LAKES AND RIVERS SUPPORT RICH and varied plant life and, although they have fresh water in common, many aspects of their ecology are different. Lakes are extensive, and some are deep. The deepest lakes in our islands are in glacially deepened troughs in the mountains. Loch Morar in western Scotland reaches a maximum depth of 310 m, while the largest lake in Britain, Loch Lomond (71 km2), has a maximum depth of 190 m, and the second largest, Loch Ness, reaches 230 m, and its average depth is over 130 m. In surface area, none of the lakes in Britain matches the larger lakes of Ireland. Lough Neagh (388 km2) is the largest, and Lough Corrib (200 km2), Lough Derg (118 km2), Lough Ree and Lough Mask (89 km2) are all larger than any Scottish lake. All these big Irish lakes are relatively shallow.
Geologically, lakes are temporary features of the landscape, because they trap sediment brought in by the rivers entering them, which in the course of time fills the lake basin. Many of our mountain lakes have flat alluvial ground bordering the river at their heads; sometimes former lakes are obvious as flat areas in the bottoms of valleys, as in the Nant Ffrancon in Snowdonia. Most of the lakes in Britain and Ireland owe their existence to the glaciations, either through glacial scour over-deepening valleys (Scottish Highlands, Snowdonia and the English Lake District) or through moraines and other deposits left by the glaciers and ice-sheets damming the surface drainage (many Irish lakes and some in Britain); many lakes in the mountains owe something to both processes. England was less heavily glaciated, so outside the Lake District there are only a few small natural lakes in the Pennines, and in the lowlands the Shropshire and Cheshire meres. Almost all the other lakes in England are man-made, as reservoirs or sometimes as landscape features. The Norfolk Broads are medieval peat diggings (Chapter 13), and have a modern parallel in the flooded gravel pits of the Thames valley. A few natural lakes exist behind shingle-bars on the coast, such as Slapton Ley in Devon and the Loe Pool in Cornwall.
The longest rivers in our islands are the Shannon (386 km), the Severn (354 km) and the Thames (346 km). The longest river in Scotland is the Tay (188 km), closely followed by the Spey and the Clyde; all these are shorter than the Barrow (192 km) in southeast Ireland, but they all drain large tracts of rainy upland country so they are major rivers in the amount of water they discharge. However, we have a multitude of smaller rivers, no less interesting in their vegetation (Fig. 150).
Lakes may be deep, and they provide a large fetch for wind, but flow rates in the water are slow, even in lakes that have substantial rivers entering and leaving them. By contrast all rivers and streams are shallow – never more than a few metres deep – and the rate and character of their flow is a cardinal factor in their ecology and vegetation (Haslam 1978). The large vascular plants, the macrophytes, make up the conspicuous part of the vegetation of both lakes and rivers, but in lakes these may be matched or even exceeded in importance by the microscopic phytoplankton suspended in the water.
FIG 150. The River Stour below Sturminster Newton, Dorset. A slow-flowing lowland river, draining from the chalk and fertile farming country, with alder and willows, reed canary-grass (Phalaris arundinacea) and yellow iris (Iris pseudacorus) lining the banks. Emergent aquatics include common reed (Phragmites australis), arrowhead (Sagittaria sagittifolia), bur-reed (Sparganium) and club-rush (Schoenoplectus lacustris), with white water-lilies (Nyphaea alba) floating in the open water.
In both lakes and rivers the chemistry of the water (see box on p. 242) has a major influence on the vegetation. The concentration of calcium and other cations varies widely, and with that comes variation in pH. Rainwater always contains dissolved substances, in part derived from sea-spray and in part from blown dust and gases in the atmosphere, and pure unpolluted rainwater on average has a slightly acid pH from dissolved carbon dioxide (CO2) in the atmosphere. Rain that has percolated through limestone (with CO2 raised by the respiration of soil organisms) dissolves calcium as bicarbonate, so streams draining from limestone country have high calcium (Ca2+) and bicarbonate (HCO3–), and a near-neutral pH (around 7); they are ‘hard-water’ streams. By contrast, streams draining from non-calcareous rocks (notably the old hard rocks of many upland districts) contain much less Ca2+ and HCO3–, and their composition is dominated by sodium (Na+), magnesium (Mg2+), with some Ca2+, balanced by chloride (Cl–, ultimately from sea-spray) and sulphate (SO42–, partly from sea-spray, partly from pollution), and they have a mildly acid pH; they are ‘soft-water’ streams.
Bicarbonate concentration, CO2 concentration and pH in natural waters are intimately related. Over a wide range, pH rises with calcium concentration, which, because high Ca2+ generally reflects solution of limestone, is balanced by HCO3–. Beyond about 20 mg/litre Ca, there is little further increase in pH with increasing Ca, and the pH of calcareous waters is largely determined by the concentration of CO2. If the Ca concentration is 40 mg/litre, and the water is in equilibrium with atmospheric CO2, the pH will be about 8.2. Natural waters are rarely in equilibrium with the atmosphere (they come closest to that in the spray of waterfalls), because of the effect of respiration and photosynthesis on CO2 concentration. Most of the organic matter in river and fen waters comes from neighbouring land habitats, and breakdown processes predominate in the water itself, so CO2 levels are high – at the same Ca concentration of 40 mg/litre the pH may be no higher than 6.5–7. In a lake, photosynthesis by the phytoplankton can produce wide daily swings of pH. The phytoplankton (and some other aquatic plants) can deplete CO2 to far below the concentration in the atmosphere, and raise pH as high as 10. However, we are then well into the region where the carbonate/bicarbonate ratio rules, and the possible HCO3– concentration is limited by the insolubility of calcium carbonate, which precipitates out as an incrustation on the plants or as fine calcareous sediment. Paradoxically, beyond a point set by the solubility product of CaCO3, the concentration of Ca can only increase with a concomitant increase of CO2, and falling pH (Fig. 151).
FIG 151. The relation of pH to concentration of calcium in water from Darnbrook and Cowside Becks, Littondale, Yorkshire. All the points fall well below the line of equilibrium with atmospheric CO2. The broken line is the limit set by the precipitation of calcium carbonate (CaCO3). The black triangles are springhead sites at which water was emerging from deep in the limestone; crosses are sites where tufa deposition is taking place.
These mineral solutes are not the whole story, because a number of other elements are essential for the growth of all plants, especially nitrogen (N), phosphorus (P) and potassium (K). Limnologists contrast eutrophic waters, rich in plant nutrients, with oligotrophic waters in which growth is limited by nutrient supply; mesotrophic waters come between these extremes. A fourth category, dystrophic, takes in the acid peat pools and lochans in blanket-bog country (Chapter 15) in which peaty dissolved organic matter plays an important role in their water chemistry. Most lowland lakes are both moderately calcareous and moderately eutrophic. Lakes and tarns (lochans) in the mountains are generally non-calcareous (unless calcareous rocks outcrop in their catchments) and oligotrophic.
Many waterside plants can grow equally well in permanently wet soil, or rooted at the bottom in shallow water as ‘emergent aquatics’ – plants rooted under water but with their leaves and flowers above (Fig. 152). The common reed (Phragmites australis), one of the few cosmopolitan plants, can dominate tall herb fens on peat in the Norfolk Broads (and elsewhere) or grow in up to half a metre or so of water; its rhizomes often form floating rafts fringing the shore. It can tolerate a wide range of nutrient regimes, moderately acid to calcareous, avoiding only the most acid and nutrient-poor placesS4. Reed canary-grass (Phalaris arundinacea) looks rather like Phragmites in build and leaf but is easily recognised by the membranous ligule at the base of the leaf blade, where Phragmites has only a line of hairs. Phalaris is more nutrient-demanding than Phragmites, and often marks places where there is heavy silting, so perhaps does not really qualify as an ‘aquatic’S28. Another large grass of the banks of eutrophic lakes and rivers, which often forms floating rafts over open water, is reed sweet-grass (Glyceria maxima)S5.
Some big sedges (Cyperaceae) are also characteristic of this habitat. The great fen-sedge (Cladium mariscus – just ‘sedge’ in East Anglia) like Phragmites spans the boundary between open water and tall fen on firm peatS2. It has long-lived, coarse saw-toothed leaves, which effectively suppress almost all competing species. Smaller than Cladium, but still substantial sedges and important in our vegetation, are the big species of Carex (which give their name to the alliance Magnocaricion of Continental vegetation ecologists). They include the tufted-sedge (Carex elata)S1, a local pioneer at the edge of calcareous pools and lakes (Fig. 153), perhaps often associated with fluctuating water level, the greater tussock-sedge (C. paniculata), whose massive tussocks fringe the lower courses of the Broadland rivers and line valley-bottoms elsewhereS3, the greater pond-sedge (C. riparia), which often fringes the banks of eutrophic lakes, slow-flowing rivers and canalsS6, and the lesser pond-sedge (C. acutiformis), which occurs in similar but perhaps generally more calcareous habitatsS7; the latter two are commonest in the lowlands of southeast England. Bottle sedge (C. rostrata), by contrast, is a plant of the north and west and of oligotrophic waters, fringing many mountain and moorland lakes and tarns, mostly acid, but sometimes calcareous as at Malham TarnS9. In sheltered situations it is often accompanied by such species as marsh cinquefoil (Potentilla palustris), bogbean (Menyanthes trifoliata) and water horsetail (Equisetum fluviatile), in effect a transition to fenS27. The common club-rush (Schoenoplectus lacustris) grows in lakes, slow-flowing rivers and canals in water usually 0.5–1.5 m deep, and ranging from acid and base-poor to highly calcareous, and from oligotrophic to eutrophicS8. The grey club-rush (S. tabernaemontani) is much less bound to deep water and is salt-tolerant; it is mainly coastal but occurs widely inland in more or less calcareous and eutrophic sites in the English MidlandsS20.
FIG 152. Profile transect across White Moss Loch, just north of the Ochill Hills southwest of Perth. Marsh and fen plants around the lake margin give way first to emergent aquatics (common reed, bogbean, slender sedge, bottle sedge), then to floating-leaved (Potamogeton natans, P. gramineus) and submerged aquatics (the pondweeds Potamogeton trichoides, P. obtusifolius, P. perfoliatus, P. filiformis and Canadian waterweed Elodea canadensis). From Tansley 1939, after Matthews 1914. Carex filiformis = C. lasiocarpa, C. inflata = C. rostrata, Comarum = Potentilla palustris, Potamogeton heterophyllus = P. gramineus.
FIG 153. Tufted-sedge (Carex elata) swamp on the northwest shore of Lough Bunny, Co. Clare, at a time of low water level, September 1975.
The bulrushes (Typha) are locally conspicuous reedswamp plants. Typha latifolia occurs in standing or slow-moving, circum-neutral and more or less eutrophic waters on silty substrates throughout Britain and Ireland; it is often common around lowland lakes, ponds and reservoirs and in sluggish streams and river backwatersS12. The seeds are dispersed by wind, and T. latifolia is often quick to colonise newly excavated pools. Lesser bulrush (T. angustifolia) tends to grow in deeper water and in less eutrophic conditionsS13, but there is a great deal of overlap between the two species.
A shorter but striking plant, often a prominent feature of moderately deep still or slow-flowing waters, is arrowhead (Fig. 154a), with long ribbon-like submerged leaves and arrow-shaped emergent leaves held, like the three-petalled white flowers, well clear of the surface of the waterS16.
The remaining emergent aquatics all make less impact on the landscape than the reedswamp dominants just discussed, but one common and often dominant species, branched bur-reed (Fig. 154b), provides the occasion to mention some of the others. Branched bur-reed commonly fringes eutrophic ponds, ditches, canals and rivers on mineral soil, and like a number of other reedswamp plants can grow either in permanently wet soil or in shallow water. Common associated species include water plantain (Alisma plantago-aquatica), common spike-rush (Eleocharis palustris), water mint (Mentha aquatica), water-cress (Rorippa nasturtium-aquaticum), fool’s-water-cress (Apium nodiflorum), water forget-me-not (Myosotis scorpioides), gypsywort (Lycopus europaeus), yellow iris (Iris pseudacorus), meadowsweet (Filipendula ulmaria), skullcap (Scutellaria galericulata) and hemlock water-dropwort (Oenanthe crocata), a western plant with us (and in Europe as a whole) and one of the most poisonous plants in our floraS14. Muddier, shelving water-margins are often dominated by floating sweet-grass (Glyceria fluitans) with a similar range of associates, which sometimes include marsh foxtail (Alopecurus geniculatus) and false fox-sedge (Carex otrubae)S22. Two rather rare plants of these water-margin communities are the greater spearwort (Fig. 155b), a magnificent spear-leaved buttercup, and the beautiful ‘flowering-rush’ (Fig. 155a) – nothing to do with the true rushes despite its rush-like flower stems, but more nearly related to the water-plantains.
FIG 154. Emergent aquatics: (a) arrowhead (Sagittaria sagittifolia), Fiddleford, Dorset; (b) branched bur-reed (Sparganium erectum), Slapton, Devon.
FIG 155. Emergent aquatics: (a) flowering-rush (Butomus umbellatus), Othery, Somerset Levels; (b) greater spearwort (Ranunculus lingua), Woodford, Co. Galway, July 1966.
Many water plants are rooted but have floating leaves, most obviously the waterlilies. The white water-lily (Nymphaea alba) can tolerate a very wide range of waters in pools and slow-flowing rivers. Because it has no submerged leaves it is easily damaged by boat traffic. It occurs throughout Britain and Ireland, but is probably most naturally at home in acid, often peaty, oligotrophic lakes and tarns of the north and west, where it grows with occasional pondweeds (Potamogeton natans and P. polygonifolius), bulbous rush (Juncus bulbosus) and a thin scattering of other aquatic plantsA7. The yellow water-lily (Nuphar lutea, Fig. 158a), by contrast, prefers at least moderately calcareous and eutrophic waters, so is commonest in the southeast and the Midlands of England and the midland plain of Ireland. It occurs widely in slow-flowing rivers, little-used canals (as it has submerged leaves it is less vulnerable to disturbance by boats), dykes, lakes and pools, with a longer and more diverse list of associates than is usual for the white water-lily, of which perhaps the most frequent are the common water-starwort (Callitriche stagnalis), the duckweeds Lemna minor and L. trisulca, Canadian waterweed (Elodea canadensis) and amphibious bistort (Persicaria amphibia)A8.
FIG 156. Pondweeds: (a) broad-leaved pondweed (Potamogeton natans), Exminster, Devon; (b) shining pondweed (P. lucens), Exeter Canal; (c) fennel pondweed (P. pectinatus), Fiddleford, Dorset; (d) beaked tasselweed (Ruppia maritima), Cuckmere Haven, Sussex.
Amphibious bistort itself is common and sometimes dominant in the shallows of still and slow-flowing, sometimes fluctuating, waters (it is frequent in some of the Irish turloughs). It occurs widely in Britain and Ireland, mostly in the lowlands in moderately base-rich and moderately eutrophic water. It can grow in wet ground but flowers much more freely as an aquatic plant, with the flower spikes held stiffly above the surfaceA10. A number of pondweeds (Potamogeton) can have floating leaves. Much the most widespread is the broad-leaved pondweed (Fig. 156a), which occurs throughout Britain and Ireland in still or slowly flowing waters (and even occasionally in a fully submerged form in fast-flowing streams). It has extraordinarily wide ecological tolerance, and can grow in acid or calcareous, oligotrophic or eutrophic waters, and from near-terrestrial conditions to water 5 m deepA9.
Most of the other pondweeds have only submerged leaves. The perfoliate pondweed (Potamogeton perfoliatus) is a broad-leaved species, which, with other submerged aquatics, often dominates the macrophyte vegetation forming bulky masses in still or very gently flowing, mesotrophic and usually rather base-poor waters over the whole of Britain and Ireland, from Kent and Kerry to the Shetlands. The dominant P. perfoliatus is most often accompanied by alternate water-milfoil (Myriophyllum alterniflorum), a range of pondweeds including P. gramineus, P. berchtoldii, P. obtusifolius, P. natans and P. pusillus, Canadian waterweed, amphibious bistort, the moss Fontinalis antipyretica and charophyte algae of the genera Chara and NitellaA13. In the most base-poor and oligotrophic pools and streams alternate water-milfoil may dominate alone, with occasional Juncus bulbosus, intermediate water-starwort (Callitriche hamulata), Potamogeton gramineus and the moss Fontinalis antipyreticaA14. In base-rich, mesotrophic to eutrophic conditions in clear, unpolluted waters, this is replaced by a community dominated by fennel pondweed (Fig. 156c) and spiked water-milfoil (Myriophyllum spicatum), with a long list of associates of which the most frequent are the pondweeds Potamogeton pusillus, P. lucens (Fig. 156b), P. crispus, P. natans, P. filiformis, P. berchtoldii and P. perfoliatus, Canadian waterweed, amphibious bistort and unbranched bur-reed (Sparganium emersum)A11. Where the water is turbid or polluted the fennel pondweed may dominate alone or with only a few associatesA12.
The white-flowered water-crowfoots, like the pondweeds, have both broad floating leaves and finely dissected submergerged leaves, and several are important in our aquatic vegetation. The common and pond water-crowfoots (Ranunculus aquatilis and R. peltatus) have been much confused; both are probably important (and interchangeable?) in the fringing vegetation of stagnant waters in ponds, ditches and slow-flowing streams along with such species as floating sweet-grass, water-cress, fool’s-water-cress, broad-leaved pondweed, spiked water-milfoil, various water-starworts (Callitriche spp.) and the duckweeds Lemna minor and L. trisulcaA19, A20. In coastal brackish waters these two common water-crowfoots are replaced by R. baudotii, typically in water half-a-metre or so deep accompanied by such species as horned pondweed (Zannichellia palustris), soft hornwort (Ceratophyllum submersum), fennel pondweed, the duckweeds Lemna minor and L. gibba, the water-starworts Callitriche stagnalis and C. obtusangula, mare’s-tail (Hippuris vulgaris) and beaked tasselweed (Fig. 156d)A21. Two water-crowfoots are characteristic of faster-flowing waters. The stream water-crowfoot (Fig. 157) is a very characteristic dominant of calcareous, moderate to quite fast-flowing rivers with sandy or gravelly bottoms, and generally draining chalk or limestone catchments. In southern England, these include many of the classic chalk streams and their equivalents on the Jurassic limestones of the Cotswolds – and farther north and west rivers and streams on Devonian and Silurian rocks in Wales, on Carboniferous limestone of Derbyshire and Yorkshire, and on Devonian rocks in the Border counties of Scotland. Apart from the dominant water-crowfoot (spectacular when in flower), this is a rather species-poor community; among the commoner associates are water-cress, brooklime (Veronica beccabunga) and lesser water-parsnip (Berula erecta)A17. Deep fast-flowing oligotrophic to mesotrophic rivers with stable stony beds are the favoured habitat of the river water-crowfoot (Ranunculus fluitans). Many of the British occurrences are clustered in the Welsh Marches or around the southern Pennines; R. fluitans is local in Scotland, and known only from Co. Antrim in Ireland. This is another species-poor community, with spiked water-milfoil the most frequent associateA18.
FIG 157. Stream water-crowfoot (Ranunculus penicillatus ssp. pseudofluitans), River Otter, East Budleigh, Devon.
There remain only some free-floating water plants to consider (Fig. 158). At first sight, it is paradoxical that these plants, free from the constraints of an attachment to firm ground, are so bound to the shelter (and constraints) of small water bodies, or of rooted aquatic vegetation on the shores of lakes. The largest of our free-floating aquatics is the water soldier (Fig. 159b), which was formerly rather widespread in eastern England but has declined over the years for reasons that are not clear, and Dutch ecologists are not unanimous about its nutrient requirements. From the available data, its surviving Broadland localities have rich aquatic floras, with such species as greater bladderwort (Utricularia vulgaris), water violet (Hottonia palustris) and greater spearwortA4. Frogbit (Fig. 158b) is much commoner and occupies similar habitats in the clear, unshaded, mesotrophic to eutrophic (but unpolluted) waters of ditches and ponds over the whole of lowland England. The most constant species beside frogbit are the floating duckweeds (Fig. 159a), and the submerged ivy-leaved duckweed (Lemna trisulca), Canadian waterweed and rigid hornwort (Ceratophyllum demersum); the tiny rootless duckweed Wolffia arrhiza occurs occasionally in this community, especially in the Somerset LevelsA3. Several of these free-floating species can behave as community dominants. Ceratophyllum demersum often dominates still or slow-moving eutrophic waters in ditches, ponds, sluggish streams and canals, with smaller quantities of Canadian waterweed, common water-starwort (Callitriche stagnalis) and the water-crowfoot Ranunculus circinatus, and is perhaps favoured by eutrophication with agricultural run-offA5. By contrast, the rarer soft hornwort, with more finely divided brighter-green leaves, usually grows in company with Potamogeton pectinatus and Ranunculus baudotii; it is mainly coastal in eastern England, but comes inland in the Fens and Somerset Levels, and in the southwest MidlandsA6. The floating Lemna species can both form extensive quasi-pure stands on their own, accompanied by a rather predictable list of common associatesA1, A2. The little introduced American water-fern Azolla filiculoides floats like a duckweed and can cover the surface of an unshaded ditch or pond in a remarkably short space of time; its dense, buoyant and vigorous growth can crowd out virtually all competitors.
FIG 158. Floating-leaved aquatics, a rooted and a free-floating species: (a) yellow waterlily (Nuphar lutea), Fiddleford, Dorset; (b) frogbit (Hydrocharis morsus-ranae), Shropshire Union Canal.
FIG 159. A contrast in free-floating aquatics. (a) Two duckweeds, Spirodela polyrhiza (left) and Lemna minor (right), Exminster, Devon; (b) water soldier (Stratiotes aloides), Capellen a. d. Ijssel, the Netherlands.
A very distinctive ‘isoetid’ life-form predominates in clear, shallow oligotrophic waters (Fig. 160), a suite of unrelated plants all forming ‘shuttlecock’ rosettes of quill-like leaves. The most widespread is shoreweed (Littorella uniflora), in the plantain family. It has a typical upland distribution, with outliers in the New Forest and other heathland areas in the southeast. The two quillworts (Isoetes lacustris and I. echinospora) and water lobelia (Fig. 161a) are more closely bound to lakes and tarns in the mountains and moorlands of the north and west, and awlwort (Subularia aquatica, a crucifer) has a similar but slightly more restricted distribution. Lastly, pipewort (Fig. 161b) sits within the distribution of Littorella, Isoetes lacustris and the water lobelia, but is confined to Kerry, Connemara, Donegal, the Inner Hebrides and on the Scottish mainland only in Ardnamurchan. A fascinating feature of these isoetids is that they all rely largely on CO2 taken in through their roots and diffusing through the copious air spaces of the plant to the leaves for photosynthesis. This is a habit they share with no other plants; it enables them to grow alongside algae that can deplete CO2 in the water to very low levels.
Arguably, in their classic oligotrophic lake habitat in the north and west the isoetids could be seen as a single variable communityA22, A23. Littorella has a wider distribution than the others, and can form an understorey to some of the aquatic communities already considered. In the calcareous lakes of the Burren, Littorella forms characteristic communities on flat surfaces of calcareous marl, flooded most of the year but dry in summer, with lesser water-plantain (Baldellia ranunculoides), lesser spearwort (Ranunculus flammula), Juncus bulbosus, marsh bedstraw (Galium palustre), Potamogeton gramineus, the big rich-fen moss Scorpidium scorpioides and species of Chara. On the better-drained areas of marl around some of the larger lakes, lesser water-plantain, marsh bedstraw, Juncus bulbosus and Potamogeton gramineus are missing, and their place is taken by the (normally calcifuge) spike-rush Eleocharis multicaulis.
FIG 160. Loch nan Eilean, Skye looking towards the Cuillins, June 1981. An oligotrophic lake on hard acid rocks, with emergent bottle sedge (Carex rostrata) and bogbean (Menyanthes trifoliata), and floating-leaved white water-lily (Nymphaea alba) and pondweeds (Potamogeton spp.).
The clear water allows quillwort (Isoetes lacustris) and plants of similar growth-form to carpet the bottom.
FIG 161. Isoetids, Costelloe, Co. Galway, July 1971: (a) water lobelia (Lobelia dortmanna) has rosettes of blunt strap-shaped leaves on the bottom; only the flower-spikes appear above water; (b) pipewort (Eriocaulon aquaticum) has submerged flat rosettes of pointed leaves, and aerial flower spikes like knitting-needles.
Most streams in the uplands, and some in the lowlands, have rocky beds, and even slow-flowing lowland rivers have bridges, weirs and other masonry or concrete lapped by the water where aquatic mosses can get established. The trailing streamers of Fontinalis antipyretica (Fig. 162a) adorn probably most of the river bridges in the country, and are common in slow-flowing streams. Another big moss, Cinclidotus fontinaloides (Fig. 166), is characteristic of the flood zone, submerged during periods of high water level, exposed when the river is low in summer, when it forms conspicuous untidy dry blackish masses – especially on limestone, but often on concrete and sometimes brickwork. Like many mosses of the flood zone, Cinclidotus can tolerate severe and prolonged desiccation. Two species are common on brick or stonework in mill-leats and in streams with stony beds: Brachythecium rivulare (Fig. 162b) at or a little above normal water level and intermittently submerged, and Platyhypnidium riparioides (Fig. 162c), which forms a zone below it, exposed only when water level is particularly low. A third common (but rather nondescript) lowland species, Leptodictyum riparium, grows in wet places including pools, ditches and slow-flowing streams and rivers. There are a number of mosses very characteristic of riverside tree-bases in the flood zone, including Syntrichia latifolia, Orthotrichum rivulare, O. sprucei, Leskea polycarpa and Scleropodium cespitans. Riverbanks, particularly if they are rocky, can be very rich in bryophytes, but most of the species are not truly aquatic.
In upland streams, mosses (and sometimes liverworts) come into their own and make up most of the plant cover. Brachythecium rivulare and Platyhypnidium riparioides are still common, particularly where the water is at least reasonably base-rich, but they are often joined by Brachythecium plumosum and Hygroamblystegium fluviatile (Fig. 162d). On limestone, Cinclidotus fontinaloides continues to be conspicuous in the flood zone, largely replaced in fast-flowing streams by Schistidium platyphyllum and Orthotrichum cupulatum. The submerged zone is the preserve of Platyhypnidium riparioides, Hygroamblystegium fluviatile and two calcicole mosses, Hygrohypnum luridum and (usually where the water is saturated with calcium bicarbonate) Palustriella falcata.
FIG 162. Stream bryophytes: (a) Fontinalis antipyretica, common in slow-flowing rivers and streams; (b) Brachythecium rivulare; (c) Platyhypnidium riparioides, often dominant on rocks or weirs in moderately fast-flowing waters; (d) Hygroamblystegium fluviatile, on rocks in fast-flowing rivers and streams; (e) Racomitrium aciculare, common on rocks in upland streams; (f) Scapania undulata, a leafy liverwort often dominant in acid headwater streams.
In waters draining from more acid rocks (with less calcium but pH not far below neutral), the greatest diversity is to be found. Flood-zone specialists include such species as Racomitrium aciculare (Fig. 162e) and Schistidium rivulare. The more-or-less permanently submerged species are joined by Fontinalis squamosa (which lacks the characteristic keeled leaves of the larger and commoner F. antipyretica), Hygrohypnum ochraceum, locally Platyhypnidium alopecuroides, sometimes the ‘red’ alga Lemanea, and the leafy liverworts Marsupella emarginata and Nardia compressa. Many of these species drop out in the higher reaches of upland rivers. A common pattern on Dartmoor is a threefold zonation, with sparse black cushions of Andreaea rothii on the granite boulders extending down into the zone of occasional flooding, a prominent belt of Marsupella emarginata spanning the normal variation in water level, and a dense cover of Nardia compressa covering the permanently submerged stream-bed.
In headwater streams draining from acid rocks the bryophyte flora is limited to a few species. Philonotis fontana and Dichodontium palustre are species characteristic of springs – waterside mosses rather than true aquatics. The same could be said of Dichodontium pellucidum and the common thalloid liverwort Pellia epiphylla, and the Sphagnum species and Polytrichum commune from the surrounding moorland (Chapter 17). True stream bryophytes are often confined to Racomitium aciculare topping the boulders, and the leafy liverwort Scapania undulata (Fig. 162f), bright green or tinged with red, covering the wet rock surfaces.
The larger aquatic plants we have just considered are mostly confined to the shallow water around the margins of lakes. By contrast, the phytoplankton, the photosynthesising organisms that live suspended in the water, can be everywhere. At our latitudes, the phytoplankton is sparsest in the cold short days of winter. Diatoms, such as Asterionella formosa (Fig. 163a), Stephanodiscus hantzschianus (Fig. 163c) or Melosira species, with their silicified ‘frustules’, typically dominate the plankton in the late winter and early spring. As the days lengthen and temperatures rise, the diatom populations start to grow. At many sites the beautiful stellate colonies of Asterionella formosa are prominent in the phytoplankton in the early months of the year. In the Cumbrian lakes the spring rise of Asterionella generally begins in March or April, peaking in late May and June, but in smaller and shallower lakes this increase may begin as early as January, as at Slapton Ley in south Devon (Van Vlymen 1980) and Malham Tarn in the Pennines (Lund 1961), rising to a peak in March or April. The spring diatom maximum is brought to an end largely by depletion of essential nutrients – phosphate, and the silica essential for the diatom frustule. In Slapton Ley, a eutrophic lake, the most numerous organism in the early summer phytoplankton was the cyanobacterium Anabaena flos-aquae, but diatoms such as Melosira, Stephanodiscus and Fragilaria remained common, accompanied by green algae such as Scenedesmus, Pandorina and Pediastrum, and by cryptomonads whose numbers varied greatly from year to year. In late summer, with high temperatures and exhaustion of nitrogen (while phosphate remained in adequate supply), Slapton often saw dense ‘blooms’ of nitrogen-fixing cyanobacteria, such as Microcystis aeruginosa, Anabaena spiroides and Gloeotrichia echinatula, but the green algae such as Pediastrum boryanum (Fig. 163f), the diatom Fragilaria and cryptomonads remained a significant and sometimes major (but variable) component. As autumn leads to winter diatoms once again dominate the phytoplankton.
FIG 163. Phase-contrast photomicrographs of some phytoplankton: (a) a colonial diatom, Asterionella, Malham Tarn, Yorkshire; (b) a cyanobacterium, Anabaena sp. (Culture coll.); (c) a centric diatom, Stephanodiscus hantzschianus, Slapton Ley, Devon; (d) a colonial motile chrysophyte, Synura uvella, Fernworthy Reservoir, Devon; (e) a motile colonial chlorophyte, Eudorina elegans, Slapton Ley; (f) a non-motile colonial chlorophyte, Pediastrum boryanum, with empty urn-shaped thecae of the colonial chrysophyte Dinobryon, Malham Tarn.
Shallow lakes are mixed throughout their entire depth by turbulence generated by the wind across their surfaces. Even moderate turbulent mixing ensures that some cells remain in suspension, and during stormy periods some re-suspension of cells from the bottom can take place, mainly in shallow water. The temperature profile of a shallow lake is gradual from top to bottom. Water expands and becomes lighter with rising temperature. In a deep lake in summer turbulence generated by the wind can no longer mix the warmer and lighter surface water into the deep body of the lake. The lake becomes stratified, into a well-mixed, warmer epilimnion, floating above a colder hypolimnion which extends to the depths of the lake; the two are separated by a thermocline (around 10 m below the surface in Windermere in summer), in which the temperature gradient (which may be around 5 °C) is concentrated. Cells that fall through the thermocline are lost to the zone where there is enough light for photosynthesis, so in periods when the lake is stratified motile cells with flagella and cyanobacteria with gas vacuoles (which can regulate their position in the water column) are at an advantage – and non-motile cells quickly disappear from the population. This results in a more complete summer crash of the diatom populations in stratified lakes and, for the same reason, stratification leads to the surface layers becoming depleted of nutrients. Declining temperature at the end of summer, along with autumn storms, brings the breakdown of stratification, and the lake again becomes turbulently mixed through the whole of its depth. For more on the complexities of phytoplankton ecology see Macan (1970) and Reynolds (1984).
We hear so much about the dire consequences of rising atmospheric CO2 that it may be hard to remember that for plants CO2 remains a scarce resource. The complex mesophyll of a flowering-plant leaf is an adaptation that can increase the area for CO2 uptake 20-fold or more; the CO2 compensation point (at which photosynthesis is balanced by respiration) for most temperate flowering plants (and bryophytes) is 50–100 parts per million by volume. The diffusion rate of gases in water is slower than that in air by a factor of about 10,000 to 1. Most of the phytoplankton organisms and many of the submerged macrophytes counter this by having a carbon-concentrating mechanism (CCM), enabling them to reduce the CO2 in the water to very low levels. ‘There is no such thing as a free lunch’, and CCMs come at a price in energy terms, but when CO2 is limiting an organism with a CCM can out-compete an organism without one. Emergent aquatic plants can take up CO2 from the air in the ordinary way, but the submerged macrophytes, the phytoplankton and the algae growing on rocks on the lake bottom and around the shore, or epiphytically on the macrophytes, are all competing for the CO2 in the water.
The vast majority of bryophytes do not have a CCM, so bryophytes are usually sparse in lakes. The exceptions are around lake shores where wave action keeps the water better aerated, in clear, unproductive oligotrophic lakes, where mosses can grow to surprising depths, and around inflow streams bringing in CO2-rich water and organic sediment. In rivers most of the organic matter comes from the land along their banks, so CO2 levels in rivers are generally much higher than in air (or in lakes). Bryophytes are much commoner in rivers, and become the dominant plants in mountain streams and the upper reaches of rivers with stony or rocky beds.
Light is the other sine qua non of photosynthesis. Even clear water absorbs light, but light attenuation in water is much increased by turbidity, by plankton in suspension, by submerged macrophytes, or by a cover of floating macrophytes at the surface. Dense growth of phytoplankton, particularly summer ‘blooms’ of cyanobacteria, can depress the growth and impoverish the diversity of the submerged macrophyte flora.
Growth in most plants and ‘algae’ is limited by the availability of nitrogen and phosphate, and for diatoms by silica (SiO2). The effect of nutrient depletion on the spring diatom peaks in the phytoplankton has been considered already. In large deep lakes (particularly when they are stratified in summer) the plankton is much more nearly independent of the rest of the lake ecosystem than in smaller, shallow lakes, where the phytoplankton, the shore and the submerged macrophytes are in closer juxtaposition and more direct competition for nutrients.
Added nutrients (from treated sewage effluent or agricultural run-off) shift the trophic status in the eutrophic direction. In moderation this may do little harm, but large additions lead to excessive growth of a few vigorous species – common reed, branched bur-reed, fennel-leaved pondweed, rigid hornwort, filamentous green algae and Enteromorpha, and cyanobacteria such as Microcystis, Anabaena and Gloeotrichia – and can be a part of the process leading to a switch from a clear-water species-rich canal or lake to one dominated by phytoplankton and a few tolerant macrophytes. Toxic pollutants, such as herbicides, pesticides, heavy metals and industrial waste products, have no redeeming features.
Over much of mid-western Ireland the bedrock is nearly level-bedded Carboniferous limestone, often covered by a layer of clayey glacial drift, but in which the drainage is mostly underground. The limestone is honeycombed with crevices and larger passages, through which water can move relatively freely. The scouring of the ice-sheets during the glaciations left an irregular surface in which the depressions had no relation to the surface drainage. As a consequence, the western part of the Irish midland plain is dotted with numerous seasonal lakes, turloughs, closed depressions that fill with water during winter or following heavy rain, but drain out more or less completely in dry summers (Figs 164, 165). Sheehy Skeffington et al. (2006) listed 304 turloughs in Ireland. They traditionally provide summer grazing for livestock, and they have long fascinated biologists, hydrologists and geomorphologists alike. Although some analogous sites can be found on limestones in other parts of the world (as at Pant-y-llyn in Carmarthenshire (Blackstock et al. 1993), in seasonal pools on the Swedish island of Öland (Albertson 1960), and the lacs à niveau variable of the Île d’Anticosti in the Gulf of St Lawrence (Coté et al. 2006)), typical turloughs have a good claim to be seen as a distinctively Irish phenomenon. Wide expanses of flat-bedded limestone, recently glaciated, and in a rainy oceanic climate, are a rare coincidence worldwide.
FIG 164. Newtown turlough, west of Gort, Co Galway, part of 194 ha liable to intermittent flooding; 180º panorama from the road, (a) 22 September 2008, after an exceptionally wet August, (b) 19 June 2010, after a long dry spring.
FIG 165. Turloughnagullaun, south of Bell Harbour, Co. Clare, a small turlough (20.4 ha): (a) 21 September 2008; (b) 20 June 2010. The lowermost hawthorns, flooded for long periods in 2008 and 2009, are dead or dying.
Different hollows in the limestone have different patterns of seasonal variation in water level. Hollows floored by impermeable clayey glacial drift, in which water cannot fall below the level of the outflow stream, are occupied by permanent lakes, usually with more or less extensive Schoenus (black bog-rush) fens around them; smaller hollows have often become completely filled with Schoenus fen peat. These lakes and fens may flood to a depth of a metre or more in winter, but the water level during the growing season is generally stable to within half a metre or so. Hollows containing turloughs are in relatively free communication throughout their depth with the water-table in the limestone through springs, swallow-holes, or estavelles (openings into the underground drainage, which may function as either), so water level during the growing season may fluctuate widely (Proctor 2010). Between 2001 and 2004 water level in Skealoghan Turlough, near Balinrobe in Co. Mayo, varied over a range of 2.2 m (Moran et al. 2008); elsewhere the annual fluctuation may be 6–7 m or more. In July 1966, the water level in Hawkhill Lough west of Gort dropped by several metres in the course of 10 days – this is the turlough at which (in the dry summer of 1950) the sight of the aquatic moss Fontinalis antipyretica growing on top of a limestone field wall is said to have drawn from the eminent Swiss ecologist Josias Braun-Blanquet the astonished comment unmöglich! – impossible! In wet weather in summer in other turloughs I have seen autumn hawkbit (Leontodon autumnalis) and biting stonecrop (Sedum acre) still in full flower under water that had risen during the night.
The maximum winter water level in turloughs is roughly marked by the lower limit of hawthorn and hazel scrub and by the upper limit of the coarse blackish moss Cinclidotus fontinaloides (Fig. 166); flood debris can be left a metre higher than this. In the hollows occupied by lakes and Schoenus fens, the limestone grassland–fen transition is usually only a little below the lower limit of scrub. In turlough hollows there may be a zone of fen or fen-like grassland below the limit of the hazel and hawthorn bushes, especially if there is much lateral seepage of water from the surrounding country. However, in typical turloughs most of the summer grazing is provided by broad zones of pasture that recall grasslands in the flood zones of rivers (Chapter 9), or in seasonally flooded dune slacks (Chapter 19). Four species are particularly prominent and characteristic. Creeping buttercup (Ranunculus repens) and silverweed (Potentilla anserina) are almost ubiquitous in turloughs. At upper levels carnation sedge (Carex panicea) is abundant and often dominant in the turf along with common pasture and wet-meadow species. Common sedge (Carex nigra) typically forms a belt below this, still with abundant silverweed and creeping buttercup, but with a rather more ‘weedy’ associated flora, which commonly includes hairy sedge (Carex hirta) and creeping cinquefoil (Potentilla reptans). In shallower turloughs (as at Skealoghan) the bottom may be occupied by pools with relatively ‘normal’ emergent aquatic vegetation of reeds and sedges. At the lowest levels in the deeper turloughs the Carex nigra zone often grades down into open annual vegetation of permanently wet mud, with such species as needle spike-rush (Eleocharis acicularis), yellow-flowered Rorippa species, marsh foxtail-grass, curled dock, amphibious persicaria and other persicarias, red goosefoot (Chenopodium rubrum), marsh cudweed (Gnaphalium uliginosum), mudwort (Limosella aquatica), the tiny ephemeral moss Physcomitrella patens and the liverwort Riccia cavernosa.
FIG 166. The big desiccation-tolerant moss Cinclidotus fontinaloides, which grows on rocks in untidy-looking blackish masses near the flood limit in turloughs. Leaves of dewberry (Rubus caesius) to the right.
This is a mere sketch of a complex and fascinating habitat. Turloughs have been called ‘the callows of underground rivers’, and they have all the complexities of river floodplains, with associated fens, marshes and aquatic habitats, plus a few of their own. The turloughs, especially those in the bare limestone country of the Burren, are home to some notable rare plants. Shrubby cinquefoil (Potentilla fruticosa) grows in the upper part of the fen zone around lakes and turloughs of the eastern Burren, and water germander (Fig. 167) just a little lower – both in much the same situations as in their more extensive occurrences on the bare limestone alvar of the Swedish island of Öland (Albertson 1960). Fen violet (Fig. 168) is another species of similar ecology and geographical distribution frequent in the Carex panicea–Potentilla anserina zone of the Burren turloughs, but otherwise very local in Ireland and with only a few localities in Britain.
FIG 167. Water germander (Teucrium scordium), a rare plant of seasonally flooded calcareous places, in a turlough south of Mullagh More, Co. Clare, July 1971.
FIG 168. Fen violet (Viola persicifolia), in a turlough near Killinaboy, Co. Clare, July 1971, with creeping bent, carnation sedge, marsh bedstraw and bird’s-foot trefoil. Fen violet is characteristic of sites with large annual fluctuations in water level.