Temporal trends in fish mercury concentrations in an Adirondack Lake managed with a continual predator removal...

Mercury is a neurotoxic pollutant and contamination in remote ecosystems due to atmospheric mercury deposition coupled with watershed characteristics that influence mercury bioavailability. Biological interactions that affect mercury bioaccumulation are especially relevant as fish assemblages change in response to species introductions and lake management practices. We studied the influence of shifting food web dynamics on mercury in fisheries of Little Moose Lake in the southwestern Adirondack Mountains of New York, USA. Annual removal of non-native Smallmouth Bass (Micropterus dolomieu) has been used as a management strategy since 2000 to restore the native fish assemblage and food web in favor of Lake Trout (Salvelinus namaycush). Changes in total mercury, stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotopes, and growth were evaluated for Lake Trout and Smallmouth Bass. Growth rates increased for both predators and trophic position increased for Lake Trout post-removal. Mercury concentrations in Lake Trout increased over the 16-year study period influenced by a diet shift from invertebrates to higher trophic level prey fish, regardless of increased growth. Smallmouth Bass mercury concentrations decreased with compensatory growth from a reduced population size. These contrasting trends indicate that changes in mercury deposition were not the primary driver for mercury bioaccumulation responses in Little Moose Lake. Stable isotope values changed for both predators and for several lower trophic level organisms, likely reflecting changes in nutrient cycling and/or inputs. Our findings emphasize the potential role of fisheries management on whole-lake and predatory fish responses to mercury contamination in temperate lakes.