Soil Mercury Accumulation Delays Fish Recovery from Atmospheric Deposition Declines

Marked decreases in U.S. mercury emissions over the last 30 years reflect successful air quality management, but data documenting the responses of ecosystems and biota over this same period are limited. We synthesized long-term data on mercury cycling at Arbutus Lake in the Adirondack Park, NY, and evaluated whether brook trout (Salvelinus fontinalis) from five regional lakes responded to changing watershed inputs. We find that the rate of watershed mercury inputs from dry deposition, wet deposition, and litterfall significantly decreased over the monitoring record (2003-2021), but the watershed continued to accumulate mercury by 25.2 ± 2.8 μg m^-2 yr^-1 (mean ± interannual SD). Annual fluxes of total mercury from the major tributary inflow to Arbutus Lake declined early in the record (-3.3% yr^-1, 2005-2010) but have increased more recently (1.8% yr^-1, 2015-2019), possibly in response to changing climate and soil chemistry. No significant trends were observed in the net total or methylmercury loading to Arbutus Lake, nor in fish mercury concentrations in nearby lakes. These findings suggest that legacy mercury in soils from historical deposition may delay the response of lake biota to declining mercury deposition and highlight the potential impacts of climate change on future mercury cycling within forested ecosystems.