ECOSYTEMS

Biodiversity, or biological diversity, is ‘the variability among living organisms … and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.’ This definition comes from the international Convention on Biological Diversity (CBD), a key document regarding sustainable development, which seeks to conserve biodiversity and encourages nations to develop strategies for its conservation and sustainable use. The CBD was adopted at the Earth Summit in Rio de Janiero in 1992. South Africa was one of the original signatories.

A diversity of ecosystems and species on Earth is important for many reasons. Biodiversity promotes, and is an indicator of, ecological stability, and therefore the sustainability of the Earth’s ecosystems, and is necessary for efficient biogeochemical cycling (biological and environmental pathways through which elements are circulated). Additionally, a biodiverse Earth provides us with useful species such as crops and medicines. Biodiversity also provides us with the pleasure of natural beauty. We should conserve biodiversity not only for these reasons, but also to bequeath these benefits and this heritage to future generations. What is more, since we are utterly unable to recreate biodiversity when it has been destroyed, we have a moral duty to conserve it.

The preservation of certain ecologically important species, sometimes called ‘keystone species’, may be crucial for the normal functioning of some ecosystems. For example, small redfin minnows are keystone species in some mountain streams in the CFR. These minnows feed largely on invertebrates, which in turn graze on algae and other epilithon from the surfaces of the rocks in the riverbed. When redfins are present, the surfaces of the rocks are thickly coated with algae, because numbers of grazing invertebrates are kept under control by the redfins. When redfins are absent (usually because bass or trout have eliminated them), populations of grazing invertebrates increase and keep the surfaces of the rocks virtually clear of algae.

Modifications to ecosystems are often accompanied by large and surprisingly rapid changes in community structure. It seems that the community structure of ecosystems tends to be fairly stable and that a change, be it biotic or abiotic, can ‘flip’ the system into a different but still stable state. This point of change, when the ecosystem flips from one stable state to another, is sometimes called a ‘knickpoint’. As an example, many coastal lakes have either clear waters and rooted weeds, or pea-green alga-laden waters with no waterweeds. When in the alga-rich state, so little light penetrates to the bottom that waterweeds are unable to establish themselves. Conversely, when waterweeds flourish, they take up so many nutrients that the algae cannot bloom. Each of these states is fairly stable. The environmental conditions acting as the knickpoint at which the lake flips from one state to the other are not well understood.

ECOSYSTEM SERVICES PROVIDED BY FRESHWATER SYSTEMS

Intact ecosystems provide the fundamental requirements for life, including clean water, oxygen and food. They also decompose dead biomass, contributing to the cycling of carbon dioxide and nutrients. The provision of these substances and the processes involved are known as ‘ecosystem services’. Some ecosystem services, such as the production of oxygen by plants, are provided by all ecosystems, but aquatic ecosystems provide certain services that others do not.

Aquatic ecosystems primarily provide us with clean water. Rivers do this first by washing solid and dissolved materials downstream. Water running in rivers is also cleansed, because rivers provide habitats for decomposer bacteria under and between stones and in the bed sediments. These bacteria break down organic matter, releasing carbon dioxide (most of which escapes into the air) and nutrients (which are washed downstream). Wetlands are even more efficient than rivers at providing clean water. Large dense stands of wetland plants act as filters, slowing down water movement and trapping particles (including sediments, organic matter and bacteria). These particles settle out on the bottom, so water flowing out of the wetland is clear and not muddy. Particles of organic material (including faeces) are decomposed by bacteria in the sediments, being converted to carbon dioxide or methane and nutrients. Wetlands are nature’s water-purification works. Artificial wetlands are easy to construct and are very effective in cleansing polluted water.

Wetlands are also remarkably effective for flood control. Because of the presence of dense stands of rooted plants, wetlands ameliorate the force of floodwaters, forcing the water to spread out and reducing its damaging effects. Furthermore, the beds of wetlands often contain deep layers of peat, which is dead, but not decayed, plant material. Peat acts as a sponge, taking up water and releasing it slowly after a flood has passed. In this way, wetlands reduce flash flooding of the type that devastated Laingsburg in the southern Cape during 1981. Some of the retained water also percolates into the soil, recharging ground water in aquifers.

It is often said that water running to the sea is wasted, but this is untrue: rivers provide the most valuable materials required by the coastal zone for normal functioning. The nutrients transported to the sea by rivers are key. The sea itself is very poor in nutrients, and so populations of coastal fishes often depend largely on river-borne nutrients to support the base of their food chain. In a similar way, sediments transported by rivers sometimes supply sand to beaches. When rivers are dammed, and sediments can no longer reach the sea, coastal erosion can become problematic.

Furthermore, wetlands and river floodplains form some of the most productive lands on Earth, turning more carbon into living tissue than do rainforests, area for area. In this way, they not only perform the vital role of absorbing vast quantities of carbon dioxide, the main ‘greenhouse’ gas, but also produce huge quantities of food. Finally, wetlands provide vital habitat, food and shelter for an enormous variety of plants and animals, from tiny to huge, and from aquatic to terrestrial.

Aquatic ecosystems in general are also valuable resources for recreation and tourism, biodiversity and conservation, and have cultural and spiritual value. In all, the value to humans of ecosystem services is incalculable. Everything we need for life is provided by the Earth and its resources, and yet we continue to exploit them and damage the Earth in an entirely unsustainable way. Unless the services provided by the planet and its biota are properly managed, Earth will not be able to sustain the increasing billions of people that rely on it for life and the pursuit of happiness.

MANAGEMENT OF WATER AND AQUATIC ECOSYSTEMS IN SOUTH AFRICA

The South African National Water Act (NWA) 36 of 1998 was one of the first in the world to recognise the importance of protecting rivers and wetlands in order for them to provide adequate quantities of clean water for human use. Implementation has been slow, and imperfections in the Act have become apparent over the years. Nonetheless, the legislation is important, because it recognises that rivers and wetlands are unable to provide the ecosystem services that humans expect from them if they are not properly managed. Two major provisions of the NWA are the requirement to keep an ‘ecological reserve’, an amount of water that must be left in a river or wetland to allow it to continue to provide water sustainably; and that the unit of management of rivers and wetlands is to be the catchment.

While the NWA deals with proper management of rivers and wetlands for providing water for human use, other acts, particularly the National Environmental Management Act (NEMA) 107 of 1998 and its amendments, deal with other aspects of the environment. The provisions of the NEMA relate to the constitutional right of South Africans

‘to an environment that is not harmful to their health or well-being; and to have the environment protected, for the benefit of present and future generations, through reasonable legislative and other measures that

This legislation does not provide protection or conservation for specific ecosystems such as rivers and wetlands, so the South African National Biodiversity Institute (SANBI) has recently developed a database of Freshwater Ecosystem Priority Areas (FEPAs). While a great deal still needs to be done to undertake detailed surveys of these FEPAs, and while they are not part of environmental legislation in South Africa (and therefore have no legal standing), their designation is an important step towards better protection and conservation of our wetlands and rivers. Currently, except for the St Lucia wetlands, no national or provincial reserves have been developed specifically for rivers or wetlands.

CLASSIFYING PLANTS AND ANIMALS

Unless scientists know precisely which species of organism they are working on, they cannot reliably compare their results with those of other scientists. The people who describe, name and identify species are known as taxonomists. Without the efforts of taxonomists there could have been little progress in almost any field of biology.

Each species known to science has been described and given a Latin binomial that is always italicised. Although these double-barrelled names may seem quite long and foreign, this system is necessary to avoid the confusion that can arise from the use of common names. For example, while the same fish is called the Elf in the Cape, the Shad in KwaZulu-Natal, the Tailor in Australia and the Bluefish in the United States, its scientific name, Pomatomus saltatrix, is the same everywhere. The formal system includes both a generic (‘genus’) name, which always has its first letter capitalised (e.g. Homo) and a species name (the ‘specific epithet’), which is not capitalised (e.g. sapiens). The formal scientific name for the human species is Homo sapiens, derived from Homo (‘human’) and sapiens (‘wise’ or ‘shrewd’). After the Latin binomial has been written in full in a text for the first time, the generic name may later be abbreviated to the first letter (e.g. H. sapiens). The specific epithet is never abbreviated.

Since there may be as many as 30 million species living today, it is essential to be able to classify or organise them into groups, based on common characters and on common descent from distant ancestors. The scheme laid out opposite is one of many possible classification systems. We have chosen it because it is relatively simple, it has stood the test of time and it is found in many textbooks. Other classification systems differ from this one, but which to use is a matter of personal preference until such time as we know a great deal more about the genetics and interrelationships of the groups, particularly the more obscure ones.

Note that some botanists use the term ‘division’ rather than ‘phylum’. Note also that fungi are common in fresh waters, but that so little is known about them that they are not included in this book.

CLASSIFICATI ON SCHEME FOR THIS BOOK
DOMAIN	KINGDOM/SUBKINGDOM	PHYLUM/DIVISION	CLASS/SUBCLASS	ORDER	COMMON NAME
Bacteria		Various			bacteria
		Cyanobacteria			blue-greens (blue-green ’algae’)
Eukaryota	‘Protists’	Chlorophyta			green algae
		Euglenophyta			euglenoids
		Bacillariophyta			diatoms
		Dinophyta			dinoflagellates
		Rhodophyta			red algae
		Charophyta			stoneworts
		Ciliophora			ciliates
		Amoebozoa			amoebas
		‘Mastigophora’			flagellates
	Plantae	Bryophyta			mosses
		Anthocerotophyta			hornworts
		Marchantiophyta			liverworts
		Pteridophyta			ferns
		Angiospermae	Magnoliopsida		flowering plants
	Animalia	Porifera			sponges
		Cnidaria		Hydrida	hydras
				Limnomedusae	jellyfish
		Platyheminthes			flatworms and flukes
		Nemertea			nemerteans
		Nematomorpha			wireworms
		Gastrotricha			hairybacks
		Tardigrada			water bears
		Nematoda			roundworms
		Rotifera			rotifers
		Bryozoa			moss animals
		Annelida	Oligochaeta		earthworms
			Hirudinea		leeches
		Mollusca	Bivalvia		mussels and clams
			Gastropoda		snails and limpets
		Crustacea	Branchiopoda	Anostraca	fairy shrimps
				Conchostraca	clam shrimps
				Notostraca	shield shrimps
				Cladocera	water fleas
			Ostracoda		seed shrimps
			Copepoda		copepods
			Branchiura		fish lice
			Malacostraca	Amphipoda	amphipods
				Isopoda	isopods
				Decapoda	crabs and shrimps
		Chelicerata	Arachnida	Acarina	mites
				Araneae	spiders
		Hexapoda	Collembola		springtails
			Insecta	Ephemeroptera	mayflies
				Odonata	dragonflies and damselflies
				Plecoptera	stoneflies
				Hemiptera	bugs
				Megaloptera	alderflies, dobsonflies and toe-biters
				Orthoptera	crickets and grasshoppers
				Dermaptera	earwigs
				Trichoptera	caddisflies
				Coleoptera	beetles
				Diptera	flies, midges and mosquitoes
		Chordata	Pisces		fishes
			Amphibia		frogs
			Reptilia		reptiles
			Aves		birds
			Mammalia		mammals

INTRODUCTION