Response of Bosmina size structure to the acidification and recovery of lakes near Sudbury, Canada

In response to biotic and abiotic cues, the cladoceran genus <em>Bosmina </em>can<em> </em>undergo changes in body size and appendage length and shape over successive generations. To improve our understanding of the environmental controls on <em>Bosmina</em> size structure, we used paleolimnological techniques to examine <em>Bosmina</em> size responses to the extreme acidification and metal contamination, and then subsequent chemical recovery, of lakes in the vicinity of mining and smelting operations near Sudbury, Canada. During the acidification period, <em>Bosmina </em>antennule and carapace length significantly increased in three of the five study lakes, while mucro length significantly decreased in four of the five lakes. However, despite the recent return to pre-impact pH levels, the size structure of the present-day <em>Bosmina</em> community still differs from the pre-impact size distributions. We suggest that the continued dominance of the food webs by small invertebrate predators (<em>e.g.</em>, cyclopoid copepods) is responsible for the persistent changes to <em>Bosmina</em> size structure.

Separating the effects on water chemistry of climate variation and experimental manipulation in the long-term acidification and recovery of lakesThis paper is part of the series “Forty Years of Aquatic Research at the Experimental Lakes Area”.

Changes in climate, in particular significant changes in precipitation and evaporation and thus runoff, as well as changes in regional atmospheric deposition can affect the changes in chemical masses or concentrations in lakes. We have examined the changes in alkalinity, sulfate, and dissolved organic carbon (DOC) during a 20-year period when Lake 302S and Lake 302N at the Experimental Lakes Area (Ontario, Canada) were acidified with sulfuric, nitric, and hydrochloric acids and returned to natural pH. We used the corresponding 20-year history in Lake 239 to build chemical mass balance models for the experimental lakes. With these models, we assessed the production of alkalinity and the loss of sulfate and DOC in the context of atmospheric and watershed inputs and our experimental acid additions. Alkalinity production was greatest during the period of highest acid inputs and not detectable during pH recovery even in the early stages when pH was still low. Sulfate loss was also greatest while sulfate concentrations were increasing but counter to expectations not directly related to sulfate concentrations. Losses of the mass of DOC showed little change in response to acid additions over the 20 years, but first-order losses increased at low pH.