Phosphorus and eutrophication of fresh waters. Mechanisms and consequences in large lakes

Phosphorus is a nutrient limiting the primary production of fresh water environments. Uncontrolled inputs supplement the natural production of algae and macrophytes. When primary production becomes excessive, it is no longer sufficiently consumed by the zooplankton. This excess of algae reduces the transparency of water; it does not enter the pelagic trophic network and sinks. This organic matter accumulates at depth, where its degradation by bacteria consumes oxygen, and the sediment surface may become anoxic. Survival by the eggs of certain fish is reduced or even eliminated. Changes take place in the entire structure of the trophic network, going as far as to cause serious dysfunction: blooms, in some cases toxic (cyanobacteria) occur, and some indigenous species having heritage value, such as the Arctic charr, disappear. The restoration of the quality of such environments requires considerable time and effort. A reduction in inputs is not always sufficient to restore the original quality to the ecosystem when the internal phosphorus load is too high. The chemical processes applied in situ are expensive, which limit the immediate symptoms of eutrophication albeit without modifying their causes. Biomanipulation methods consist in decreasing predation by carnivorous fish on zooplanctonophagic fish, thereby increasing the density of the zooplankton and its predation on algae, leading to an increase in the transparency of the water. This type of biomanipulation of the trophic network is really only effective as a palliative measure in aquatic environments where eutrophication is not too advanced.