Bottom-trawling signals lost in sediment: A combined biogeochemical and modeling approach to early diagenesis in a perturbed coastal area of the southern Baltic Sea

Trawl-fishing is broadly considered to be one of the most destructive anthropogenic activities toward benthic ecosystems. In this study, we examine the effects of bottom-contact fishing by otter trawls on the geochemistry and macrofauna in sandy silt sediment in an area of the Baltic Sea where clear spatial patterns in trawling activity were previously identified by acoustic mapping. We calibrated an early diagenetic model to biogeochemical data from various coring locations. Fitting measured mercury profiles allowed for the determination of the sediment mixing and burial velocity. For all sites, independent of the trawl mark density, good fits were obtained by applying the model with the same organic matter loading and parameter values, while iron fluxes scaled linearly with the burial velocity. A sensitivity analysis revealed that the fitted sulfate reduction rate, solid sulfur contents, ammonium concentration, and both the isotopic composition and concentration of dissolved inorganic carbon provided reliable constraints for the total mineralization rate, which exhibited a narrow range of variability (around +/- 20 % from the mean) across the sites. Also, the trawling intensity did not significantly correlate with total organic carbon contents in surficial sediment, indicating limited loss of organic matter due to trawling. The fits to the reactive iron, acid volatile sulfur, chromium(II) reducible sulfur contents, and porewater composition demonstrate that sediment burial and mixing primarily determine the redox stratification. The mixing depth did not correlate with trawling intensity and is more likely the result of bioturbation, as the analyzed macrofaunal taxonomy and density showed a high potential for sediment reworking. The extraordinarily long-lived Arctica islandica bivalve dominated the infaunal biomass, despite the expectation that trawling leads to the succession from longer-lived to shorter-lived and bigger to smaller macrofauna. Our results further suggest that a clear geochemical footprint of bottom-trawling may not develop in sediments actively reworked by tenacious macrofauna.