Differential effects of nutrient enrichment on carbon metabolism and dissolved organic carbon (DOC) fluxes in macrophytic benthic communities

Abstract Nutrient enrichment can alter negatively benthic communities and reduce their ecological services. This study explores in two contrasting seasons (winter and summer), the effects of in situ nutrient enrichment at the community level on carbon metabolism and dissolved organic carbon (DOC) fluxes in two benthic communities dominated by the seagrass Cymodocea nodosa and by the macroalga Caulerpa prolifera. Under nutrient enrichment, C. nodosa increased total community biomass and diversity in summer, while net community production shifted from net autotrophic to net heterotrophic in winter. In contrast, C. prolifera became heterotrophic in summer, while no significant effects were found in winter. Regarding DOC fluxes, nutrient enrichment shifted C. nodosa from net DOC consumer in winter to a strong net DOC producer in summer, while C. prolifera seemed unaffected. Therefore, nutrient enrichment can promote conditional effects (positive, negative or neutral) in coastal areas depending both on macrophyte assemblages and season.