Nutrients limitation in water bodies
Phytoplankton biomass in the eutrophic zone is often controlled by the availability of certain nutrients that, in addition to light, are essential for growth. If the supply of these nutrients is increased an increase in biomass roughly proportional to the increase in nutrient supply can be expected.
A list of chemical elements which are essential for plants is shown in the following table :
Realizing the relative requirements of plants for the essential nutrients, one may ask which of these elements are more likely to limit phytoplankton biomass. Carbon, hydrogen and oxygen are needed in the largest amount, but these elements are readily obtained from H2O (hydrogen) and CO2 (carbon and oxygen). Obviously there is no lack of H2O in an aquatics environment. Carbon dioxide is a gas that is found in the atmosphere and dissolves in water reaching the equilibrium. However the CO2 concentration in some freshwater lake can be extremely low. It was proved by Schindler (1974) that the atmosphere acts as a CO2 reservoir of an aquatics system, and the flux of CO2 from the atmosphere into the water may easily provide the carbon dioxide needed for photosynthesis, even if the ambient concentration of CO2 in water is low. Phytoplankton requires nitrogen and phosphorous in large amounts. It was observed that when there is adequate light to support photosynthesis their concentration is very low and in this case the phytoplankton biomass is less abundant. Thus, based on observed concentrations, nitrogen and phosphorous are the most likely of the macronutrients to be limiting photosynthesis and thus organic biomass. However, the prevention of excessive input of these nutrients is hardly achievable since most of it is from non-point sources.
The elimination of phosphorous as an essential component of detergents could remove about 50% of the total phosphorous entering some lakes. Three treatments could be applied to phosphorous sewage: the tertiary treatment may involve chemical, physical or biological removal.
Phosphates may be precipitated using coagulants of lime of compounds of aluminium or iron. The precipitate is then separated in a sedimentation unit and combined with the other sludge generated in the treatment of the sewage. Biological removal uses the ability of some microorganism to take up phosphorous in excess of their immediate nutritional requirements and store it within the cells in the form of polyphosphates. Phosphorous may also be removed by passing water into treatment ponds, where much of it is absorbed on to particulate matter which settle down in the pond.
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