Bladder wrack (Fucus vesiculosus) as a multi-isotope bio-monitor in an urbanized fjord of the western Baltic Sea

The spatial variations in the elemental and stable carbon, nitrogen, and sulphur isotope composition of bladder wrack (Fucus vesiculosus) growing along the shore line of the semi-enclosed urbanized Kiel Fjord (western Baltic Sea) was investigated at more than 60 sites. The analyses of the carbon-nitrogen-sulphur (CNS) stoichiometry and C and N stable isotope signature of F. vesiculosus displayed substantial differences between the north-western and the south-eastern parts of the Kiel Fjord. Different size classes displayed in part differences in C:N and C:S ratios, and the carbon isotope composition, reflecting the impact of the boundary conditions during growth. Whereas the sulphur isotope composition was controlled by the assimilation of seawater sulphate, the carbon isotope composition reflected the difference in the composition of surface waters. The δ¹⁵N values of the organic tissue tend to be an integrated monitor of anthropogenic impacts on the fjord. Results are compared to the composition of surface waters.