Reconstructing the various facets of dissolved organic carbon bioavailability in freshwater ecosystems

We explored various aspects of freshwater dissolved organic carbon (DOC) lability by comparing short‐term (< 2 d) bacterial C consumption (STCC; derived from bacterial respiration measurements) with long‐term (28 d) C consumption (LTCC) in DOC bioassays in lakes, rivers, and marshes located within the same complex drainage basin in southern Québec. We also combined STCC and LTCC measurements to estimate the proportion of DOC removed, and to derive a first‐order decay constant ( k ). STCC rates were, on average, 25% higher than LTCC, and both parameters showed distinct patterns, reaching their lowest and highest values in lakes and marshes, respectively. STCC and LTCC were correlated to DOC concentration across these freshwater ecosystems, whereas in lakes, STCC was positively correlated to chlorophyll and LTCC to terrestrial C inputs. k showed no ecosystem‐specific patterns but was negatively correlated to chlorophyll across systems. The size of the DOC pools supporting STCC and LTCC, as well as k , were related to distinct components of the DOC pool, as revealed by a parallel factor analysis of fluorescent dissolved organic matter excitation‐emission spectra. Short‐ and long‐term lability and C consumption, and the resulting k , are shown to be complementary facets of DOC bioavailability, which may play very different roles on aquatic ecosystem functioning.