Hydrology controls the carbon mass balance of a mountain lake in the eastern European Alps

Abstract

Lakes and streams in mountain regions are important contributors to carbon (C) fluxes. However, detailed carbon balances of the stream-lake continuum are rare. Combining eddy covariance (EC) measurements of lake-atmosphere net ecosystem CO2 exchange with measurements of fluvial C fluxes (dissolved organic C, DOC; particulate organic C, POC; dissolved inorganic C, DIC), and in-lake sedimentation, we here present a detailed annual C balance of an oligotrophic clearwater lake in the eastern European Alps. The C flux into the lake was 1522 Mg C year−1 by DIC (93%), DOC (6%), and POC (0.7%). The largest C losses were fluvial exports (1595 Mg C year−1) of DIC (93%), DOC (6%), and POC (0.8%), while sedimentation accounted for 7.3 Mg C year−1. The residual of all fluvial and sedimentation fluxes revealed the lake as a net sink of atmospheric CO2 of 77 Mg C year−1. The EC measurement confirmed a small positive or negative contribution of atmospheric exchange to the lake C balance. In-lake transfer among C pools was only significant for the flux from DIC to POC (8.4 Mg C year−1), which, following our model, is the transfer through primary production in summer. Fluvial DOC and DIC fluxes were controlled by discharge; POC retention and sedimentation depended on the meteorological season and in-lake residence time. Following our findings, we conclude that hydrology acted as the most important control for the C balance of this clearwater mountain lake by controlling inflow, outflow, and sedimentation fluxes.