Résumé
Mapping of the nutrient content and isotopic composition of the native, rooted, green macroalgaCaulerpa proliferaand the seagrassCymodocea nodosawas conducted in the summer of 2006 within the inner Bay of Cádiz , SW Spain. Isotope values of suspended solids from aquaculture (δ15N = 9 ± 0.88) and urban effluent (δ15N = 7 ± 1.97) were similar to the isotopic composition of the macrophytes (δ15N range 3.4 to 8.1), suggesting the influence of anthropogenic nutrient sources throughout much of the bay. The N content of tissues (C. prolifera3.0 to 4.9%;C. nodosa1.8 to 3.0%) was highly associated with their isotopic composition, indicating that nutrient supply strongly influenced plant stoichiometry. Distance from the source, i.e. variation in water column nutrient concentration, which was also highly correlated with median current velocities (0.03 to 0.18 m s–1) and to a lesser extent benthic light dose (1.8 to 10.7 mol quanta m–2d–1), appeared to determine the mass transfer of N to the benthos. Significantly lower values ofC. nodosaN content and δ15N hint at the role strong gradients in environmental conditions may play in promoting the stable coexistence of seagrasses andC. proliferawithin the bay.C. proliferawas a useful indicator species that, via the analysis of its tissue composition, can provide a highly effective tool to aid in the management of nutrient inputs within shallow, coastal zones.