Eutrophication

Phosphorus is the most limiting nutrient for primary production in many ecosystems, particularly aquatic systems. However, P can cause environmental damage if it is applied in excess. Excess P in soil may move in soluble form in runoff water or attached to eroded soil particles to receiving surface waters. Eutrophication is an increase in the fertility status of natural waters that causes accelerated growth of algae or water plants. It is not caused by inputs of P alone, but by a complex interaction between N, P, environmental conditions such as light, and the hydro-logic characteristics of surface waters. Since P limitations are common in many receiving waters, the addition of P is the most direct factor in increased eutrophi-cation (Correll, 1998). As P input to surface waters increases, the trophic state of the water body goes from oligotrophic (low productivity), through mesotrophic, to eutrophic (high primary productivity). Once eutrophic conditions are established dissolved O2 concentrations will decrease, turbidity will increase, and excessive water plant growth and algal blooms will become more frequent. The increased input of P permits increases in algal biomass, which is visible in eutrophic lakes or ponds as a green surface scum. When the algae die, they settle to the bottom and become substrate for heterotrophic benthic organisms. The decomposition of the dead algae results in oxygen depletion. In extreme cases, eutrophication may cause severe ecological changes such as fish kills due to the oxygen depletion and cause impacts on recreational and navigational uses.

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