Jan Karlsson

Inland waters play a critical role in the carbon cycle by emitting significant amounts of land‐exported carbon to the atmosphere. While carbon gas emissions from individual aquatic systems have been extensively studied, how networks of connected streams and lakes regulate integrated fluxes of organic and inorganic forms remain poorly understood. Here, we develop a process‐based model to simulate the fate of terrestrial dissolved organic carbon (DOC) and carbon dioxide (CO 2 ) in artificial inland water networks with variable topology, hydrology, and DOC reactivity. While the role of lakes is highly dependent on DOC reactivity, we find that the mineralization of terrestrial DOC is more efficient in lake‐rich networks. Regardless of typology and hydrology, terrestrial CO 2 is emitted almost entirely within the network boundary. Consequently, the proportion of exported terrestrial carbon emitted from inland water networks increases with the CO 2 versus DOC export ratio. Overall, our results suggest that CO 2 emissions from inland waters are governed by interactions between the relative amount and reactivity of terrestrial DOC and CO 2 inputs and the network configuration of recipient lakes and streams.

Metabolism is one of the most fundamental ecosystem processes, but the drivers of variation in metabolic rates among lakes dominated by benthic primary producers remain poorly constrained. Here, we report the magnitudes and potential drivers of whole‐lake metabolism across 43 Swedish arctic–alpine lakes, based on the free‐water diel oxygen technique with sondes deployed during the open‐water season near the surface and bottom of the lakes. Gross primary production (GPP) and ecosystem respiration (R) were strongly coupled and ranged from 0.06 to 0.45 mg and 0.05 to 0.43 mg L −1 d −1 among lakes. On average, GPP and R decreased eightfold from relatively shallow to deep lakes (mean depth 0.5–10.9 m) and twofold from concave to convex lakes (mean depth: maximum depth 0.2–0.5). We attribute this to light limitation and shape‐specific sensitivity of benthic GPP to disturbance by lake ice. Net ecosystem production (GPP‐R) ranged from −0.09 to 0.14 mg L −1 d −1 and switched, on average, from positive to negative towards deeper lakes and lakes richer in dissolved organic carbon (DOC; 0.5–7.4 mg DOC L −1 ). Uncertainties in metabolism estimates were high (around one and three times mean R and GPP), especially in deep lakes with low insulation and diurnally variable wind speed. Our results confirm the role of DOC in stimulating net heterotrophy and highlight novel effects of lake shape on productivity in benthic‐dominated lake ecosystems and its response to changes in lake ice cover.