Introduction and Problem Identification

Because of industrialization and intensified agriculture many lakes in urban areas have received high external loading of antropogenically derived nutrients, especially phosphorus (P) and nitrogen (N). These nutrients sooner or later transform pristine lakes into water bodies with dense algal blooms, bad odor, and mucky bottoms; a process called eutro-phication (from Greek eutrophus: highly nutritious). In most lakes, algal growth is limited by phosphorus availability, and thus, eutrophication results in a massive increase in phytoplankton growth. The increased biomass of phytoplankton leads to a reduction in water transparency and, further, when the algae die they sink to the bottom, resulting in an increased amount of dead organic matter accumulating as sediment. Bacteria mineralizing this organic material consume large amounts of oxygen, which they derive from the water, leading to reduced oxygen concentrations. Because of wind-induced mixing, the oxygen concentration in the entire column is reduced to very low levels often leading to massive fish kills. Moreover, because visually hunting predatory fish become less efficient when turbidity (caused by massive algal growth) increases, a common feature of eutrophic lakes is an increasing proportion of planktivorous prey fish (Figure 1). These fish feed efficiently on large-bodied zooplankton, resulting in reduced abundances and size of grazing zooplankton, which in turn leads to an increase in the abundance of planktonic algae (phytoplankton), further deteriorating the underwater light climate. Hence, the eutrophication process leads to considerable food-web changes in the lake ecosystem, and also reduces the potential of using these lakes for recreation, fishing, and as a source of drinking water.

The logical solution to prevent the aforementioned water quality changes is to divert the runoff nutrient-rich waste water away from the polluted lake or to treat it by removing nutrients before they enter the lake, i.e., to prevent the factors causing eutrophication and pollution of lakes. Such preventive measures may, in some cases, lead to improvement of the water quality. One important process preventing lakes from recovering from eutrophication is that large amounts of nutrients are released from the mineralization of dead organic matter accumulated at the lake bottom. This release of nutrients from the sediment into the water column is called internal nutrient loading, a process that may proceed at high rates for decades. Hence, there is often a need for restoring eutrophi-cated lakes by, in addition to nutrient reduction, bringing about changes in food-web interactions by using biological methods. However, as for most restoration attempts, such measures rather rehabilitate than completely restore lakes to their pristine state.

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