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 dissolved organic matter (DOM) plays a key role in the carbon cycle. There is growing interest for its chemical and ecological properties but its variability in the very heterogeneous coastal environments is poorly documented. In this study, we assess the spatial and temporal variations in marine coastal DOM chemical properties and its potential to fuel the growth of the heterotrophs in the planktonic food-web. We sampled two northwestern Mediterranean coastal stations under contrasting terrestrial and human influences. From January to July 2022, dissolved organic carbon (DOC) concentration progressively increased, although highly variable spatially. DOM composition varied temporally rather than spatially. These variations appeared related to a combination of direct freshwater inputs (tracked by salinity variations), direct human contaminations (tracked by concentrations in copper and lead), and primary production (tracked by concentrations in chlorophyll a). An incubation experiment was used at each sampling date to evaluate the growth potential of heterotrophic prokaryotes and compare it to variations in DOM properties. Significantly higher growth was observed with DOM from a site under higher terrestrial 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. This work suggests that human activities and constructions in harbors have the potential to strengthen the heterotrophic basis of the planktonic food web.