Adirondack (NY, USA) reference lakes show a pronounced shift in chrysophyte species composition since ca. 1900

Researchers are increasingly tasked with identifying biological recovery targets as ecosystems recover from anthropogenic stressors. Attempts to define such recovery targets are often hampered by two problems: (1) a lack of long-term monitoring data; and (2) the confounding influence of multiple stressors, especially regional stressors like climate warming. Paleolimnological studies of minimally disturbed reference sites can help address both these issues. Using paleoecological techniques, researchers can isolate the long-term impact of regional stressors like climate change on species assemblages largely independent of other confounding stressors such as acidification, eutrophication, and land-use change, thereby providing a framework to assess biological recovery in lakes that are recovering from acid deposition or other stressors. This manuscript provides a theoretical paleolimnological framework for the use of reference lakes in studying biological recovery from acidification, and provides an example of how assemblages of scaled-chrysophytes have changed in Adirondack-region reference lakes (NY, USA) from pre-ca. 1900 to present. The thirty-one reference lakes were selected from a database of over 1400 lakes, using criteria to minimize the influence of acidification, eutrophication, road-salt seepage, and piscivore introductions. As such, these lakes provide a unique opportunity to examine the effects of regional stressors in the Adirondack ecological region, which can inform biological recovery in lakes that have acidified historically. The modern chrysophyte assemblages from the reference lakes were significantly related to modern limnological variables including pH, dissolved organic carbon and ionic concentration as well as important physical variables including lake depth, which were used to help understand changes in the chrysophyte assemblages over the last century. Changes in chrysophyte assemblages from pre-1900 to present were determined by comparing the modern surface assemblages to a sediment interval representing pre-1900 conditions, revealing significant increases in the abundance of colonial chrysophyte taxa, especially S. petersenii, S. sphagnicola, and S. echinulata, and corresponding decreases in the relative abundance of many Mallomonas taxa. These changes suggest that regional warming and/or oligotrophication have influenced the species assemblages of minimally disturbed reference lakes, suggesting that lakes currently recovering from acidification are unlikely to return to their pre-disturbance assemblages.