RESEARCH PAPER Limnology (2007) 8:121–130 © The Japanese Society of Limnology 2007 DOI 10.1007/s10201-007-0202-8 Barbara Leoni · Giuseppe Morabito · Michela Rogora Davide Pollastro · Rosario Mosello · Silvia Arisci Elena Forasacco · Letizia Garibaldi Response of planktonic communities to calcium hydroxide addition in a hardwater eutrophic lake: results from a mesocosm experiment Abstract 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 me- socosm 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. Redissolu- tion of the precipitating calcium carbonate as it came into contact with the deep water determined a phosphorus in- crease about 2 weeks after the liming. The liming had a strong impact on the plankton assemblage, as was demon- strated by the sharp decrease in phytoplankton and zoo- plankton 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 re- garded as a suitable treatment for accelerating recovery of the lake. Multiple treatments will be necessary to improve the trophic level over a longer period. Key words Eutrophication · Restoration · Liming Ca(OH) 2 · Phytoplankton · Zooplankton Introduction Eutrophication as a result of anthropogenic impact is one of the commonest causes of the deterioration of the quality of water bodies located in the most populated and industri- alized areas of the world. Since the end of the 1960s, many ecotechnological methods have been proposed and used to tackle the negative effects of the eutrophication processes (see Straskraba 1996 for a review). Management at water- shed level is essential to reduce nutrient loading to the lake so that further interventions within the water body can be effective. Most of these recovery actions are primarily designed to reduce the amount of phosphorus available to the phyto- plankton, by adding to the lake water substances that determine the precipitation of phosphorus through its in- corporation into insoluble compounds. This results can be achieved using aluminium or ferric compounds; however, both treatments may have negative side effects, such as the aluminium poisoning of aquatic organisms in the absence of neutralizing agents, or an unacceptable pH decrease in low buffered systems (Cooke 1993). In the last few years, liming with CaO, Ca(OH) 2 , and CaCO 3 has been successfully used to treat natural ponds (Sosnowska 1990), man-made ponds (Murphy et al. 1990), experimental enclosures (Koschel 1997), and hardwater lakes (Prepas et al. 2001; Zhang 2001). In hardwater lakes, the addition of lime simulates the natural process of calcite biogenic precipitation, also known as “whiting.” This proc- ess, which usually occurs during periods of high productiv- ity, is caused by the precipitation of calcium carbonate when the lake water is supersaturated with it. The situation is favored by increasing pH resulting from the CO 2 consumption by phytoplankton. During calcium carbonate precipitation, phosphorus is incorporated into the calcite crystals and precipitates out of solution. Insofar as it imitates a natural process, liming has fewer negative effects on biota than other chemicals used to con- trol algal biomass. Moreover, lime addition can also induce phytoplankton flocculation and decrease water transparen- cy, further reducing phytoplankton biomass and slowing down the photosynthetic activity. There are few studies describing the effect of liming on freshwater biotic communities, mostly restricted to acidic lakes (Olem 1991), where the planktonic assemblages are quite different from those commonly found in hardwater eutrophic lakes. Some information on planktonic response to liming in hardwater lakes comes from the experiments B. Leoni (*) · E. Forasacco · L. Garibaldi Dipartimento di Scienze dell’Ambiente e del Territorio, Università degli Studi di Milano Bicocca, Piazza della Scienza 1, 20126 Milano, Italy Tel. +39 0264482737; Fax +39 0264482795 e-mail: barbara.leoni@unimib.it G. Morabito · M. Rogora · D. Pollastro · R. Mosello · S. Arisci CNR Istituto per lo Studio degli Ecosistemi, Verbania Pallanza, Italy Received: October 10, 2006 / Accepted: January 24, 2007 / Published online: July 7, 2007