Variations in Zooplankton Communities as Indicators of Biological Responses to Climate Change and Recovery From Acidification in Northeastern and Maine Mountain Lakes

Since the Clean Air Act Amendments of 1990, lakes in Maine and much of the Northeastern US have seen significant shifts in response to reduced atmospheric deposition as well as climate change. The organisms that inhabit lakes are susceptible to environmental change, but our understanding about how biological communities react to simultaneous changes in geochemistry and climate is incomplete. This research investigates how zooplankton communities respond to geochemical changes and warming, on both long-term and seasonal scales. These small organisms are sensitive to changes in physical lake conditions, and variation within zooplankton communities could indicate larger ecosystem shifts. We evaluated historical water chemistry and zooplankton composition data from 143 lakes throughout the northeastern US from the EPA’s Eastern Lake Survey (ELS) to understand how recovery from acidification and subsequent chemical shifts influenced zooplankton. Surface water sulfate concentrations decreased by 22% between the two sample years of 1986 and 2004. Sites with closer proximity to development saw significant increases in chloride concentration likely due to increased use of road deicers, which caused a subsequent increase in calcium and magnesium base cations. Zooplankton body size increased significantly between 1986 and 2004 and Daphnia species, which have high calcium requirements, increased in correlation with increases in calcium. Zooplankton community structure, however, was most strongly influenced by variation in ANC, sulfate, and dissolved organic carbon (DOC). Our findings indicate that, though surface water acidity influences zooplankton community structure, recovery of zooplankton populations to a pre-acidification composition is unlikely because other chemical shifts along with changes in climate and land-use, are also eliciting responses in zooplankton communities. A significant result of climate change is shifts in the timing of seasonally reoccurring events which have already been observed in Northeastern lakes including earlier ice break-up and longer periods of thermal stratification. Biotic responses to shifting seasons in lakes depends on species specific life-history traits and the seasonal events that drive them. We examined zooplankton phenology in lakes across a range of elevation and climate zones in Maine to understand the importance of spring surface water warm-up in high elevation versus low elevation sites. We collected zooplankton, chlorophyll-a, and water temperature in eight remote Maine lakes from the extremes of elevations (94m – 955m above sea level), representing the endmembers of climate variability in the state. High elevation lakes were distinguished by a short and rapid period of spring warming compared to more gradual warm-up in low elevation lakes. Zooplankton abundances increased in correlation with spring warm-up, most significantly in high elevation lakes. The link between surface water warming and zooplankton phenology varied among taxa, with rotifers and calanoid copepods more dependent on spring temperature than cladocerans and cyclopoid copepods. The variable response among zooplankton taxonomic groups to spring water warmup indicates that responses to climate change will be taxa specific. Disentangling the drivers of zooplankton populations will help provide a mechanistic understanding of longer-term climate and geochemical shifts observed in northeastern lake ecosystems and potential consequences of climate change