Strength of coupling between phyto- and zooplankton in Lake Lucerne (Switzerland) during phosphorus abatement subsequent to a weak eutrophication

The strength of coupling between phyto- and zooplankton was measured from 1961 to 1995 by comparing the grazing effect of zooplankton (visible as clear-water phase only in 1968-1994) and also by excluding zooplankton in limnocorral experiments (1980-1984). Although long-term (1961-1995) measurements show little evidence of temporal changes in total biomass of phytoplankton or zooplankton, there is strong evidence of changes in the strength of coupling due to top-down effects. The ratio of change in biomass caused by cladocerans in the intensive grazing period of each year (May/June) and the recovery of netplankton after this period seems to be strongly influenced by the trophic state of the lake. When Lake Lucerne was mesotrophic (1971-1982), the annual mean of monthly changes in phytoplankton biomass was in the range of +/-1-2, indicating that the biomass more than doubled (or halved) from month to month (no change = 0). Under oligotrophic conditions, the annual average of monthly changes in biomass was below 0.5. Grazing measurements in limnocorrals at 2 m depth with labelled food (Rhodomonas lacustris) showed distinct diel rhythms, with maximum community grazing rate at dusk and dawn. These diel changes were caused by vertical migration of the zooplankton. Grazing rate and zooplankton biomass were strongly coupled, with a maximum rate of similar to 100-200 ml day(-1) mg(-1) (zooplankton biomass) when daphnids were dominant. The decrease in biomass caused by excessive grazing shows parallel trends in nanoplankton and netplankton. However, the increase in biomass after the clear-water phase was largely caused by netplankton.