TOP-DOWN CONTROL IN FRESH-WATER 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 t op-down control is more important in shallow lakes than in deep lakes, excepting lakes with a high abundance of submerged macrophytes. The e vidence in support is: (1) That at a given epilimnion total phosphorus concentration (TP) the biomass of fish per m(2) is independent of dep th, which means that biomass per m(3) is markedly higher in shallow la kes. (2) That the biomass of benthic invertebrates is higher in shallo w lakes, which means that the benthi-planktivorous fish are less depen dent on zooplankton prey than in deep lakes. By their ability to shift to zooplankton predation their density can remain high even in period s when zooplankton is scarce and they can thereby maintain a potential ly 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 p resumably then also the grazing pressure on phytoplankton. (5) That nu trient 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 ea sily broken (at least in Northern temperate lakes), and (7) That top-d own 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 pr ey fish. We conclude that manipulation of fish and submerged macrophyt es may have substantial impact on lake ecosystems, in particular in sh allow eutrophic lakes. On the contrary, if the conditions for more per manent changes in plant abundance or fish community structure are lack ing the feed-back mechanisms that endeavour a return to the original t urbid state will be particularly strong in shallow lakes.