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Land-to-water hydrological connections represent a key regulatory mechanism of carbon transport, controlling carbon dioxide (CO2) emissions from lakes; however, as of yet, there is no assessment of its role at a pan-Arctic scale across large climatic and topographical gradients. We hypothesized that hydrologically well-connected lakes in wetter regions are CO2 sources fueled by stronger lateral fluxes of external carbon relative to drier regions. However, based on data from >200 Arctic lakes, we found that lakes in drier regions have higher and more variable annual CO2 emissions (37.06.2146.0 gC m-2 yr-1, medianQ1Q3) compared to lakes in wetter regions (8.01.717.3 gC m-2 yr-1), with both the lowest and the highest fluxes recorded among dryland lakes. We hypothesize that with increasing wetness, the relative proportion of fluvial emissions increases, whereas in drier landscapes where lakes often have limited stream export, carbon inputs can be retained and more efficiently emitted from lakes.

Inland waters emit and export large quantities of carbon (C), and the balance between emission and export determines the role of inland waters in the global C cycle. Modeling studies of stream-lake networks suggest that C emission-to-export ratios (emission:export) are higher in lake-rich networks with longer network residence times (NRT), yet empirical evidence supporting these patterns remains limited. We sampled 362 stream segments and 42 lakes across 32 nested arctic stream-lake networks, repeated across four seasons with contrasting environmental conditions. The data show that lake-rich networks have much longer NRT and higher C emission:export regardless of environmental conditions, while overall C emission:export increased during warmer low-flow periods. Our findings stress the importance of accounting for lakes within stream networks and simultaneously quantifying multiple C fates when assessing the role of inland waters in the C cycle.