Stable nitrogen isotopes in coastal macroalgae: Geographic and anthropogenic variability Original Research Article

Growing human population adds to the natural nitrogen loads to coastal waters. Both anthropogenic and natural nitrogen is readily incorporated in new biomass, and these different nitrogen sources may be traced by the measurement of the ratio of stable nitrogen isotopes (δ15N). In this study δ15N was determined in two species of macroalgae (Ascophyllum nodosum and Fucus vesiculosus), and in nitrate and ammonium to determine the relative importance of anthropogenic versus natural sources of nitrogen along the coast of NW Spain. Both algal species and nitrogen sources showed similar isotopic enrichment for a given site, but algal δ15N was not related to either inorganic nitrogen concentrations or δ15N in the water samples. The latter suggests that inorganic nitrogen inputs are variable and do not always leave an isotopic trace in macroalgae. However, a significant linear decrease in macroalgal δ15N along the coast is consistent with the differential effect of upwelling. Besides this geographic variability, the influence of anthropogenic nitrogen sources is evidenced by higher δ15N in macroalgae from rias and estuaries compared to those from open coastal areas and in areas with more than 15 × 103 inhabitants in the watershed. These results indicate that, in contrast with other studies, macroalgal δ15N is not simply related to either inorganic nitrogen concentrations or human population size but depends on other factors as the upwelling or the efficiency of local waste treatment systems.