Consumer-dependent responses of lake ecosystems to nutrient loading

The nutrient loading concept proposes that algal biomass, waterclarity and the processes of lake eutrophication are a functionof nutrient loading. We hypothesized that grazers play an importantrole in determining the impacts of nutrient loading on algalbiomass and water clarity, and the overall eutrophication process.To test how the contrasting grazer communities modify the fateof nutrients, we added nutrients (nitrate and phosphate) ata known loading rate to four large enclosures, but in two ofthe four enclosures large cladoceran grazers (Daphnia >1mm mean length) were allowed to develop by removing the planktivorousfish. In the remaining two enclosures, the development of largeDaphnia was prevented by adding planktivorous fish. The concentrationsof epilimnetic total phosphorus (TP) increased at a similarrate in all four enclosures. However, the daily accumulationof added phosphate into the participate or planktonic forms,especially into plankton <20 µm, was three times fasterwhen large Daphnia were absent than when large Daphnia wereabundant. In the enclosures with large Daphnia, added phosphatewas accumulated in the dissolved pool instead. At a constantnutrient loading, algal biomass (chlorophyll a) increased fourtimes faster in the enclosures without large Daphnia than inthose with large Daphnia. Similarly, Secchi depth declined from3.5 to <1 m when Daphnia were absent, but did not declinewhen Daphnia were common. Our results demonstrate that the samenutrient loading and the resultant increase in epilimnetic TPdo not produce the same trophic conditions, as indicated byalgal biomass and water clarity, if the grazers of the majorassimilators of nutrients (the fraction of plankton edible toDaphnia) are different. We suggest that stratified lake ecosystemshaving functionally dominant large grazer communities may beless prone to eutrophication than those lacking large grazers.Consistent with the nutrient loading concept, epilimnetic concentrationsof phosphorus increase proportionately with increased loadingof phosphorus, but the trophic conditions of ecosystems indicatedby algal biomass and water clarity do not follow the same patternsunder contrasting conditions of grazer communities. We suggestthat models predicting algal biomass from loading rates shouldaccount for the role of grazers.