Metabolism is of minor importance for under-ice DIC dynamics in hardwater lakes

Our knowledge of inland lakes is largely based on samples collected during the ice-free “growing” season, even though lakes in continental climates may be ice-covered for several months during winter. Studies that have characterized under-ice conditions were predominantly conducted in dilute boreal and temperate lakes, while information on hardwater lakes (>120 mg L−1 CaCO3) is still limited. Yet, hardwater lakes represent ∼50% of standing waters on earth, contain large quantities of dissolved organic and inorganic carbon (DOC, DIC) and are considered hotspots of carbon processing. Here, we present results from a 9-lake survey of Canadian hardwater lakes, conducted over 3 consecutive winters, with preceding summer conditions characterized to contrast seasonal differences. Despite their 10-fold differences in morphometry and water chemistry, hardwater lakes showed similar trajectories from fall to winter to spring: (1) physical freeze-out increased salinity and DOC; (2) metabolic activity controlled algal biomass, dissolved oxygen (DO), pH, and soluble reactive phosphorus (SRP); and (3) DIC dynamics were influenced predominantly by nonmetabolic factors such as hydrology (groundwater) and water chemistry (CaCO3 dissolution), relative to metabolic factors (aerobic/anaerobic respiration). While temporal patterns of chlorophyll a, DO, SRP, and pH were comparable to boreal and temperate lakes, DIC dynamics were starkly different. Hence, we propose that hardwater lakes should be considered separately, particularly pertaining to their role in the global carbon cycle.