Annual winter water level drawdowns limit shallow‐water mussel densities in small lakes

Abstract

Regulated water level fluctuations alter the physical, chemical, and biological environments in lakes. However, few studies have measured the effects of repeated annual winter drawdowns on freshwater mussel populations (Bivalvia: Unionida), and it is unknown whether drawdowns permanently constrain mussel populations to deeper depths or are resilient to the annual disturbance. We quantified mussel densities during normal water levels and their mortality after the initiation of drawdown in lakes with annual winter (December–March) water level drawdown regimes that have existed for several decades. We used systematic quadrat sampling and visual snorkel surveys to estimate the number of live mussels at the surface and buried, and shell length in six drawdown and three control lakes at exposed (0.5‐m) and unexposed (1‐m) depths during winter drawdown. We also estimated mortality of mussels along the exposed lakebed for nine drawdown lakes. Study lakes were dominated by Elliptio complanata and Pyganodon cataracta. During normal water levels, mussel densities were much lower in drawdown lakes than control lakes at 0.5‐m depths. When mussels were present in drawdown lakes at 0.5‐m depths, they were often burrowed in the substrate and smaller than buried mussels in control lakes. At unexposed 1‐m depths, mussel densities were primarily explained by geographic region (western versus central Massachusetts), and found higher densities in drawdown lakes compared to control lakes. Mussel mortality after a single drawdown event in nine drawdown lakes was not correlated with drawdown magnitude or rate. Concordance of shell length‐frequency distributions between dead and buried living mussels in drawdown lakes suggests annual mortality of colonising individuals, which are typically small and presumably of young cohorts. Annual winter drawdowns still limit densities of E. complanata and P. cataracta in shallow‐water habitats in the following autumn (September–October) after 4–5 months of potential recovery, and hence constrain their distribution in lakes exposed to drawdowns. The development of sustainable water level management practices will be essential to minimise impacts to native mussel populations.

Hydrology of annual winter water level drawdown regimes in recreational lakes of Massachusetts, United States

“…However, even relatively small WD magnitudes can have substantial ecological impacts. For example, within a subset of the current study lakes, Carmignani et al (2019) found WD regimes with <1 m magnitudes limited freshwater mussel distributions deeper than stable phase water levels, presumably due to their low mobility and susceptibility to desiccation. Also, short pulses of large and rapid water level recession as observed in our study may expose high mussel densities on shallow benthic shelves (e.g., Onota).…”

“…Galbraith et al (2015) found most mussels were stranded under 4 cm/d and 8 cm/d recession rates but with variable species-specific mortality after stranding. Given that many WD events in the current study possessed net daily recession rates >4 cm/d, increases in magnitude with similar recession rates will likely impact existing mussel assemblages, for which the distributions are already restricted by ongoing WD regimes (Carmignani et al 2019). Also, rapid recession rates may trap fish in shallow pools, leading to mortality via stranding or because of stressful overwintering conditions (Nagrodski et al 2012).…”

“…Due to differences in climate, watershed and lake hydromorphological features, and water level management goals, WD regimes in recreational lakes in the northeastern United States likely differ compared to hydroelectric lakes in north temperate and boreal zones. Consequently, ecological responses to WDs in recreational lakes may differ, requiring quantification of water level fluctuation to reliably estimate potential ecological impacts (e.g., Carmignani et al 2019). Furthermore, while most studies focus on WD magnitude, other hydrological components, such as timing, duration, water level recession and refill rates, and degree and duration of lakebed exposure, may help to better predict ecological responses Roy 2017, Hirsch et al 2017).…”

Towards control of dam and reservoir systems with forward–backward stochastic differential equations driven by clustered jumps

“…16 Bivalve populations are constrained by annual water level drawdowns. 17 Rapid water level changes of a reservoir perish fish species living in it while some other species increased abundance, suggesting that changing water levels serves as a new technique to control fish assemblages in reservoirs. [18][19][20] Operation purpose of dam and reservoir systems sometimes include hydropower generation and water quality management as well.…”

“…In the Three Gorges Reservoir, spawning areas of carp species depend on the water level where low water level is preferred for human lives while high water levels are required to sustain fish population 16 . Bivalve populations are constrained by annual water level drawdowns 17 . Rapid water level changes of a reservoir perish fish species living in it while some other species increased abundance, suggesting that changing water levels serves as a new technique to control fish assemblages in reservoirs 18–20 .…”