Seasonal and spatial variation in the organic carbon and nitrogen concentration and their stable isotopic composition in Zostera marina (Denmark)
Limnol. Oceanogr., 50(4), 2005, 1084-1095 | DOI: 10.4319/lo.2005.50.4.1084
ABSTRACT: 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 Øresund, Denmark. Organic carbon (Corg) and nitrogen (Norg) 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‰ to -8‰ (δ13Cleaf) and +7‰ to +20‰ (δ15Nleaf). The δ13Cleaf correlated weakly (r2 ~ 0.3) with the δ13C of total dissolved inorganic carbon (DIC) in the surface waters (δ13CDIC range: -5.4‰ to -0.3‰). The temporal change was more pronounced than the spatial change for Norg, the atomic C:N ratio, and δ13Cleaf but was much smaller for Corg and δ15Nleaf. Eelgrass plants had higher Norg concentrations, lower C:N ratios, and more depleted δ13Cleaf 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 (εeelgrass-DIC range: -12.1‰ to -5.3‰) 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 δ15Nleaf reflected changes in the isotopic composition of the source of assimilated inorganic nitrogen. The strongly positive δ15Nleaf values measured inside the fjord indicated an anthropogenic origin for inorganic nitrogen, which comes from riverine input (riverine δ15N - NO3- = +11.8‰ ± 1.1‰).