Trade-offs between light and nutrient availability across gradients of dissolved organic carbon concentration in Swedish lakes: implications for patterns in primary production

Dissolved organic carbon (DOC) limits primary production in lakes when present at high concentrations by reducing light availability, but stimulates primary production at lower concentrations by releasing nutrients through photolysis. These dual influences create the potential for threshold relationships between DOC and primary production, but empirical tests for the prevalence of thresholds are scarce. We used Box-Cox regression and environmental monitoring data from 703 subarctic and boreal lakes to assess patterns and potential threshold relationships between light and nutrient availability along gradients of DOC in northern Sweden's six major watersheds. We found consistent patterns of increasing nutrient concentration and light attenuation with DOC. Further, we identified thresholds (mean = 5.96 mg.L-1) below which nutrient concentrations increased more rapidly than light extinction and above where the opposite occurred. These results suggest consistent patterns in primary production with shifts from nutrient to light limitation with increasing DOC. Accordingly, the thresholds agree with the vertex of the curvilinear relationship between lake primary production and DOC. We estimated that most lakes in Sweden are within +/- 3 mg.L-1 of the threshold, indicating high potential for changes from positive to negative influences of DOC on primary production if forecasted increases in DOC concentrations due to climate and land cover change are realized.