Zooplankton monitoring of the Darß-Zingst lagoon – long- term changes and inter-annual variability

We studied a long time series, dating from 1969-2004, of zooplankton biomass in the shallow and wind-exposed Darß-Zingst lagoon (Northern Germany). Our aim was to investigate long-term changes and inter-annual variability. Copepods and rotifers occurred over the total time of investigation. Other groups, phyllopods and the larvae of an invaded polychaete, developed considerable biomass values only during restricted periods. All groups showed a stable pattern of seasonal change with a dominance of copepods in spring and late summer, the phyllopods during early summer, the rotifers in mid-summer and the poychaete larvae during late autumn and winter. Besides these tendencies, a considerable inter-annual variability of the biomass level was observed. This resulted mainly from climatic differences between the years. Deterministic chaos as a source of variability could not be identified in the data sets. In the long-term run, two cases of change were observed in the dominance structure and overall biomass level during a rather short period of a few years. (1) In the beginning of the 1980s, a pronounced decrease of copepod biomass occurred. This resulted from a shift in food quality, which probably was caused by a switch of the Darß-Zingst lagoon from a macrophyte dominated system to a system dominated by phytoplankton. The phyllopods disappeared completely in connection with this event. (2) In the beginning of the 1990s, a clear decrease of the rotifer biomass was observed at the same time as the alien polychaete species Marezellaeria neglecta invaded the Darß-Zingst lagoon. Whether or not the decline of rotifers is causally linked to the polychaete invasion is not yet clear.