Top-down control in freshwater lakes: The role of nutrient state, submerged macrophytes and water depth

Based on data from 233 Danish lakes, enclosure experiments, full-scale experiments and published empirical models we present evidence that top-down control is more important in shallow lakes than in deep lakes, excepting lakes with a high abundance of submerged macrophytes. The evidence in support is: (1) That at a given epilimnion total phosphorus concentration (TP) the biomass of fish per m(2) is independent of depth, which means that biomass per m(3) is markedly higher in shallow lakes. (2) That the biomass of benthic invertebrates is higher in shallow lakes, which means that the benthi-planktivorous fish are less dependent on zooplankton prey than in deep lakes. By their ability to shift to zooplankton predation their density can remain high even in periods when zooplankton is scarce and they can thereby maintain a potentially high predation pressure on zooplankton. (3) That the possibilities of cladocerans to escape predation by vertical migration are less. (4) That the zooplankton:phytoplankton mass ratio per m(2) is lower and presumably then also the grazing pressure on phytoplankton. (5) That nutrient constraints appear to be weaker, as evidenced by the fact that at a given annual mean TP, summer TP is considerably higher in shallow lakes, especially in eutrophic lakes lacking submerged macrophytes. (6) That negative feedback on cladocerans by cyanobacteria is lower as cyanobacterial dominance is less frequent in shallow lakes and more easily broken (at least in Northern temperate lakes), and (7) That top-down control by benthi-planktivorous fish is markedly reduced in lakes rich in submerged macrophytes because the plants serve as a refuge for pelagic cladocerans and encourage predatory fish at the expense of prey fish. We conclude that manipulation of fish and submerged macrophytes may have substantial impact on lake ecosystems, in particular in shallow eutrophic lakes. On the contrary, if the conditions for more permanent changes in plant abundance or fish community structure are lacking the feed-back mechanisms that endeavour a return to the original turbid state will be particularly strong in shallow lakes.