INVESTIGATING THE EFFECTS OF WINTER DRAWDOWNS ON THE ECOLOGICAL CHARACTER OF LITTORAL ZONES IN MASSACHUSETTS LAKES

Anthropogenic alteration of water levels in lakes is a major stressor to the ecological integrity of littoral zones, which provide critical heterogenous resources that support diverse biological communities. Annual winter drawdowns have been practiced in Massachusetts (MA) for several decades; however, few studies have estimated impacts to littoral zone habitat and biological communities, particularly at relatively mild magnitudes (i.e., m) and in lakes that co-occur with other anthropogenic pressures (e.g., lakeshore development) as seen in MA lakes. My dissertation reviewed the winter drawdown literature and collected empirical data in MA lakes to characterize winter drawdown hydrological regimes and estimate responses of physical habitat (macrophytes, sediment texture, coarse wood), macroinvertebrate assemblages, and mussel assemblages to variable levels of drawdown magnitude. Through a stratified random selection approach, I selected 21 MA lakes (18 drawdown, 3 non-drawdown) based on drawdown information from an email survey to local conservation commissions and lake and pond associations. I continuously monitored water levels for 3–4 years within these lakes that represented a drawdown magnitude gradient. Drawdown regimes displayed considerable inter- and intra-lake variability in the timing and duration of annual drawdown events. The majority of winter drawdown events were incongruous to MA state issued timing guidelines, particularly for April 1st refill dates. In the same set of lakes, I found increased drawdown magnitude was correlated with coarser substrates and reduced silt, reductions in macrophyte biomass and biovolume, and proportional increases of macrophyte taxa with annual longevity strategy and amphibious growth form. During normal water levels, I found markedly lower freshwater mussel densities at drawdown-exposed depths compared to the same depths in non-drawdown lakes. I also found drawdown magnitude significantly structured macroinvertebrate taxonomic and functional composition with evidence that suggests several drawdown-sensitive taxa (e.g., Amnicola) and traits (e.g., semivoltinism). To minimize losses to lake ecological integrity, winter drawdown management should consider the extent of lakebed and littoral zone area exposed during drawdowns, incorporate depth-specific monitoring efforts for susceptible biota (e.g., mussels), and anticipate water level responses to climate change