Concurrent warming and browning eliminate cold-water fish habitat in many temperate lakes.

Cold-water species in temperate lakes face two simultaneous climate-driven ecosystem changes: warming and browning of their waters. Browning refers to reduced transparency arising from increased dissolved organic carbon (DOC), which absorbs solar energy near the surface. It is unclear whether the net effect is mitigation or amplification of climate warming impacts on suitable oxythermal habitat (<20 °C, >5 mgO/L) for cold-loving species because browning expands the vertical distribution of both cool water and oxygen depletion. We analyzed long-term trends and high-frequency sensor data from browning lakes in New York's Adirondack region to assess the contemporary status of summertime habitat for lacustrine brook trout. Across two decades, surface temperatures increased twice as fast and bottom dissolved oxygen declined >180% faster than average trends for temperate lakes. We identify four lake categories based on oxythermal habitat metrics: constrained, squeezed, overheated, and buffered. In most of our study lakes, trout face either seasonal loss (7 of 15) or dramatic restriction (12 to 21% of the water column; 5 of 15) of suitable habitat. These sobering statistics reflect rapid upward expansion of oxygen depletion in lakes with moderate or high DOC relative to compression of heat penetration. Only in very clear lakes has browning potentially mitigated climate warming. Applying our findings to extensive survey data suggests that decades of browning have reduced oxythermal refugia in most Adirondack lakes. We conclude that joint warming and browning may preclude self-sustaining cold-water fisheries in many temperate lakes; hence, oxythermal categorization is essential to guide triage strategies and management interventions.

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

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.

Chemical and biological recovery from acid deposition within the Honnedaga Lake watershed, New York, USA.

Honnedaga Lake in the Adirondack region of New York has sustained a heritage brook trout population despite decades of atmospheric acid deposition. Detrimental impacts from acid deposition were observed from 1920 to 1960 with the sequential loss of acid-sensitive fishes, leaving only brook trout extant in the lake. Open-lake trap net catches of brook trout declined for two decades into the late 1970s, when brook trout were considered extirpated from the lake but persisted in tributary refuges. Amendments to the Clean Air Act in 1990 mandated reductions in sulfate and nitrogen oxide emissions. By 2000, brook trout had re-colonized the lake coincident with reductions in surface-water sulfate, nitrate, and inorganic monomeric aluminum. No changes have been observed in surface-water acid-neutralizing capacity (ANC) or calcium concentration. Observed increases in chlorophyll a and decreases in water clarity reflect an increase in phytoplankton abundance. The zooplankton community exhibits low species richness, with a scarcity of acid-sensitive Daphnia and dominance by acid-tolerant copepods. Trap net surveys indicate that relative abundance of adult brook trout population has significantly increased since the 1970s. Brook trout are absent in 65 % of tributaries that are chronically acidified with ANC of <0 µeq/L and toxic aluminum levels (>2 µmol/L). Given the current conditions, a slow recovery of chemistry and biota is expected in Honnedaga Lake and its tributaries. We are exploring the potential to accelerate the recovery of brook trout abundance in Honnedaga Lake through lime applications to chronically and episodically acidified tributaries.