Response of planktonic communities to calcium hydroxide addition in a hardwater eutrophic lake: results from a mesocosm experiment

The main focus of this study was to investigate the effects of lime treatment with Ca(OH)2 on a hypereutrophic hardwater lake (Lake Alserio, Northern Italy), focusing on its impact on plankton communities. We performed a mesocosm experiment using two large enclosures, one for treatment and one control, in the lake. The addition of Ca(OH)2 proved to be effective in reducing phosphorus concentration, but the effect was limited in time. Redissolution of the precipitating calcium carbonate as it came into contact with the deep water determined a phosphorus increase about 2 weeks after the liming. The liming had a strong impact on the plankton assemblage, as was demonstrated by the sharp decrease in phytoplankton and zooplankton biomass and density, and 1 month after the treatment we observed a different taxonomic composition inside the enclosure from that of the lake. The result of the experiment showed that liming with Ca(OH)2 can be regarded as a suitable treatment for accelerating recovery of the lake. Multiple treatments will be necessary to improve the trophic level over a longer period.