Paleolimnological comparison of algal changes in a clear-versus a brown-water lake over the last two centuries in the northeastern USA

We reconstructed historical diatom and algal pigment records from a clear-water versus a brown-water lake in the northeastern U.S. to compare ecological responses to reduced acid deposition in recent decades in the context of a 140-year record, during which time multiple external drivers influenced both lakes. In the clear-water lake, diatom community structure changed continually from the beginning of the record, while it was more static in the brown-water lake over time, with the period of greatest change occurring after 1990. Concentrations of algal pigments were low in the clear-water lake until 1940, then increased during the 1940 to 1990 period, after which they slightly declined. The opposite pattern occurred in the brown-water lake-algal pigments were high until 1940, decreased in the period from 1940 to 1990, and increased after 1990. The clear-water lake was more responsive to long-term climate warming beginning at the end of the Little Ice Age. During the period of higher acid deposition, light availability was more important for controlling algal responses in the clear-water lake, while nutrient subsidies from allochthonous DOC were likely the primary control in the brown-water lake. With reductions in atmospheric sulfate deposition, both lakes showed signs of recovery toward pre-acidification conditions, but these changes were dampened in the clear-water lake, suggesting greater sensitivity to acid deposition and other external drivers such as effects of climate change. Our study highlights strong and divergent algal responses in a clear-water vs. a brown-water lake during a period of higher acid deposition, and considers these changes in the context of multiple drivers of environmental change over the past 140 years.