Anthropogenic Impact on Zostera noltei Seagrass Meadows (NW Iberian Peninsula) Assessed by Carbon and Nitrogen Stable Isotopic Signatures

Seagrass meadows provide valuable ecosystem services for human well-being. They are threatened by increasing human development on coastal areas, which results in eutrophication and ecosystem degradation. The negative effects of anthropogenic pressures on Zostera noltei meadows in NW Spain are unknown. This study aims to explore the relationship between watershed human development (i.e., the demographic temporal evolution and land cover indicators of human pressure) and the C and N isotopic signatures determined in Z. noltei seagrass meadows located in the related estuarine areas. We measured δ15N, δ13C, and the C:N content of sediment cores, Z. noltei leaves, and epiphytes collected from three seagrass meadows located at NW Iberian Peninsula characterized by well-differentiated watersheds in terms of the intensity of the anthropogenic pressures (Caldebarcos, Lourizán, and A Ramallosa). Ages and sedimentation rates were estimated by 210Pb/137Cs dating of one sediment core from the A Ramallosa seagrass meadow, corresponding to the most populated and urbanized watershed. Magnitudes of anthropogenic pressure on the watersheds were determined by the analysis of historic demographic data and the quantification of land cover changes obtained from CORINE Land Cover database. The intense anthropogenic transformation observed in the A Ramallosa watershed resulted in increases of sedimentation rates in the Z. noltei meadow. δ15N and δ13C signatures revealed the effects of anthropogenic nitrogen inputs. Sediment δ15N was the variable that best performed as an early warning eutrophication indicator, whereas δ15N in Z. noltei and epiphytic material were less coupled to the magnitudes of artificial land and population density on watersheds.