Seasonal and spatial variation in the organic carbon and nitrogen concentration and their stable isotopic composition in Zostera marina (Denmark)

The spatial and seasonal variation in the concentration and stable isotopic composition of carbon and nitrogen was studied in Zostera marina (eelgrass) meadows in Roskilde Fjord and circle minus resund, Denmark. Organic carbon (C-org) and nitrogen (No,,) concentrations in new tissues (leaf, rhizome, roots) ranged from 11 to 40 mmol g(-1) C and 0.4 to 3.6 mmol g(-1) N, whereas their stable isotopic composition in new leaf ranged from -22 parts per thousand to -8 parts per thousand (delta(13)C(lenf)) and +7 parts per thousand to +20 parts per thousand (delta(15)N(leaf)). The delta(13)C(leaf) correlated weakly (r(2) similar to 0.3) with the delta(13)C of total dissolved inorganic carbon (DIC) in the surface waters (delta(13)C(DIC) range: -5.4 parts per thousand to -0.3 parts per thousand). The temporal change was more pronounced than the spatial change for N-org, the atomic C:N ratio, and delta(13)C(leaf), but was much smaller for C-org and delta(15)N(leaf). Eelgrass plants had higher N, concentrations, lower C: N ratios, and more depleted delta(13)C(leaf) in winter than summer, reflecting the temporal imbalance between inorganic nutrient supply and plant demand imposed by seasonality in the growth rate. The apparent carbon isotope enrichment factor of new leaf relative to external DIC (epsilon(eelgrass.DIC) range: -12.1 parts per thousand to 5.3 parts per thousand) indicated that eelgrass must be reliant on direct HCO3- uptake, especially during the period of high growth rate (May to September). The steep spatial gradient of delta(15)N(leaf) reflected changes in the isotopic composition of the source of assimilated inorganic nitrogen. The strongly positive delta(15)N(leaf) values measured inside the fjord indicated an anthropogenic origin for inorganic nitrogen, which comes from riverine input (riverine delta(15)N - NO3- = +11.8 parts per thousand +/- 1.1 parts per thousand).