Estimating historical in-lake alkalinity generation from sulfate reduction and its relationship to lake chemistry as inferred from algal microfossils

Sediment cores were used to estimate in- lake alkalinity generation resulting from sulfate reduction relative to inferred changes in lakewater pH and trophic status over the last century in three Connecticut lakes. Despite being situated in geological settings with crystalline bedrock and thin, poorly buffered soils, and being impacted with high rates of acidic precipitation, none of the study lakes have declined in inferred pH based on scaled chrysophyte and diatom remains. In fact, the pH of one of the lakes, Coventry Lake, has significantly increased over the last century. Over the last similar to 44 to 69 years the amount of sulfur stored in the sediments from each lake increased from similar to two to three times resulting in mean rates of alkalinity generation ranging from 78 to 145 meq m(-2) yr(-1), significantly higher than the 45 to 48 meq m(-2) yr(-1) of hydrogen ions falling directly on the lake surfaces. In- lake alkalinity generation resulting from sulfate reduction has been sufficient to neutralize all of the acid falling directly onto the lake surfaces, as well as between 9% and 25% of the acid deposited onto the surrounding watersheds. Despite the increased importance of in- lake alkalinity generation, our findings support the hypothesis that significant amounts of alkalinity are also being generated in the catchments of the study lakes. The bulk of the increases in stored sulfur in all three lakes were as Fe sulfides and not in the form of organic sulfur, suggesting that the increases were the result of dissimilatory bacterial reduction of sulfate. As a result of the large increases in storage of Fe sulfides the ratio of total iron to chromium reducible sulfur ( Fe: CRS) has declined in all cores over time. Despite the overall decline in Fe: CRS in recent sediments, values are still largely above 3 in more recent sediments of two of the lakes. However, values of Fe: CRS have dropped below 1 in surface sediments of Uncas Lake, suggesting that in- lake loading of phosphorus may be responsible for a recent shift in the algal flora towards a slightly more eutrophic condition.