A Multi-isotope Hydrobiogeochemical Characterization of a Subterranean Estuary Before and After Changes in Coastal Protection

Abstract The coastal area, the interface between the terrestrial and marine environment, is a dynamic critical zone influenced by hydrological, biogeochemical, and physical processes. This study aimed to reveal the influence of the change in coastal protection (groyne removal) on the composition of submarine groundwater in front of a coastal peatland at the southern Baltic Sea. Multi-stable isotopes (H, C, O, S) and the concentrations of major and trace ions were followed using two 5-m-long permanent pore water lances between years 2022 and 2024. Results from the two sites showed strong spatial and vertical differences in the hydrobiogeochemical characteristics of the subterranean estuary. This is caused by the variability in mixing of seawater with land-derived water, high in dissolved organic matter and solute concentrations. High concentrations of redox-sensitive species (Mn 2+ , Fe 2+ , and H 2 S) in the pore water suggest high in situ diagenesis. The concentration profiles and isotope values revealed the consistent existence of a sulfate-methane transition zone at the upper 3 m depth at one site. Substantial accumulations of dissolved elements are observed below. The submarine groundwater compositions differ from those of peatland surface and groundwater and extrapolated freshwater components. The pore water biogeochemistry changed due to sediment ventilation in response to the groyne removal, but also to storm surges and Baltic inflows, which had a more pronounced impact. This resulted in enhanced microbial sulfate reduction and changes in concentrations of redox-sensitive elements. We conclude that the removal of coastal protection structures caused disruptions in the biogeochemical processes, but effects are less important than those of storm surges and Baltic inflows.