| Summary: | Increased estuarine input and subsequent degradation of dissolved organic carbon (DOC) is a potentially important cause of rising CO2 concentrations that contribute to coastal marine acidification. However, it is not clear whether or not the evolved CO2 originates from the decomposition of terrestrially-derived DOC. We recorded the changes in δ13C and δ2H of dissolved organic matter during laboratory incubations with water from 18 river mouths, plus one estuary (northern Sweden) sampled with spatial and temporal resolution. The Keeling plot method was used in combination with isotope mixing models to estimate the source of the decomposed dissolved organic matter (DOM). At the river mouths, freshwater algae preferentially contributed as a source of DOC being degraded, although a substantial fraction also had terrestrial origin. However, in the estuary marine algae supported most DOM degradation. In fact, we only found clear evidence for degradation of terrestrially-derived DOC during high flow conditions in shallow estuarine waters close to the river mouth. Thus, terrestrial export of DOC does not appear to strongly contribute to the release of CO2 by coastal plankton communities.
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