Résumé
Mapping of the nutrient content and isotopic composition of the native, rooted, green macroalga Caulerpa prolifera and the seagrass Cymodocea nodosa was conducted in the summer of 2006 within the inner Bay of Cadiz, SW Spain. Isotope values of suspended solids from aquaculture (delta N-15 = 9 +/- 0.88) and urban effluent (delta N-15 = 7 +/- 1.97) were similar to the isotopic composition of the macrophytes (delta N-15 range 3.4 to 8.1), suggesting the influence of anthropogenic nutrient sources throughout much of the bay. The N content of tissues (C. prolifera 3.0 to 4.9%; C. nodosa 1.8 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(-2) d(-1)), appeared to determine the mass transfer of N to the benthos. Significantly lower values of C. nodosa N content and delta N-15 hint at the role strong gradients in environmental conditions may play in promoting the stable coexistence of seagrasses and C. prolifera within the bay. C. prolifera was 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.