THE ZOOPLANKTON STORY OF HUMIC LAKE SKJERVATJERN DURING WHOLE CATCHMENT ACIDIFICATION

Major trends in macrozooplankton development were monitored in humic l ake Skjervatjern from 1988 to 1994. No changes were detected following division into an acidified basin (A) and reference basin (B) in 1989. Acidification was initiated in autumn 1990. During the summer of 1991 , there was a normal development of zooplankton in the B-basin, while an initial increase and subsequent collapse of the dominant crustacean Holopedium occurred in the A-basin. Here, total zooplankton biomass d ropped from a July maximum of 72 mu g C L(-1) to a September minimum o f less than 4 mu g C L(-1). Holopedium remained dominant in the B-basi n, while it was absent from the A-basin during the following years. Th e zooplankton perturbations occurred in spite of marginal changes in p H and Al-concentrations. Survival tests did not reveal any acute toxic ity of the acidified water, nor did climatic variables offer any expla nation for the zooplankton changes. There were pronounced oscillations in phytoplankton biomass and species composition. Measured grazing ra tes indicated that these oscillations were not caused by macrozooplank ton grazing, while disappearance of key phytoplankton species, notably Oocystis, apparently contributed to the zooplankton decline.