Constraints on the biological recovery of the Bohemian Forest lakes from acid stress

Abstract

Summary The response of planktonic (phytoplankton, ciliates, rotifers and crustaceans) and littoral (Ephemeroptera, Plecoptera, Trichoptera and Heteroptera: Nepomorpha) assemblages to chemical recovery was studied over a twelve‐year period (1999–2011) in eight glacial lakes in the Bohemian Forest (central Europe). The region suffered from high atmospheric pollution from the 1950s to the late 1980s, but has since been recovering from acidification due to 86% and 44% decrease in sulphur and nitrogen deposition, respectively, during the 1990s–2000s. Despite the rapid improvement in water chemistry of all the eight studied lakes, only four have partly recovered so far (low‐aluminium lakes), while the other four lakes still remain strongly acidic (high‐aluminium lakes). All present lake assemblages are dissimilar by 40–90% from those during the early phase of chemical recovery as a result of species (re)colonisation (ciliates, crustaceans and insects) in the high‐Al lakes, and considerable species replacement (zooplankton) and (re)colonisation (insects) in the low‐Al lakes. Phytoplankton remained very similar in the high‐Al lakes, but changed (loss and/or replacement of some acid‐tolerant species) in the low‐Al lakes. Aluminium (Al) concentrations were dominant in structuring the assemblages of phytoplankton, rotifers and Nepomorpha, but also affected crustaceans through the seston carbon to phosphorus (P) ratio. Both direct (toxicity) and indirect (P availability) effects of Al control biological recovery in the Bohemian Forest lakes. A concentration of 200 μg L−1 of total Al is the main barrier preventing the high‐Al lakes from recovery. In contrast, pH and total P rather than Al significantly influenced Ephemeroptera, Plecoptera and Trichoptera. Although biotic responses (especially in the low‐Al lakes) showed important signs of recovery, such as reappearance of some indigenous or acid‐sensitive species, decline in eurytopic acid‐tolerant species and colonisation by vagile species, the assemblages of all the lakes still suffer from acid stress. Our results also indicate an increasing role of biotic interactions between colonisers and residents leading to the reconstruction of aquatic food webs in the low‐Al lakes.