Coastal habitats are important for global ocean production and provide important ecosystem services. The contribution of coastal ecosystem dominated by macrophytes to the global sequestration and burial of carbon may be important. The aim of this study is to determine the burial rate of C and N of characteristic habitats from Cadiz Bay and the sources of the organic matter being buried. The experimental design involved the analysis of key biogeochemical properties, like plant aboveground and belowground biomass, organic matter contents, C and N, and isotopic fractionation of C and N contents of the sediment, along a transect in the Trocadero Island saltmarshes, from a Zostera noltii dominated bed (S1) in the low saltmarsh, bare sediment (S2), an area of bare sediment with scattered Spartina maritima plants (S3), to a Spartina maritima dominated sediment in the high saltmarsh (S4). This transect represent a sea to land gradient in marine influence. These four habitats are characteristic of the Cadiz Bay Natural Park and of many other temperate saltmarshes. The horizontal heterogeneity in the biogeochemical characteristics within each habitat was high. The major differences in the biogeochemical characteristics of the sediment were related to the tidal height of each habitat, i.e., their position in the sea to land gradient. No significant differences were found in the content of organic matter between the different habitats. However, the content in carbonates was significantly lower in the most terrestrial habitat, S3 and S4, than in S1 and S2, being these two habitats more influenced by marine conditions. On the contrary, the organic C and total N content of the sediment tended to increase towards the land. In general, the vertical profiles of the biogeochemical properties did not show a clear trend with depth that might be due to intense mixing of the sediment surface. The analysis of δ13C and δ15N and the comparison with previous data suggest that the sediment organic matter seems to have multiple sources, although the organic matter derived from macroalgae and suspended particulate matter represented an important fraction. Our calculation indicates that between 73 - 123 g OM m-2 y-1 are buried in the inner bay, which represents organic C and total N burial rates of between 15.6 – 26.4 g C m-2 y-1, and 2.1 – 3.5 g N m-2 y-1, respectively. Thus, the total annual C and N burial rates for the inner bay, which has an area of 30 km2, of which the intertidal area is about 13 km2, are estimated to be about 630 t-C y-1 and 84 t-N y-1.This work was supported by the project of excellence “Funciones de la diversidad de macrófitos marinos bentónicos: mecanismos de control en el ciclo de nutrientes de zonas costeras someras” (FUNDIV, P07-RNM-02516) funded by the Junta de Andalucia and the European Union (FEDER) and by the National proyects “Interacciones entre las praderas de macrófitos marinos y la hidrodinámica: desde el organismo al ecosistema” (IMACHYDRO, CTM2008-0012/MAR) and “Microecología y cambios en los ciclos biogeoquímicos del carbono y el nitrógeno en sedimentos intermareales ocasionados por floraciones de macroalgas (MICROCHANGE)” (CTM2009-10736), both funded by the Spanish Ministry of Science and Technology and the European Union
