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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.2. Eutrophication as an Environmental Problem

1.2.3. Internal Supply

Internal recycling of nitrogen and phosphorus from sediments of lakes and reservoirs can sustain eutrophic conditions for long periods after external loading is reduced. Models that incorporate biogeochemical and physical processes and empirical studies are usually employed to evaluate the likelihood of internal supplies compensating for lowered external inputs. Shallow, warm water lakes with a history of receiving nutrient-rich inflows are especially likely to maintain high rates of internal recycling.

Transfer of phosphorus to the lake sediments through deposition of particulate matter usually exceeds the release of soluble phosphorus. However, the seasonal release of dissolved inorganic phosphorus can be an important source, particularly in shallow lakes and in lakes with anoxic hypolimnion. Numerous environmental controls on phosphorus release from sediments have been identified. The onset of seasonal anoxia in the water overlying the sediments has been most consistently linked to an increase in phosphorus release from the sediments.

Ammonium is produced by decomposition of organic matter in the lake sediments. A portion of the ammonium may be nitrified to nitrate, which may be partially denitrified to nitrous oxide and gaseous nitrogen, depending on the oxidation-reduction status in the sediments. Typically, high concentrations of dissolved inorganic nitrogen in the surface sediments result in diffusion of these nutrients into the overlying water.

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