If the problem with eutrophication is not solved pro-phylactically, i.e., by reducing the external nutrient input, it can be therapeutically approached by different rehabilitation methods, sometimes with good, but sometimes with less satisfactory results. Such rehabilitations involve physical removal of nutrient-rich sediment by dredging or chemically binding with e.g., ferric chloride or aluminium sulphate, or inactivating the phosphorus at the sediment and thereby considerably reducing the release rate in order to also reduce algal growth.
Another therapeutic measure is to use biomanipulation, which has its theoretical roots in food-web theory. The term biomanipulation refers to the manipulation of biota in order to make a water body more desirable for humans. In general, the goal of biomanipulation is to reduce the algal biomass in eutrophic lakes. Biomanipulations are generally performed by reducing the abundance of planktivorous fish (i.e., fish eating zooplankton), either by addition of piscivorous fish (fish eating other fish), or by manually removing or reducing the biomass of undesired fish (e.g., by trawling). If the amount of planktivorous fish is sufficiently reduced, the predation pressure on zooplankton should decrease and the grazing rate on algae should increase. In this way, the likelihood of algal blooms will decrease and water transparency increase.
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