Chapter IX.

THE AERIAL ADVANTAGE

The ‘aerial advantage’ is bestowed on all aquatic plants growing partially in air. Plants that can or do grow in air are shown in Table IX-1.

Table IX-1. Aquatic Plants with the Aerial Advantage.

Category	Examples
emergent plants	cattails, reeds, pickerelweed
amphibious plants	species of Anubias, Bacopa, Cryptocoryne, Echinodorus, Hygrophila, Ludwidgia, Myriophyllum, Potamogeton
floating plants	duckweed, Watersprite, waterhyacinth, Water lettuce, Salvinia, Azolla
plants with emergent leaves	waterlilies, lotus plants, ‘banana’ plants

In comparison to fully submerged plants, emergent plants are characterized by:

•Much faster growth

•More efficient use of CO₂ and light

•More efficient oxygenation of the root area

•Enhanced biological activity (in the root masses of floating plants)

The richness of an aquatic ecosystem is often based on the aerial advantage. Thus, lake areas containing emergent plants (wetlands and lake shallows) are characterized by enormous productivity; they support at least three times greater biological activity than the open water.1 And invariably, plants used for wastewater treatment– waterhyacinth, duckweed, pennywort, water lettuce, pickerelweed, and cattail– are emergent or floating aquatic plants [2,3]. Faster growth means faster contaminant removal.

Although submerged plants may appear to grow quickly, much of that growth may simply be water. Submerged plants have often been found to contain only 6.7% dry matter, whereas a terrestrial leaf usually contains 20% dry matter [5]. This means that a terrestrial plant might represent three times more actual photosynthetic output– real growth– than a submerged plant of similar size and fresh weight.

The aquatic environment presents plants with several problems: (1) not enough CO₂ (see page 93); (2) too much oxygen²; and (3) anaerobic substrates (see page 132). Submerged aquatic plants have apparently adapted to these constraints by becoming permanently handicapped. These handicaps are genetically fixed, so that no matter how much light or CO₂ is available, they will not grow as well as plants growing in air.

A.Aerial Advantages

Submerged aquatic plants can greatly overcome the difficulties they have in obtaining sufficient CO₂ from water by producing emergent growth that can tap into air CO₂. Thus, the amphibious plant Hygrophila polysperma reportedly grew 4 times faster when it was grown in air than in water [10]. The stream plant Callitriche cophocarpa reportedly grew 4 - 9 times better when it sprouted aerial leaves than when it grew fully submerged [7]. For five Potamogeton species, the average photosynthesis was ten times faster for emergent leaves than submerged leaves [6]. Not surprisingly, floating and emergent plants obtain most of their CO₂ from the air, not the water [4,7]. Indeed, the floating plant Spirodela polyrhiza obtains only 5% of its CO₂ from the water; the rest is from the air [8].

1.Aerial Growth Uses CO₂ More Efficiently

When aquatic plants break the water surface, they not only obtain more CO₂, but they appear to be released from their own internal handicaps. Figure IX-1 compares the photosynthetic response of aerial leaves and submerged leaves of Pomamogeton amplifolius to increased CO₂ fertilization. The floating leaves responded much better to increased CO₂ than the submerged leaves. For example, at 0.12% CO₂, which is about 4 times more than air’s current CO₂ level of 0.035%³, floating leaves were photosynthesizing 10 times faster than the submerged leaves (e.g. ~300 v. ~30 µm CO₂/mg Chl/h). Thus, even under ideal conditions and plenty of CO₂, submerged leaves still photosynthesized much more slowly than aerial leaves. This is because submerged leaves are internally handicapped.

Figure IX-1. CO₂‘s Effect on Floating and Submerged Leaves of Potamogeton amplifolius.

Photosynthesis was measured in water-saturated air (to prevent drying out of the more delicate submerged leaves).

‘Photosynthesis’ represents µmoles CO₂/mg chlorophyll/h of net photosynthesis. ‘Carbon Dioxide’ is the percentage (by volume) of CO₂ gas in the humidified air.

Fig. 4 from Lloyd [11] redrawn and used with the permission of the Canadian Journal of Botany.

Bigleaf Pondweed Potamogeton amplifolius.

P. amplifolius is found throughout the Eastern states north of Georgia.

Investigators showed that its floating leaves responded to CO₂ fertilization much better than its submerged leaves. This is typical, because all submerged growth is basically handicapped.

Drawing from Hellquist [12].

2.Aerial Growth Uses Light More Efficiently

Submerged plants and leaves also cannot use light as effectively as aerial growth. Figure IX-2 compares the effect of increasing light on the photosynthesis rate of the aerial leaves and submerged leaves of Myriophyllum brasiliense. In very low light (~45 µmol/m²/s), both leaves photosynthesized at the same rate. However, as the light intensity increased above 300 µmol/m²/s, the aerial leaves photosynthesized faster, whereas the submerged leaves did not. The submerged leaves quickly became ‘light saturated’.

Figure IX-2. Effect of Light on the Aerial and Submerged Leaves of M. brasiliense.
Investigators collected 4” apical segments from both emergent and submerged forms of Myriophyllum brasiliense from a Florida lake. During measurements for net photosynthesis, emergent segments were incubated in humidified air, while submerged segments were incubated in solution. Plant segments were provided with equal amounts of CO₂. ‘Photosynthesis’ represents micromoles CO₂ fixed per mg chlorophyll per hour. Time points represent the mean of three separate experiments.

(Figure from Salvucci [13] redrawn and used with permission of Elsevier Science Publishers.)

Big differences in response to light were also found between the aerial and submerged leaves of Myriophyllum spicatum and Potamogeton amplifolius [11]. The submerged leaves of both species were light saturated at 200 µmol/m²/s, whereas the aerial leaves showed light saturation at or above 1,200 µmol/m²/s. Indeed, P. amplifolius had a maximum photosynthesis rate 20 times faster for its floating leaves than its submerged leaves.

In general, submerged plants are considered to be shade plants, able to use only a fraction of full sunlight. In contrast, aerial growth can be adapted (gradually) to use full sunlight [5].

3.Emergent Plants Ferment Better

In severely anaerobic sediments, aquatic plants may resort to fermentation to obtain energy. While fermentation yields only about 6% of the energy of aerobic metabolism (14.6 kcal v. 263 kcal per mole of glucose [15]), it may be essential for plant root survival. Several aquatic plants, both submerged and emergent, have been shown to contain the enzymes necessary for fermentation [16].

Q. How do you convert µmol/m²/s to Lux, the term most hobbyists are familiar with?

A. There is no way to precisely convert µmol/m²/s to Lux, so I’ve not done so in this book. While the term Lux is fine for hobbyists, most biologists use µmol/m²/s, which is more accurate for their purposes. This is because the light used in biological reactions like photosynthesis and human vision invariably involves pigment excitation (i.e., a ‘photochemical reaction’.) Photosynthesis is the photochemical reaction of the chlorophyll molecule. Biologists precisely measure only the light that induces photochemical reactions. They express that intensity as photon fluence rates or µmol/m²/s (micromoles per meter squared per second). (This term is equivalent to the earlier term of µEinsteins/m²/s.)

Don’t be intimidated by µmol/m²/s. All you have to know (for reading this book) is that sunlight is about 2,000 µmol/m²/s and that normal light intensity (for many aquatic plants) would be about 120 µmol/m²/s.

However, emergent plants ferment better. Thus, in an experimental study, 3 submerged species did very poorly in comparison to 3 emergent species under anaerobic conditions. For example, Isoetes lacustris (a submerged species) produced ethanol at a slow rate (0.041 mg/h/g dry wt) and showed poor viability. In contrast, Nymphaea alba, an emergent plant, released ethanol at a much faster rate (1.6 mg/h/g) and showed strong viability [17].

Wetland plants are not that inhibited by low Redox [19]. For example, Spartina alterniflora showed no inhibition of photosynthesis and N uptake when the sediment Redox was maintained at -200 mV for 20 days [20]. (Investigators lowered the Redox by bubbling N₂ gas into the sealed culture chambers.)

White Waterlily
Nymphaea alba.

N. alba, like other emergent plants, can efficiently ferment stored carbohydrates into ethanol. Thus, its roots can obtain the energy they need to grow in severely anaerobic substrates. Submerged plants, which do not ferment efficiently, are at a disadvantage in these substrates.

Drawing from Preston [18].

4.Aerial Growth Aerates the Root Area Better

a)      Root Release of Oxygen by Aquatic Plants

The roots of all aquatic plants release oxygen into their environment. This release may be small or considerable depending on the age and species of the plant. In an experimental study, oxygen release rates were measured for several aquatic plants (Table IX-2). The floating plant Pennywort released oxygen into the water faster than the other plants.

Table IX-2. Oxygen Release by the Roots of Aquatic Plants [2].

Roots were in nutrient solution in an air-tight chamber (investigators placed an air-tight seal at the crown of the plant.) An oxygen electrode measured the root oxygen release (mg O₂/h/g root dry wt.) into the nutrient solution.

Values are for young plants, which produced the most O₂.

PLANT	Oxygen Release
Pennywort (Hydrocotyle umbellata)	3.5
Pickerelweed (Pontederia cordata)	1.5
Cattail (Typha latifoia)	1.4
Waterhyacinth (Eichhornia crassipes)	1.2
Water lettuce (Pistia stratiotes)	0.30

Root oxygen release is critical for aquatic plant survival in anaerobic substrates. All aquatic plants have massive internal gas channels (lacunae), often exceeding 70% of the plant’s total volume [24] that conduct oxygen to the roots. Indeed, in comparison to terrestrial plants, most aquatic plants are simply a hollow, gas-filled tube.

Q. If the floating plants in my pond release so much oxygen into the water, why do I need to add ‘oxygenating’ plants like Elodea?

A. Ponds with only floating plants often have decreased oxygen levels.4 This is because the plant cover keeps oxygen from entering the water. Also, oxygen is consumed by bacteria and protozoa as dead plant matter decays within the plant cover. Thus, while floating plants are great for removing nutrients from the water, they provide little oxygen to fish. (This is why most pond keepers include submerged plants along with floating plants.)

b)      Root O₂ Release is More Efficient in Emergent Plants

Although all aquatic plants must bring oxygen to the root area, emergent plants do it better. Figure IX-3 shows the Redox profiles of three sediment samples. (See page 128 for how oxygen relates to Redox.) Some samples were not planted; others were planted with either an emergent plant (Sagittaria latifolia) or a submerged plant (Hydrilla verticillata). The Redox of all three sediments decreased with depth (i.e., sediments became increasingly anaerobic). However, sediments with no plants or submerged plants showed a very low Redox potential (about -200 mV) at 2.5 cm below the sediment surface. In contrast, the Redox potential of sediments with emergent plants was still positive (almost +100 mV), even at a 4.5 cm sediment depth.5

Figure IX-3. Effect of Submerged and Emergent Plants on Sediment Redox.

Plants were grown in separate 1.5 liter containers containing lake sediment. ‘No Plants’ represents containers with sediment but no plants. ‘Redox Potential’ was measured after 6 weeks growth.

{Fig. 2 from Chen [25] redrawn and used with permission from the Journal of Freshwater Ecology.}

Sagittaria latifolia or Arrowhead.

Investigators showed that S. latifolia, an emergent plant, released enough O₂ into the sediment to keep it Redox positive. The Redox of its sediment was much higher than unplanted sediments or those planted with Hydrilla, a submerged plant.

Drawing from Hellquist [26].

Emergent plants can oxygenate the root area more effectively than submerged plants, because they have a direct pipeline to air O₂. (Air contains a bountiful 21% O₂.) Indeed, all emergent plants use air O₂ to supply the root area [24, 27]. In contrast, submerged plants cannot use air oxygen; they depend on photosynthetic O₂ to aerate their roots. Thus, root O₂ release by the submerged plant Potamogeton perfoliatus dropped off within 2 min following light removal and the cessation of photosynthesis [28].

Potamogeton perfoliatus, being a submerged plant, depends on photosynthesis to aerate its roots.

Investigators showed that root aeration stopped abruptly when there was no light for photosynthesis.

{Drawing from Muenscher [21].}

Many emergent plants have ventilating systems where outside air enters the plant’s lacunae and actually moves within the plant (see page 151). In contrast, submerged plants depend on oxygen diffusion within a stagnant gas, a relatively slow process. (Although one investigator [29] found gas pressure build-up in the submerged plant Egeria densa, it lasted less than an hour and there was no sustained gas movement.)

Emergent plants seem to release O₂ more efficiently into the root area than submerged plants. Investigators compared the pattern of root O₂ release of an emergent plant (Nuphar lutea) and a submerged plant (Isoetes lacustris). They found that the emergent plant supplied considerable O₂ to the root tip where it would do the most good. (Because the root tip is the growing region and the site of most nutrient uptake [30], it needs more O₂ than the rest of the root.) In contrast, the investigators found that the submerged plant released O₂ wastefully all along the root length, such that the root tip got no more than the root shaft.

Because emergent plants oxygenate their roots more efficiently, they are better adapted than submerged plants to grow in highly anaerobic sediments containing lots of organic matter. Thus, in a study where 5 different types of organic matter was added to identical sediment samples, submerged plants (Elodea canadensis, Hydrilla verticillata, and Myriophyllum spicatum) were severely inhibited whereas emergent plants (Myriophyllum aquaticum, Potamogeton nodosus, and Sagittaria latifolia) were either stimulated or much less inhibited [31].

c)      How Emergent Plants Aerate the Root Area

Emergent plants bring air oxygen to the root area efficiently. For example, the common yellow water lily brings several liters of air each day down to its roots and rhizomes (Fig. IX-4). Air enters the younger emergent leaves of the waterlily and flows internally down the petioles to the roots and rhizomes bringing O₂ to the underground tissues. The gas picks up CO₂ from the sediment and underground plant tissue and continues flowing up the petioles of the older emergent leaves and finally exits to the atmosphere. Gas flows through the plant at an impressive rate, up to 50 cm/min. The CO₂ concentration of the exiting air sometimes exceeds 3%, which is almost 100 times air CO₂ levels. The investigator showed that 85% of this CO₂ was used for photosynthesis. Thus, the gas flow system of the water lily not only aerates the root area but also provides the leaves with a rich carbon source.

Figure IX-4. Flow-through Ventilation in the Yellow Waterlily Nuphar lutea.

Investigators used a tracer gas (ethane) to show flow-through ventilation in Nuphar lutea.

{Figure from Dacey [27] modified slightly and used with permission of Physiologia Plantarum.}

Many other emergent plants have been found to have ventilating systems similar to the yellow water lily. Thus, several species of water lily and water lotus [32], reed [33], and cattail [34,35] were found to use atmospheric air for rhizosphere ventilation. Heat build-up within the plant from absorbed sunlight increases gas efflux from the older leaves of the plant in exchange for an air influx into the younger leaves. Deepwater rice uses a unique external ventilating system, which depends on a thin air layer on the surface of its leaves [36]. (Gas flow between the atmosphere and the sediment is conducted externally along the leaf surface, rather than internally as in the water lily.) All of these strategies allow emergent plants to survive and prosper in severely anaerobic sediments.

Without their gas ventilating systems, emergent plants could probably not survive in anaerobic sediments. Manual pruning or animal grazing of emergent plants below the water surface often kills the plants [37]. For example, investigators [38] showed what happened to reeds that were cut below the water and thus denied access to air. Reeds growing in an aerobic sediment (coarse sand) were relatively unaffected, but those growing in an anaerobic sediment (mud containing 50% organic matter) died. (Reeds cut above the water were unaffected or only slightly inhibited.)

d)      How Oxygen Benefits Rooted Aquatic Plants

Oxygenation of the root and root area (rhizosphere) benefits aquatic plants in three ways. First, roots need respiratory oxygen for growth, maintenance, and nutrient uptake. Plants that can best meet their oxygen demands, grow better.

Second, root oxygenation of the rhizosphere counteracts substrate toxins. For example, excessive soluble iron is potentially toxic to plant roots. But root oxygen release causes iron to precipitate as iron oxides on the outside of the root, thus preventing excessive iron from entering the roots [39]. Iron precipitation can be seen as brown stains or precipitates on the roots [40,21].

Rhizosphere oxygen also protects the plant from hydrogen sulfide (H₂S), which is a major substrate toxin (see page 133). Specific bacteria use the oxygen to oxidize H₂S to nontoxic sulfates (see page 67). This oxidation is a common bacterial process and provides considerable protection for aquatic plants against H₂S [42]. Table IX-3 shows how bacteria and plants together control H₂S in two different soils. Thus, the H₂S concentration in the Bernard clay soil was reduced from 0.46 to 0.25 µg/g. Although H₂S is not completely removed, total removal from the soil mass may not be necessary. For as long as there is a oxygenated zone around the roots where H₂S-oxidizing bacteria can destroy the toxic H₂S, the plant will be protected, even if the bulk of the soil still contains toxic levels of H₂S (Figure IX-5).

Figure IX-5. How Rhizosphere Ecology Protects Plant Roots from Toxic H₂S.

O₂ is released from the plant’s roots into the rhizosphere (shaded area surrounding roots). Within this oxygenated zone, various H₂S -oxidizing bacteria proliferate and remove H₂S.

Thus, while the bulk of a soil or sediment may contain toxic levels of H₂S, the rhizosphere may be free of H₂S and the roots perfectly safe.

Table IX-3. Effect of Plants and Bacteria on H₂S in Two Soils [43].

Each treatment was done in triplicate and in jars containing 300 g moist soil. ‘Bacteria’ were from a purified culture of Beggiatoa, a common soil bacterium that oxidizes H₂S to SO₄^2-. ‘Plants’ were freshly planted rice seedlings. H₂S was measured after 2 weeks. Numbers were significantly different from each other.

Treatment	H2S Concentration (µg/g of soil)
	Bernard Clay	Crowley Loam
Soil only	0.46	0.33
Soil + Bacteria	0.32	0.31
Soil + Plants	0.35	0.30
Soil + Bacteria + Plants	0.25	0.27

In a manner similar to the oxidation (and destruction) of H₂S, many ordinary bacteria could use oxygen released into the root area to metabolize and degrade organic acids.

Q. When I transplanted my waterlily, the soil had a foul odor. Should I plant my waterlily in something more aerobic, like sand? I’m also concerned that H₂S might poison the fish.

A. I wouldn’t worry about the foul odor unless the waterlily’s growth is poor and its roots are stunted. Waterlilies have a ventilating system that protects them.

Fish might be affected, depending on how much H₂S is being released, and how much oxygen is in the water. Make sure that you are not over-fertilizing the plant with sulfate-containing fertilizers.

Third, root oxygen release can acidify the rhizosphere by oxidizing iron (Fe²⁺ + 3 H₂O ⇒ Fe(OH)3 + 3H⁺ + e^-). This acidification dissolves metal oxides thereby bringing nutrients into the soil solution. Thus, investigators working with rice showed that root oxygen release coupled with Fe²⁺ oxidation could increase plant uptake of zinc and phosphate [44,45].

Finally, oxygen also provides the required aerobic environment for various symbiotic fungi (mycorrhizae), which assist plants by greatly increasing nutrient absorption.6

B.Floating Plants Increase Biological Activity

Many bacteria and zooplankton, including most rotifers, are sessile by nature, in that they require surfaces for attachment [49,50]. Also, nutrients accumulate at surfaces, thereby attracting microbes (see page 69). Thus, investigators studying aquatic microbes often suspend glass slides in the water upon which microbes readily attach and colonize.

Floating plant roots function in a similar way to glass slides– only better. One investigator [52] showed that over a 100 times more bacteria and other microorganisms colonized duckweed roots than glass slides. Floating plants encourage biological activity in the water, because the roots release both oxygen and organic matter.⁷

Just as lake areas of plant growth are enormously more active biologically than the open water, so too are the roots of floating plants. Thus, I would not discount the importance of floating plants. Attached microorganisms may be critical to nutrient cycling, nitrification, denitrification, decomposition, and the consumption of algae.

C.Aerial Growth in the Aquarium

I try to keep some floating plants in all my tanks and encourage the aerial growth of amphibious aquarium plants such as Ludwigia, Hygrophila, Bacopa, etc. Sometimes just a small adjustment will help. For example, I removed the top plastic strip on the back of one tank, which allowed Bacopa monnieri to grow out. Because the tank was next to a sunny window, eventually the Bacopa formed a large mat on the back of the tank. In another tank, I decreased the water level by about an inch, so that Cryptocoryne would have room to sprout aerial leaves.

Water Sprite (Ceratopteris thalictroides) seems to grow best in shallow (12” high) tanks with its roots partially buried in soil and its leaves above the water surface. I cannot always get it to grow as a floating plant.

When I do my routine plant pruning, I am very careful not to prune or damage aerial growth. Thus, for Water Sprite, I carefully remove adventitious (baby) plants and the submerged (but never aerial) branches. I cut the stems of amphibious plants like Bacopa caroliniana above the water line. As for duckweed and water lettuce, I thin it out regularly, such that new growth is continuously encouraged.

I suspect that aerial growth is less important in tanks with CO₂ fertilization. CO₂ fertilization probably provides enough CO₂ so that amphibious plants don’t need to resort to aerial strategies to increase their carbon uptake. In aquariums without CO₂ fertilization (such as mine), using aerial growth and/or just allowing amphibious plants some emergent growth becomes much more critical.

Just as floating plants are used to remove nutrients efficiently from wastewater, aerial growth can be used in aquariums to efficiently remove excess nutrients from the water. By combining aerial growth with submerged plants, the hobbyist greatly increases total plant growth in the same volume of water. Not only does enhanced plant growth contribute to fish health by removing nutrients and pollutants from the water, but it also discourages algal growth. Finally, the roots of floating plants provide a home for bacteria and protozoa– organisms that may be useful in reducing ammonia, digesting organic matter, and consuming algae. Aerial growth enhances the health and functioning of aquarium ecosystems.

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56.Ibid., p. 545.

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1Littoral and wetland zones, which contain emergent, amphibious, and floating plants and their associated bacteria and algae, are more productive than the pelagial zone (open water), which contains only submerged plants and phytoplankton (i.e., green-water algae). The difference is enormous– 30-80 mT/ha/yr for the littoral and wetland zones versus a mere 0-10 mT/ha/yr for the pelagial zone [1]. (Also, see Table VI-1 on page 93)

²Oxygen (the waste product of photosynthesis) diffuses 10,000 times slower in water than in air. Because oxygen cannot readily escape from the plant, it inhibits photosynthesis by stimulating photorespiration, a wasteful process for the plant that releases fixed CO₂. The loss of fixed CO₂ may reduce photosynthetic efficiency by about 20-25% [9]. Submersed plants, most of which have C_3^-type photosynthetic metabolism, are particularly vulnerable.

³Percentage in this instance represents gas volume, not weight. Thus, 0.035% CO₂ means 350 ul (microliters) of CO₂ gas per liter (or 1 million ul) of air.

⁴Experimental ponds with a waterhyacinth cover but no submerged plants were shown to have only 0.2 to 3.0 mg/l DO (dissolved oxygen) [22]. In contrast, ponds with algae or Elodea had 5 to 20 mg/l of DO during midday and 2 to 8 mg/l at night. (Fish require a minimum of 2 mg/l DO for survival [23].)

⁵Although this experiment shows that submerged plants did not affect sediment Redox, they do release O₂ into the immediate (<0.5 mm) rhizosphere. Depending on plant species, the amount ranges from 0.01 to 0.2 ug O₂/cm² root/min [55]. The released O₂ is consumed so rapidly by bacteria and chemical processes that a Redox probe (platinum electrode) placed into the bulk soil often cannot detect it.

O₂ release by submerged plant roots often profoundly affects substrate ecology (see pages 135-136).

⁶The fungal hyphae extending outward from inside the roots into the surrounding soil bring nutrients to the plant. In exchange, the fungus obtains carbohydrates from the plant [46]. Several aquatic plants, both emergent and submersed, have been shown to possess endomycorrhizae [56]. Mycorrhizae are especially helpful to aquatic plants in nutrient-depleted environments [47]. Sharma [48] describes the mycorrhizal association found in three species of Isoetes.

⁷Oxygen release by aquatic plant roots is discussed on page 148. Organic matter released by floating plants would include root excretions, cell lysates, and whole cells sloughed off from growing root tips. Aquatic plants often release 1-10% of the their photosynthetic carbon as DOC [53]. Terrestrial plant roots also give off a great deal of DOC [54]. Not all of these compounds represent the passive release from dead cells; some may be actively released and play a role in allelopathy or nutrient uptake.