Reservoir ecosystems recover from atmospheric acidification: II. Signs of biological recovery

Most of the atmospherically acidified drinking-water reservoirs in the Erzgebirge (Germany, southern Saxony) responded more rapidly to the strong reduction of oxidized sulfur and nitrogen emissions than surface waters from other siliceous mountain ranges in Germany (Ulrich and Meybohm 2005). The substantial decrease of proton concentration and toxic forms of aluminum provided a basis for the recovery of sensitive aquatic organisms. Three of four reservoirs which were monitored over a decade showed significantly (p<0.01) falling trends of chlorophyll a and phytoplankton biovolume, mainly due to the decline of dinophyceae. Biovolumes of diatoms and picoplankton slightly increased in the Werda and Muldenberg Reservoir. The zooplankton biocoenosis was dominated by rotifers and small phyllopods, the genus Daphnia was still lacking.
Since 1999 and 2002, small perch (Perca fluviatilis) and sunbleak (Leucaspius delineatus) occurred in the Muldenberg and Werda Reservoir, respectively. The zooplanktivorous sunbleak population showed an extremely high fluctuation of abundance (1,000 67,000 individuals per hectare) and biomass (1 29 kg ha-1, Fig.). Even a population crash due to increased natural mortality happened. The fish individuals revealed a very low factor of corpulence, which indicates limited energy supply and high intraspecific competition on food resources (i.e. on zooplankton, Fig.). Severe top-down control may further delay the recovery of bigger zooplankton species (e.g. Daphnia) and consecutive planktonic groups of the food web. Introduction of predatory fishes could help to control zooplanktivorous fish populations and to prevent mass mortality.

References
Ulrich, K.-U. and Meybohm, A. (2005) Reservoir ecosystems recover from atmospheric acidification: I. Trends of chemical reversal. ACID RAIN 2005, Conference Proceedings, 000.