Significant chemical shifts have been detected in northeastern lakes since the 1980s, when surface water acidification was widely reported and subsequently, programs were established by the US Environmental Protection Agency (EPA) to monitor environmental responses to emissions regulations.  Since the Clean Air Act Amendments of 1990 intensified regulations to limit emissions of sulfur and nitrogen, lakes in Maine and much of the Northeast have recovered from acidification, just as other chemical and physical changes have been detected as a result of a changing climate. The organisms that inhabit lakes are susceptible to these shifts, but our understanding about how different biological communities react to simultaneous changes in geochemistry and climate is incomplete. This research investigates how zooplankton communities respond to these changes, on both long-term and seasonal scales. Due to their short life cycles, these small organisms are sensitive to changes in physical lake conditions, and variation within zooplankton communities could indicate larger ecosystem shifts. We are evaluating how zooplankton size and community composition has changed across decades, using data from US EPA’s Eastern Lakes Survey (ELS-II), 145 lakes throughout the Northeast that were sampled in 1986 and resampled in 2004, a decade after implementation of the Clean Air Act Amendments. Additionally, we are assessing how zooplankton respond to annual lake phenology in seasonally ice-covered lakes across a range of elevation and climate zones in Maine. Beginning in winter 2019, we are collecting zooplankton, chlorophyll-a, water chemistry samples, and water temperature in eight remote Maine lakes from the extremes of elevations (94m – 955m above sea level) and climate divisions (mean annual T=2˚C–12˚C), in order to capture variation in climate and seasonal duration. The lakes are US EPA Long-Term Monitoring (LTM) study sites with a record of geochemistry data beginning in the 1980s. Samples were collected under ice in the winter, at 10-day intervals over the course of spring warming and will be collected during summer stratification and fall mixing. A chain of thermistors measure temperature in the epilimnion and hypolimnion throughout the year, including under ice. Zooplankton phenology and community structure will be compared among seasons and among lakes to understand how variation in climate and water chemistry influence community structure. Disentangling the drivers of zooplankton populations will help provide a mechanistic understanding of longer-term shifts observed in northeastern lake ecosystems and can provide a basis for understanding consequences of environmental change.