Possible impact of mild winters on zooplankton succession in eutrophic lakes of the Atlantic European area

1. We report on a long-term study (1975-94) of water temperatures and plankton in a eutrophic lake (Heiligensee, Berlin, Germany). Using a phenomenological approach, we use historical data to infer how an increase in air temperature has influenced a natural zooplankton community. 2. Air temperatures in Berlin showed a significantly rising trend between 1975 and 1994. Mean winter air temperatures in the last 8 years always exceeded the long-term mean. 3. A rising trend was also found for April water temperature, which increased significantly beginning in 1988-89. An increase of 2.58 degrees C in the last 21 years was recorded using a linear model. A significantly decreasing trend was found in June but no trend was noted for the other summer months. 4. Phytoplankton composition shifted from a dominance of diatoms and cryptophytes during winter and spring in the 1980s towards a dominance of cyanobacteria in 1990-94. 5. The dominant zooplankton species in spring shifted in recent years from the large-bodied Daphnia galeata to the smaller D. cucullata. Cyclops kolensis, previously the only invertebrate predator during winter, decreased in abundance while C. vicinus, usually present during spring and autumn, increased in abundance and was numerous during winter, a season passed in diapause in the earlier years. 6. Because direct and indirect temperature effects are species specific, we put forward the hypothesis that zooplankton species, rather than functional groups, are the nexus between environmental stress, such as rising air temperatures, and ecosystem changes.