RESEARCH ARTICLE Examining the effects of climate change, acidic deposition, and copper sulphate poisoning on long-term changes in cladoceran assemblages Jennifer B. Korosi • John P. Smol Received: 22 November 2011 / Accepted: 5 April 2012 / Published online: 25 April 2012 Ó Springer Basel AG 2012 Abstract We analyzed cladoceran remains in dated sed- iment cores from four lakes in Nova Scotia, Canada, to assess the potential effects of climate warming, acidic deposition, and a major fish kill caused by copper sulphate poisoning on assemblage composition and Bosmina size structure. In three of the four lakes, we observed a decline in Daphnia in the early 20th century that might be indic- ative of limnological changes in response to acidic deposition or increased fish predation. The appearance of the softwater zooplankter Holopedium glacialis in Hirtle Lake *1995 might be linked to declining aqueous [Ca], a consequence of acidic deposition. No shifts in subfossil Cladocera were identified in this study that could be linked to climate warming. The application of copper sulphate as a fish poison to Trefry Lake (the lake was later re-stocked with trout fry) in 1938 resulted in dramatic and persistent changes in the cladoceran assemblage, where littoral Cladocera declined in relative abundance and small, pela- gic Bosmina increased. In addition, we observed a reduction in the mean body size of Bosmina in post-1938 sediments, suggesting that planktivorous fish abundance increased and/or predatory copepods and other invertebrate predators decreased. No recovery of Cladocera to pre-dis- turbance conditions was observed. Overall, our data suggest that acidic deposition and calcium decline may have had a modest impact on the cladoceran communities in these lakes, but the effects of copper sulphate poisoning on the food web in Trefry Lake were widespread, and still persist over 70 years following this intervention. Keywords Cladocera Á Nova Scotia Á Biomanipulation Á Copper sulphate poisoning Á Bosmina morphology Á Paleolimnology Á Acidic deposition Introduction The importance of Cladocera as sentinels of environmental change in lakes has been well documented (summarized in Jeppesen et al. 2011). In particular, their intermediate position in a lake food web makes them sensitive to shifts in both top–down and bottom–up forces, and, as a result, changes in the cladoceran community can be indicative of changes in the physical and chemical lake environment, and/or changes in food web structure. Consequently, long- term monitoring of Cladocera can be an effective method for tracking the ecological status of a lake through time. Unfortunately, at present, only a few such datasets exist, but where they are absent, indirect paleolimnological techniques can be used to assess the effects of multiple- stressors on lake ecosystems (Smol 2008, 2010). Cladoceran exoskeletons are composed of chitin, a chemically inert substance that is relatively resistant to degradation. Although there are species-specific differ- ences in the preservation of remains (depending on the degree of hydration of the chitin polymers), most taxa will leave some form of identifiable remains in lake sediments (Korhola and Rautio 2001). Analyzing temporal changes in the species assemblage of subfossil Cladocera has previ- ously been successful for tracking a number of environmental perturbations, including acidification (Nils- sen and Sandoy 1990), eutrophication (Manca et al. 2007), J. B. Korosi (&) Á J. P. Smol Department of Biology, Paleoecological Environmental Assessment and Research Lab (PEARL), Queen’s University, Kingston, ON K7L 3N6, Canada e-mail: Jennifer.Korosi@queensu.ca Aquat Sci (2012) 74:781–792 DOI 10.1007/s00027-012-0261-8 Aquatic Sciences 123