A basic Introduction to the physiology and ecology of aquatic plants
By Dave Huebert
Dissolved Inorganic Carbon (DIC) in freshwater occurs as four different species in equilibrium with one another. The four species of DIC are; carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-), and carbonate (CO3=). The total amount of DIC largely determines the buffering capacity of freshwater, and the ratio of these species with one another largely determines the pH. Carbon dioxide dissolves readily in water. At air equilibrium, the concentration of CO2 in air and water is approximately equal at about 0.5 mg/L. Unfortunately, CO2 diffuses about ten thousand times slower in water than in air. This problem is compounded by the relatively thick unstirred layer (or Prandtl boundary) that surrounds aquatic plant leaves. The unstirred layer in aquatic plants is a layer of still water through which gases and nutrients must diffuse to reach the plant leaf. It is about 0.5 mm thick, which is ten times thicker than in terrestrial plants. The result is that approximately 30 mg/L free CO2 is required to saturate photosynthesis in submerged aquatic plants.
The low diffusivity of CO2 in water, the relatively thick unstirred layer and the high CO2 concentration needed to saturate photosynthesis have prompted one scientist to state, “For freshwater submerged aquatic macrophyte plants, the naturally occurring DIC levels impose a major limitation on photosynthesis … The DIC limitations on aquatic macrophytes and its corollary, the need to conserve carbon, are becoming increasingly apparent as important ecological features of aquatic environments (George Bowes, ‘Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms, 1985).”
Aquatic plants have adapted to CO2 limitation in several ways. They have thin, often dissected leaves. This increases the surface to volume ratio and decreases the thickness of the unstirred layer. They have extensive air channels, called aerenchyma, that allow gases to move freely throughout the plants. This allows respired CO2 to be trapped inside the plant and in some species even allows CO2 from the sediment to diffuse into the leaves. Finally, many species of aquatic plants are able to photosynthesize using bicarbonate as well as CO2. This is important, since at pH values between 6.4 and 10.4 the majority of DIC in freshwater exists in the form of bicarbonate.
For the aquarist, the supply of CO2 can be augmented in two ways. Both methods work by increasing the rate of diffusion of CO2 into the plants. First, the rate of water movement in the aquarium can be increased. This will decrease the thickness of the boundary layer and ensure that CO2 levels are at air equilibrium. This method is inexpensive, easy to implement and will produce excellent growth of aquatic plants under most conditions. Secondly, CO2 can be injected into the aquarium. This method can be expensive, and if done improperly, can be lethal to fish. This latter method is only essential, however, if there is a significant daily pH fluctuation in the aquarium, or if the species of plants being cultured are completely unable to use bicarbonate (such as Cabomba sp.).
Light Plant chlorophyll absorbs light at wavelengths of 400 to 700 nm. This is termed Photosynthetically Active Radiation (PAR). The intensity of full, natural sunlight is approximately 2,000 umoles/m2/s, or 100 klux, of PAR. Light is attenuated rapidly in freshwater, however, so that submerged aquatic plants receive far less than this amount.
Submerged aquatic plants are adapted to the low light levels found in freshwater, and are classified as shade plants on the basis of these adaptations. For instance, aquatic plant chloroplasts, which are the organelles that contain chlorophyll, are often located in the top cell layer of leaves to ensure that as much light as possible is absorbed. Additionally, photosynthesis is saturated at only 15 to 50% full sunlight intensity. Aquatic plants also have a low light compensation point (LCP). The LCP is the point at which the rate of photosynthesis equals the rate of respiration and growth stops. This allows them to grow to depths that receive only 1 to 4% full sunlight (20 to 80 umoles/m2/s PAR).
For the aquarist, high light intensities are those which saturate photosynthesis. Only metal halide bulbs can provide this level of intensity. Medium intensities can be provided by 2 to 4 Watts per gallon of fluorescent lights. At this level of intensity, photosynthesis will not be at its highest level but will still be greater than respiration. Anything less than 2 Watts per gallon is low light. At this level of lighting, light compensation points will be approached for many aquatic plants and only the most light tolerant species will flourish. The attenuation of light in water is wavelength specific. Water absorbs light in the infrared and ultraviolet bands of the spectrum, organic solutes absorb blue, violet and ultraviolet light, phytoplankton absorb blue and orange-red light, and suspended silt absorbs light fairly uniformly at all wavelengths. Aquatic plants are therefore exposed to light that is vastly different in quality than incident radiation. Moreover, light quality underwater can change rapidly depending on water depth, turbidity, algal blooms and the level and type of organic solutes present. These data suggest that aquatic plants are flexible as to their light requirements and that the pursuit of ‘full spectrum’ light is unnecessary in the freshwater aquarium.
There is in fact clear evidence in the scientific literature that freshwater plants can sustain high growth rates under simple cool-white fluorescent light. Full spectrum lighting may perhaps be useful, however, for true color rendition, and for attempts by the hobbyist to achieve flowering in ‘difficult’ aquatic plants.
Plants are sensitive to daylength. The pigment that senses light in plants is called phytochrome, and it absorbs light in the red/far-red end of the spectrum. Research has shown that some aquatic plants are short-day plants, some are long-day plants, and some are indifferent to daylength. When exposed to the ‘ wrong’ daylength, plants will continue to photosynthesize in the presence of light, and grow vegetatively, but will not complete their lifecycle and flower. This is true of both terrestrial and aquatic plants. Generally, it is safest to assume that tropical aquarium plants are short-day plants, which means they are more likely to flower with a duration of 10 to 12 hours of light per day. Plants which grow in temperate zones are generally long-day plants and are most likely to flower with 14 to 16 hours of light per day.
Essential mineral nutrients are conveniently separated into two categories. Nutrients used by plants in relatively large amounts are termed macronutrients. They are nitrogen (N), phosphorus (P), sulfur (S), calcium (Ca), magnesium (Mg) and potassium (K). Nutrients used by plants in small amounts are termed micronutrients. They are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co), and boron (B). Other mineral elements, such as sodium (Na), are also present in plants, but there are at present no definite roles for them and so they are not classified as essential nutrients.
Aquatic plants, unlike their terrestrial counterparts, can absorb mineral nutrients both from the water through their leaves and from the sediment through their roots. Unfortunately, it is often assumed that rooted aquatic plants can obtain all their mineral nutrient requirements through their leaves. This is, however, incorrect. As early as 1905 a researcher by the name of Raymond H. Pond stated that, ” … a soil substratum is requisite for normal growth.” and that, ” [rooted aquatic plants] make a better growth on a good loam soil, just as many land plants do.” Since then, the dramatic and consistently superior growth of plants rooted in soil compared to plants rooted in sand has been shown repeatedly for many different aquatic plant species from many different types of habitat.
While the reasons for this superior growth are not completely understood, certain facts are clear. First, submerged soils are generally lacking in oxygen. This is of benefit to rooted aquatic plants since under anoxic conditions Fe, P and N are more readily available than under aerobic conditions. Second, nutrient concentrations are higher in a fertile soil than in the overlying water. Third, there is no competition with phytoplankton for available nutrients. This latter point is important because with water based nutrition, too much fertilizer and the algae bloom, and too little and the plants stop growing.
Rooted aquatic plants are well adapted to growing in an anaerobic substrate. They are able to ‘pump’ enough oxygen to the roots so that in many cases the oxygen actually diffuses into the surrounding sediment. They can also respire anaerobically if necessary and produce lactic acid or ethanol instead of CO2 as a byproduct. The root meristems (growing tips) of some species are even inhibited in the presence of oxygen.
Aquatic plants also have requirements for certain nutrients in the overlying water. Most rooted aquatic plants need Ca, Mg, K and a carbon source in the water if they are to thrive. I say most, since some aquatic species such as Isoetes sp. and Lobelia dortmanna actually obtain even their carbon dioxide from the sediment. These plants are adapted to growing in acidic softwater lakes that have extremely low levels of DIC in the water and so absorb CO2 from the sediment through their roots.
Aquatic plants grow in an environment that is often poor in mineral nutrients. Perhaps for this reason, these plants can absorb and store large quatities of nutrients for later use. Concentrations of some mineral nutrients in plants, most notably micronutrients such as Fe and Cu, can exceed the level in the water by 1,000 to 1,000,000 times. Regular additions of mineral nutrients, particularly Fe, are therefore essential for the sustained growth of aquatic plants in the aquarium.