Natural and anthropogenic influences on modern salt-marsh foraminifera across Wadden Sea Islands

Salt-marsh foraminifera are widely used as indicators of environmental gradients and relative sea level, yet their spatial consistency across contrasting coastal settings in the Wadden Sea remains uncertain. Therefore, we examined live foraminiferal populations and dead assemblages across five German-Danish Wadden Sea islands (Scharhörn, Nigehörn, Neuwerk, Rømø, and Mandø), representing salt marshes with differing histories and degrees of coastal management, including less modified sites. Along 12 transects, we analyzed vegetation composition and surface-sediment properties, including grain size, organic matter (OM) content, soil pH, and soil salinity, as well as foraminiferal fauna composition. Live foraminiferal densities were highest in less intensively managed salt marshes with heterogenous, mosaic-like vegetation (e.g., Neuwerk) and lowest in areas dominated by monospecific plant communities that are subject to more intensive management (e.g., Mandø). Entzia macrescens was the dominant species among the live foraminifera, although its abundance declined in late-successional vegetation characterized by extensive Elymus athericus stands. Non-metric multidimensional scaling (NMDS) revealed significant relationships with environmental variables, particularly OM content and soil pH in live populations and dead assemblages. Cluster analysis did not indicate a clear vertical zonation of dead assemblages along the tidal-elevation gradient. Instead, clusters were structured mainly by the dominance of agglutinated or calcareous taxa, thereby comprising samples from different vegetation zones. This suggests limitations in the applicability of modern foraminiferal assemblages as proxies for reconstructing relative sea-level changes in the Wadden Sea. Our results further indicate that storm-surge related sediment reworking, salt-marsh management, and site-specific diagenetic processes have impaired natural foraminiferal assemblage patterns.