Patterns of Spring/Summer Open-Water Metabolism Across Boreal Lakes

Northern regions host the greatest density of surface water globally, but knowledge of lake metabolism in this vast yet remote landscape is limited. Here, we used an oxygen stable isotope approach to quantify patterns and drivers of surface layer metabolism in lakes throughout an approximately 10(6) km(2) tract of boreal Canada. Ecosystem gross primary production (GPP) and respiration rates (R) were much higher than previously assumed for spring and summer months. Both rates were strongly linked to nitrogen (N) concentrations, not light availability, despite earlier work showing community-level light effects. Net ecosystem production (NEP = GPP - R) was negative for most lakes. Hierarchical modeling revealed that although NEP is strongly stabilized via similar effects of N on both GPP and R, NEP decreases with increasing dissolved organic carbon (DOC). These interactive controls on NEP were not predictable from bivariate regressions linking NEP to physical, chemical or habitat-specific drivers. In contrast to expectations, NEP was higher in warmer waters due to increased temperature dependency of GPP, not R. Temperature and DOC content had opposing effects on NEP in all but the most dystrophic lakes, possibly implying a muted response of metabolic balances to future shifts in both regional climate and OC delivery.