Climate-Associated Oxythermal Squeezes Elevate Mercury in a Lacustrine Food Fish

Climate change is heating surface waters while deoxygenating bottom waters of lakes, compressing vertical habitat for cold-water fish. Resulting increases in the volume of water lacking dissolved oxygen (anoxia) could facilitate the formation of toxic, bioaccumulating methylmercury (MeHg). Oxythermal squeezes, where thermally stressful surface water pushes cold-water fish species against or into low-oxygen deep water, could enhance mercury bioaccumulation. Here, we test this hypothesis using high-resolution oxygen data and brook trout muscle mercury data for 16 neighboring lakes (Adirondack Park, New York, USA) collected in the fall of two consecutive years. In four anoxic lakes that underwent oxythermal squeezes in both years, muscle mercury concentrations were consistently elevated (0.40 μg/g mean (0.08 μg/g SD) length-adjusted wet weight concentration), and averaged more than double the levels in well-oxygenated lakes. Importantly, lakes with substantial anoxic zones yet no oxythermal habitat compression showed intermediate concentrations, indicating that forcing fish to reside next to oxygen-depleted water plays a key role in boosting their methylmercury exposure. Climate change is pushing many lakes toward such oxythermal habitat squeezes; hence, our findings suggest that higher fish mercury contamination is to be expected, even when external loading of mercury to aquatic ecosystems is stable or declining.