Changes of the plankton community composition during chemical neutralisation of the Bockwitz pit lake

Lake Bockwitz, a pit lake in a former lignite open-cast mine south of Leipzig (Germany), was neutralised (from pH 2.65 to pH 7.1) by addition of soda ash (14,620 t) from 2004 to 2007. The additions had to be continued due to ongoing inflows of acid ground and surface water. This paper reports on the changes in the plankton community accompanying the neutralisation.At the beginning, the community composition and biomass was comparable to other acidic pit lakes, i.e. the pigmented flagellates Chlamydomonas and Ochromonas dominated the autotrophs, and ciliates and rotifers were the top predators. The biomass was small (maximum 2 mg fresh weight per litre) and decreased until the end of 2008. With increasing pH, the autotrophic community became more diverse, whereby diatoms, chrysophyceans and blue greens contributed significantly to biomass. Although neutral pH conditions were achieved in autumn 2007, picocyanobacteria were not present until the end of 2008. In addition, crustaceans were under-represented in terms of biomass and diversity. Daphnids were not found. We attributed this to short-term changes of the ionic composition of the water and to the limited time available for non-acido-tolerant organisms to colonise the system.