Temporary collapse of the Daphnia population in turbid and ultra-oligotrophic Lake Brienz

The cyclical parthenogen Daphnia is a key species in aquatic food webs. Its abundance is influenced by environmental factors like food quantity and quality, predation, diseases, temperature and washout by discharge. In ultra-oligotrophic Lake Brienz (Switzerland), which is turbid from suspended glacial material, Daphnia density has continuously decreased since the 1990 s. In spring and summer 1999, during and after a severe flood, Daphnia density was below detection level, but the population recovered the following year. Simultaneously, a drastic two-year decline occurred in the yield of whitefish (Coregonus sp.), which mainly feed on Daphnia. Several hypotheses were tested to explain the collapse of the Daphnia population: a negative effect of the suspended particles, a covering of the diapausing eggs by sediment, and a combined washout/temperature effect. A direct negative effect of the particles and covering of diapausing eggs could be excluded. According to model calculations, the spring growth of the Daphnia population could not compensate the washout losses, as it was limited by poor food conditions due to re-oligotrophication and reduced by extraordinarily low water temperatures. Moreover, ephippia abundance analysed from sediment cores was consistent with the process of eutrophication and re-oligotrophication and indicated that daphnids did not persist in the lake in the period before eutrophication (until 1955). Like most peri-alpine lakes in Europe, Lake Brienz has returned to its natural ultra-oligotrophic state and is now unable to support a large Daphnia population and fishing yield.