Biological recovery of the Bohemian Forest lakes from acidification

A limnological survey of eight small, atmospherically acidified, forested glacial lakes in the Bohemian Forest (Sumava, Bohmerwald) was performed in September 2003. Water chemistry of the tributaries and surface layer of each lake was determined, as well as species composition and biomass of the plankton along the water column, and littoral macrozoobenthos to assess the present status of the lakes. The progress in chemical reversal and biological recovery from acid stress was evaluated by comparing the current status of the lakes with results of a survey four years ago (1999) and former acidification data since the early 1990s. Both the current chemical lake status and the pelagic food web structure reflected the acidity of the tributaries and their aluminium (Al) and phosphorus (P) concentrations. One mesotrophic (Plesne jezero) and three oligotrophic lakes (Cerne jezero, Certovo jezero, and Rachelsee) are still chronically acidified, while four other oligotrophic lakes (Kleiner Arbersee, Prasilske jezero, Grosser Arbersee, and Laka) have recovered their carbonate buffering system. Total plankton biomass was very low and largely dominated by filamentous bacteria in the acidified oligotrophic lakes, while the mesotrophic lake had a higher biomass and was dominated by phytoplankton, which apparently profited from the higher P input. In contrast, both phytoplankton and crustacean zooplankton accounted for the majority of plankton biomass in the recovering lakes. This study has shown further progress in the reversal of lake water chemistry as well as further evidence of biological recovery compared to the 1999 survey. While no changes occurred in species composition of phytoplankton, a new ciliate species was found in one lake. In several lakes, this survey documented a return of zooplankton (e.g., Cladocera: Ceriodaphnia quadrangula and Rotifera: three Keratella species) and macrozoobenthos species (e.g., Ephemeroptera and Plecoptera). The beginning of biological recovery has been delayed for ∼20 years after chemical reversal of the lakes.