Hypervariable DOM properties in coastal NW Mediterranean Sea -Evidences of strong human influences and potential consequences for the heterotrophic base of planktonic food webs

Marine DOM plays a key role in the carbon cycle. Though there is growing interest for its chemical and ecological properties, its variability in the very heterogeneous coastal environment is poorly documented. In this study, we assessed the spatial and temporal variations in marine coastal DOM chemical properties and its potential to support the growth of the heterotrophic basis of the planktonic food-web. For that, we considered two NW Mediterranean coastal sampling stations under contrasted continental and human influences. From January to July 2022, DOC concentration demonstrated a progressive increasing trend in addition to marked spatial differences. DOM composition presented more variations in time than in space. These variations appeared related to a combination of direct freshwater inputs tracked by salinity variations, direct human contaminations tracked by concentrations in Cu and Pb, and phytoplanktonic production. An experimental approach was also used at each sampling date to evaluate DOM ability to support the growth of heterotrophic prokaryotes. Significantly higher growth was observed with DOM from a site under higher continental and human influences. Water temperature exerted a higher control on growth than DOM properties. Correlation analysis with DOM sources suggested the uncoupling of phytoplanktonic production and growth of heterotrophic prokaryotes, which appeared better supported by human contaminations and, to a lesser extent, freshwater inputs. Sediment resuspension in harbors and antifouling paints could represent two important sources of bioavailable resources, favoring fast heterotrophic growth and higher net production, respectively.