Contrasting Patterns of Labile and Semilabile Dissolved Organic Carbon From Continental Waters to the Open Ocean

Marine and freshwater prokaryotes feed primarily on bioavailable labile dissolved organic carbon (BDOC L ), as well as the bioavailable fraction of the semilabile DOC (BDOC SL ) pool. These fractions are operationally defined here as the DOC consumed within a month and greater than a month to a year and a half, respectively. Organic matter from these different pools comes from various autochthonous and allochthonous sources, but their relative bioavailability is unknown across aquatic ecosystems. To fill this gap, we compiled literature information that included 653 batch culture DOC biodegradation experiments across eight aquatic ecosystem types over the past 20 years. We show that the proportion of BDOC L across all aquatic ecosystems was surprisingly consistent (6.1%) despite a 2 order of magnitude variation in initial DOC concentrations, suggesting an overall tight balance between carbon supply and consumption. A higher proportion of BDOC L , 16.3% on average, was observed in high productivity ecosystems. BDOC SL , on the other hand, gradually decreased from 16.0% in lakes to 7.2% in estuaries to undetectable in the open ocean, suggesting that terrestrial connectivity regulates BDOC SL across the continuum. Our results support that recent primary production fuels short‐term prokaryotic DOC needs with an increasing reliance on the abundant BDOC SL pool as ecosystems approach the land‐water interface. Batch culture experiments show that BDOC SL is metabolizable in freshwater and coastal environments but not in the open ocean. We estimate that BDOC SL can sustain 62% of total prokaryotic biomass in inland waters and coasts and an estimated total of 16.7% across aquatic biomes.