The role of terrestrial productivity in regulating aquatic dissolved organic carbon concentrations in boreal catchments

The past decades have witnessed an increase in dissolved organic carbon
(DOC) concentrations in the catchments of the Northern Hemisphere.
Increases in terrestrial productivity may be a reason for the increases in
DOC concentration. The aim of this study is to investigate the impacts of
increased terrestrial productivity and changed hydrology following climate
change on DOC concentrations. We tested and quantified the effects of
gross primary production (GPP), ecosystem respiration (RE) and discharge
on DOC concentrations in boreal catchments over three years. As catchment
characteristics can regulate the extent of rising DOC concentrations
caused by the regional or global environmental changes, we selected four
catchments with different sizes (small, medium and large) and landscapes
(forest, mire and forest-mire mixed). We applied multiple models: Wavelet
coherence analysis detected the delay-effects of terrestrial productivity
and discharge on aquatic DOC variations of boreal catchments; thereafter,
the distributed-lag linear models (DLMs) quantified the contributions of
each factor on DOC variations. Our results showed that the combined
impacts of terrestrial productivity and discharge explained 62% of aquatic
DOC variations on average across all sites, whereas discharge, GPP and RE
accounted for 26%, 22% and 3%, respectively. GPP dominated the DOC
variations in small catchments (<1 km2), but in large catchments,
DOC variations were mainly dependent on discharge. The direction of the
relation between GPP and discharge on DOC varied. Increasing RE always
made a positive contribution to DOC concentration. This study demonstrated
that terrestrial greening and changing hydrology caused by climate change
did affect the DOC export from terrestrial to aquatic ecosystems, which
improves the mechanistic understanding of surface water DOC regulation in
boreal catchments under climate change.