Anthropogenic nitrogen impacts in littoral ecosystems: Quantification with N stable isotopes using long-living macroalgae

Abstract

Coastal areas, particularly estuaries, have become the final deposits of contaminants. The utilization of the ratio of N stable isotopes (δ15N) is a useful tool to trace nutrient sources and for studying the potential risk of eutrophication of the coastal marine systems. The use of the δ15N to trace anthropogenic loadings is based on the fact that different anthropogenic sources of N alter the baseline levels of 15N in marine biota. It is generally agreed that δ15N of brown macroalgae can discriminate between anthropogenic and natural sources of nitrogen. Despite its widespread use for monitoring studies, the influence of intrinsic or external factors on the variability of the isotopic values is poorly understood. Two fucoid species were selected, F. vesiculosus and A. nodosum, to study these factors and the variability of N stable isotopes in these species. The variability of N stable isotopes in these macroalgae was studied at both regional and local scales. At regional scale, the natural and anthropogenic variability of N stable isotopes in macroalgae was analyzed along a biogeographic gradient in an upwelling area (NW Spain) to determine the influence of both sources. And at local scale, the spatial and temporal sources of variability were determined along a salinity gradient. The use of these species is based on their apical growth. The growing tip is the part of the frond that has been traditionally considered for monitoring studies, but they have also been used as retrospective biomonitors. The growing period that reflects a segment, together with the uptake capacities need to be tested to understand the isotopic values in their tissues. The growth rates of these intertidal macroalgae were determined along with other features, as the appearance of the gas bladders or bifurcations, which could also help to determine the growth rates. The equilibration of the isotopic values of the different parts of the thallus, together with the nitrate uptake capacity and the potential existence of N transport along the thallus were determined. As nitrogen is an essential element for macroalgae, the present dissertation addresses the understanding of the variability of N sources and it impact on macroalgae at different spatial and temporal scales, thus providing a template for a better exploitation of stable isotopic values in monitoring studies.