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

Abstract

Based on data from 233 Danish lakes, enclosure experiments, full-scaleexperiments and published empirical models we present evidence that top-downcontrol is more important in shallow lakes than in deep lakes, excepting lakeswith a high abundance of submerged macrophytes. The evidence in support is: (1)That at a given epilimnion total phosphorus concentration (TP) the biomass offish per m2 is independent of depth, which means that biomassper m3is markedly higher in shallow lakes. (2) That the biomass of benthic invertebratesis higher in shallow lakes, which means that the benthi-planktivorous fish areless dependent on zooplankton prey than in deep lakes. By their ability to shiftto zooplankton predation their density can remain high even in periods whenzooplankton is scarce and they can thereby maintain a potentially high predationpressure on zooplankton. (3) That the possibilities of cladocerans to escapepredation by vertical migration are less. (4) That the zooplankton:phytoplanktonmass ratio per m2 is lower and presumably then also thegrazing pressure onphytoplankton. (5) That nutrient constraints appear to be weaker, as evidenced bythe fact that at a given annual mean TP, summer TP is considerably higher inshallow lakes, especially in eutrophic lakes lacking submerged macrophytes. (6)That negative feedback on cladocerans by cyanobacteria is lower as cyanobacterialdominance is less frequent in shallow lakes and more easily broken (at least inNorthern temperate lakes), and (7) That top-down control by benthi-planktivorousfish is markedly reduced in lakes rich in submerged macrophytes because theplants serve as a refuge for pelagic cladocerans and encouragepredatory fish at the expense of prey fish. We conclude that manipulation of fishand submerged macrophytes may have substantial impact on lake ecosystems, inparticular in shallow eutrophic lakes. On the contrary, if the conditions formore permanent changes in plant abundance or fish community structure are lackingthe feed-back mechanisms that endeavour a return to the original turbid state willbe particularly strong in shallow lakes.