Response of drinking-water reservoir ecosystems to decreased acidic atmospheric deposition in central Europe II. Signs of biological recovery

Strongly decreasing atmospheric emissions and acidic depositions during the 1990s have caused a chemical reversal from acidification in several drinking-water reservoirs of the Ore Mountains, SE Germany. Responses of phytoplankton, zooplankton, and fish stock were studied in five reservoirs and in enclosure scale where a water body of 1,200 m3 was experimentally neutralized by application of a new buffering substance. Three months after this treatment, the dominating chrysophytes and dinoflagellates were substituted by diatoms and cryptomonades. The colonization by acidic-sensitive species of green algae, cryptomonades, rotifers, and Cladocera (e.g. Bosmina longirostris) is explained by the occurrence of dormant stages or by survival of individuals in very low abundances. In conformity with the enclosure experiment, three reservoirs showed significantly (p<0.01) falling trends of chlorophyll a and phytoplankton biovolume, mainly due to the decline of dinoflagellates. Picoplankton and diatoms slightly increased in two reservoirs. The zooplankton biocoenosis was dominated by rotifers and small cladocera, the genus Daphnia was still lacking. Two reservoirs were re-colonized by zooplanktivorous fish populations of either perch (Perca fluviatilis) or sunbleak (Leucaspius delineatus). The latter exhibited extremely high fluctuating abundance and biomass and even suffered from a population crash. This natural mortality was caused by limited food supply. Hence, severe top-down control may delay the recovery of bigger zooplankton species like daphnids. Fishery management comprising the introduction of predatory fishes could help to control zooplanktivorous fish populations and to prevent their mass mortality.