Abstract
Inland waters transport, transform and retain significant amounts of dissolved organic carbon (DOC) that may be biologically reactive (bioreactive) and thus potentially degraded into atmospheric CO2. Despite its global importance, relatively little is known about environmental controls on bioreactivity of DOC as it moves through river systems with varying water residence time (WRT). Here we determined the influence of WRT and landscape properties on DOC bioreactivity in 15 Swedish catchments spanning a large geographical and environmental gradient. We found that the short-term bioreactive pools (0–6 d of decay experiments) were linked to high aquatic primary productivity that, in turn, was stimulated by phosphorus loading from forested, agricultural and urban areas. Unexpectedly, the percentage of long-term bioreactive DOC (determined in 1-year experiments) increased with WRT, possibly due to photo-transformation of recalcitrant DOC from terrestrial sources into long-term bioreactive DOC with relatively lower aromaticity. Thus, despite overall decreases in DOC during water transit through the inland water continuum, DOC becomes relatively more bioreactive on a long time-scale. This increase in DOC bioreactivity with increasing WRT along the freshwater continuum has previously been overlooked. Further studies are needed to explain the processes and mechanisms behind this pattern on a molecular level.
Highlights
Despite its global importance, relatively little is known about environmental controls on bioreactivity of dissolved organic carbon (DOC) as it moves through river systems with varying water residence time (WRT)
Long WRT could favor complete oxidation of DOC removal of a source of potential long-term bioreactive DOC (LTBR) from inland waters, our results show that the role of WRT as facilitator of organic matter transformation was greater than the role of WRT as a factor leading to CDOM removal (Fig. 2)
Our study shows a broad scale pattern of short-term bioreactive DOC replenishment along the continuum through aquatic primary production, while long-term DOC bioreactivity was sustained through reprocessing of terrestrial DOC derived from upstream ecosystems
Summary
Relatively little is known about environmental controls on bioreactivity of DOC as it moves through river systems with varying water residence time (WRT). It is often assumed that WRT in freshwater systems causes a unidirectional decrease in DOC bioreactivity[5,9], but if internal loadings of bioreactive DOC compensates or even surpasses the biological consumption of DOC in the aquatic network[10], downstream coastal ecosystems would receive more bioreactive DOC than expected from current knowledge. This would have major consequences for the function of recipient coastal ecosystems, such as for CO2 emissions of coastal waters to the atmosphere, and on coastal hypoxia[6,11].
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