Dissolved organic matter uptake in a temperate seagrass ecosystem

Abstract

We assessed the utilization of inorganic and organic nitrogen compounds of different complexity by primary producers and bacteria in a seagrass ecosystem. Using double-labeled ( 13 C and 15 N) substrates, the net transfers from the dissolved nitrogen and carbon pools to phytoplank- ton, planktonic bacteria, epiphytes, seagrasses (Zostera noltii and Cymodocea nodosa), and a macroalga (Caulerpa prolifera) were quantified in field incubations. Phytoplankton was the largest nitrogen sink, followed by the epiphytic community. In contrast, carbon fixation was dom- inated by the macrophytes. Although compartment-specific variations existed, NH4 + was gener- ally preferred over NO3 � and urea. Specific uptake rates of individual amino acids were inversely related to their C:N ratio and their structural complexity (glycine > L-leucine > L-phenylalanine). In addition, biomarker-specific measurements (polar lipid-derived fatty acids and D-alanine) indi- cated increasing bacterial contributions to carbon uptake with increasing amino acid structural complexity. All primary producers acquired nitrogen from a complex pool of algae-derived dis- solved organic matter (DOM), but algae-derived dissolved organic carbon (DOC) was almost exclusively used by the planktonic compartment. In contrast, a similar complex pool of bacteria- derived DOM was not taken up in significant quantities by any of the primary producers. Our results illustrate that dissolved organic nitrogen (DON) can provide bioavailable nitrogen to pri- mary producers and bacteria in seagrass ecosystems on very short time-scales, and DON and DOC dynamics are largely uncoupled and must be investigated as 2 separate interacting pools.