Food web manipulations

Food web manipulations in lakes are man made altera tions of the lake biota and their habitats to facilitate certain interactions that lake users consider beneficial, namely reduction of algal (phytoplankton) biomass. In most cases, food web manipulation refers to the reduction of planktivorous fish that leads to a cascade effect on phytoplankton (Figure 9) and, ultimately, clearer water of lakes.

Biomanipulation

High biomass

Low biomass

Piscivorous fish

High biomass

Low biomass

Zooplanktivorous fish

Low biomass

High biomass

Zooplankton High Low biomass, biomass, large small species species

Algae

Low biomass, large species

High biomass, small species

Low biomass

Zooplanktivorous fish

High biomass

Zooplankton High Low biomass, biomass, large small species species

High biomass, small species

Figure 9 Schematic view of top-down control of phytoplankton abundance in eutrophic lakes. Effect of manipulation of zooplanktivorous fish biomass (left) as compared to an unmanipulated food web.

Scientific background. Food webs are either regulated by resources ('bottom up') or by predation ('top down'). A strong reduction of the biomass of zooplanktivorous fish such as roach (Rutilus rutilus L.) is often followed by an increase in the abundance and size of zooplankton (pre dominantly Daphnia species). This increases the grazing pressure on phytoplankton and potentially leads to the top down control of phytoplankton biomass, in which case the water becomes clear and extreme values of oxy gen and pH are avoided (Figure 9). A reduced biomass of planktivorous fish may also reduce nutrient recycling rates. The success of food web manipulation may there fore also be triggered by bottom up forces. Benthivorous fish such as bream (Abramis brama L.) or common carp (Cyprinus carpio L.) exert bottom up effects on water quality as they increase sediment resuspension, water turbidity, and internal nutrient loading. The removal of benthivorous fish may therefore also strongly determine the success of a food web manipulation. Top down con trol of phytoplankton biomass was found to occur in shallow lakes and in deep lakes of slightly eutrophic or mesotrophic state. It is unlikely in eutrophic or hyper trophic deep lakes. The substantially higher success rates of food web manipulations in shallow - as opposed to stratified - lakes can be attributed to positive feedback mechanisms triggered by the recovery of submerged macrophytes (Figure 2).

Techniques. A reduction of the biomass of zooplankti vorous and benthivorous fish can be reached by stocking with piscivorous fish such as pike (Esox lucius L.), pike perch (Sander lucioperca L.), or perch (Perca fluviatilis L.). A direct reduction can also be achieved by poisoning, fish removal by conventional fishery techniques, or a tempor ary drainage of the lake. An appropriate balance between piscivorous, planktivorous, and benthivorous fishes is required for long lasting success of food web manipula tions. The average success rate is 60%. Strongest effects of food web manipulations are predicted when both fish and nutrients are altered. Experience has shown that lake water quality can only be improved by food web manipu lation if annual loading is lower than 0.6-0.8 g of total Pm 2 of lake surface area, or the in lake P concentration is lower than 100 mg l 1 in shallow lakes. The complexity of lake food webs, however, makes scientifically sound predictions of success rather difficult.

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