Consequences of pelagic food-web changes during a long-term lake oligotrophication process

Changes in the pelagic communities of Lago Maggiore (Italy) during 1983-1995, when the lake underwent a process of oligotrophication caused by a gradual phosphorus abatement, were analyzed. Events in the plankton after a large increase of Bythotrephes longimanus starting in 1988 were strikingly similar to those in Lake Michigan after the initial Bythotrephes cederstroemii invasion. They included a decrease in numbers of Daphnia, assumed to be caused by Bythotrephes predation, and an increase in the body size of Daphnia. In Lago Maggiore, these changes took place with a stable fish stock. As Lehman initially found in Lake Michigan, herbivory did not constrain the maximum phytoplankton biomass. However, a significant negative correlation (P < 0.01) existed between Daphnia and phytoplankton cell numbers over the whole long-term period. It is concluded that, although chlorophyll and biovolume changes are dependent on phosphorus reduction, the observed increase in cell numbers of phytoplankton and the decrease in average cell size may depend not only on phosphorus decline but also on changes in the food web during lake oligotrophication.