Ecosystems dominated by seagrasses often exhibit low inorganic nitrogen concentrations. Given the high productivity in these systems, recycling of nitrogen is expected to be high. We investigated the use of inorganic and organic nitrogen compounds by co-occuring primary producers in a temperate macrophyte-dominated (seagrasses: Zostera noltii, Cymodocea nodosa, and macroalga: Caulerpa prolifera) system. Using dual isotope labeling (13C and 15N) uptake of substrates with different molecular structure and complexity by phytoplankton, planktonic bacteria, epiphytes, seagrasses, and macroalgae were quantified in field incubations. Phytoplankton was by far the largest nitrogen sink, followed by the epiphytic community. In contrast, the seagrasses and Caulerpa prolifera dominated carbon fixation. NH4+ was usually preferred over NO3− and urea, particularly by the seagrasses. Specific uptake rates of nitrogen from individual amino acids were inversely proportional to their C/N-ratio and structural complexity (glycine > L-leucine > L-phenylalanine). In addition, biomarker-specific measurements (polar lipid-derived fatty acids and D-alanine) indicated an increasing bacterial contribution to nitrogen uptake with increasing amino acids complexity. Algae derived DON was taken up to a larger extent than bacteria derived nitrogen, indicating the importance of the source to DON utilization. All primary producers acquired nitrogen derived from the algae-derived DOM, but the DOC was almost exclusively used by the planktonic biota. Our results illustrate that (1) dissolved organic nitrogen plays an important nutritional role in seagrass meadows, (2) phytoplankton in seagrass meadows is a major sink for dissolved nitrogen at short time scales, and (3) organic nitrogen and carbon dynamics are largely uncoupled and should be investigated as such.
|Publication status||Published - 2011|