Spatial and Temporal Patterns of Benthic Diatom Communities in a Coastal Peatland with Limited Connection to the Baltic Sea

The southern Baltic Sea coastline is a dynamic system, often bordered by coastal peatlands. Artificial dunes and dykes commonly separate these peatlands from the sea. Given their potential as carbon sinks and natural buffer zones against sea-level rise and flooding, interest in their restoration is increasing. Microphytobenthos, particularly diatoms, are key primary producers in coastal and peatland environments and serve as valuable bioindicators. While their use as bioindicators has focused on freshwater systems, their application in coastal contexts also holds promise. In contrast to their freshwater counterparts, the ecology, taxonomy and diversity of benthic diatoms along the southern Baltic Sea coast, particularly in coastal peatlands, remain poorly understood. Despite being separated from the Baltic Sea by a dune dyke, the coastal peatland in the nature reserve “Heiligensee und Hütelmoor” is periodically exposed to saltwater inflow during storm surges. Following a storm surge in January 2019, brackish water entered the peatland, providing a rare opportunity to study a coastal peatland with periodic salt water input. Over one year, diatom community compositions, biomass, and key environmental variables were monitored at two peatland sites and an adjacent marine site to assess spatial and temporal patterns along a marine–terrestrial gradient. Overall, 198 diatom taxa were found and the share between the Baltic Sea and the peatland was only 4% despite the recent saltwater intrusion. Diatom community compositions showed distinct seasonal and spatial variation within the peatland area. The main drivers of community differences in the dataset were salinity, temperature, pH and nitrogen-based nutrients.