Papadimitriou, S., H. Kennedy, D. P. Kennedy, and J. Borum
Limnol. Oceanogr., 50(4), 2005, 10841095
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 Ψresund, 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 (d13Cleaf) and +7 to +20 (d15Nleaf). The d13Cleaf correlated weakly (r2 ~ 0.3) with the d13C of total dissolved inorganic carbon (DIC) in the surface waters (d13CDIC range: -5.4 to -0.3). The temporal change was more pronounced than the spatial change for Norg, the atomic C:N ratio, and d13Cleaf but was much smaller for Corg and d15Nleaf. Eelgrass plants had higher Norg concentrations, lower C:N ratios, and more depleted d13Cleaf 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 (eeelgrass-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 d15Nleaf reflected changes in the isotopic composition of the source of assimilated inorganic nitrogen. The strongly positive d15Nleaf values measured inside the fjord indicated an anthropogenic origin for inorganic nitrogen, which comes from riverine input (riverine d15N - NO3- = +11.8 ± 1.1).