BIOLOGICAL-CONTROL OF EUTROPHICATION IN LAKES

Eutrophication of lakes, expressed as excess growth of planktonic alga e, is caused by excessive inputs of phosphorus and can be mitigated by many mechanisms, including grazing. However, it has been hypothesized that grazing becomes ineffective with even modest increases in P inpu t. We tested this contention directly by fertilizing lakes that had co ntrasting food webs. A lake with zooplanktivorous fishes and small gra zers accumulated algal biomass as predicted by Vollenweider's model of eutrophication. A lake with piscivorous fishes and large grazers accu mulated about half the algal biomass predicted by the model. However, blue-green algae bloomed in both lakes. Grazing may effectively contro l total algal biomass over a relatively wide range of P input rates, b ut may not suppress irruptions of nuisance algae.