Depth Moderates DOC Impact on Cold‐Water Refugia in Small, Northern Temperate Lakes

Widespread changes in dissolved organic carbon (DOC) concentrations in lakes of the Northern Hemisphere are well documented, and there is a need to understand the ecological implications for changes in cold‐water refugia. DOC controls thermal structure in small, wind‐sheltered lakes, thus influencing the volume and persistence of the hypolimnion and cold‐water habitat. We explored the relationship between the quantity and quality of DOC and the availability of cold‐water habitat over the course of summer stratification in small lakes of varying depths. The ratio of the hypolimnetic volume to total lake volume was calculated to assess the relative changes in cold‐water habitat availability in each lake over time. DOC concentration, maximum lake depth, and the interaction between DOC concentration and maximum lake depth explained 87% of the variability in percent change in hypolimnion volume ratio across studied lakes. Hypolimnion compression was greater in shallow, low DOC lakes, whereas deeper, intermediate DOC lakes had more persistent hypolimnion volumes over the course of summer stratification. These results demonstrate that depth moderates the effect of DOC on the volume and persistence of cold‐water refugia availability in north temperate lakes, advancing our understanding of differing lake sensitivity in a rapidly changing physical and chemical environment. Key Points Dissolved organic carbon concentration, maximum lake depth, and their interaction explained 87% of the variability in hypolimnion volume ratio The relative change in hypolimnion volume ratio over the course of summer stratification had a strong, negative correlation with mean Schmidt stability