Impact of benthic trawling on carbon mineralization in continental shelf sediments of the Bornholm Basin, southern Baltic Sea

Physical disturbance of the seafloor related to bottom trawling has led to widespread concern within the scientific community, as it may cause long-lasting impacts to benthic ecosystems, sediment composition, and biogeochemical cycling. Although trawling-induced effects on benthic habitats have been widely studied, the effects on sediment carbon mineralization and carbon stocks on the seabed remain unclear. Most of the catch obtained by bottom trawling is harvested from productive continental shelves, which play an important role in carbon sequestration and burial. Parts of the Baltic Sea have been trawled between one to ten times annually and thus provide a prime locality to study trawling related effects on benthic ecosystems and on the geochemical system. This study investigates the effects of trawling on carbon mineralization within the Bornholm Basin of the southern Baltic Sea, which has been consistently trawled until 2019. This was achieved through on-site sampling and analysis of porewater geochemistry, solid-phase sediment profiles, benthic fluxes and macrofaunal abundances sampled in the summer of 2023.

Comparisons between 4 study sites, paired into high-and low-trawled pairs based on fishing intensity data, revealed greater differences in measured physical and chemical properties than those between high- and low-trawled locality pairs, suggesting that environmental variability significantly influences carbon mineralization. However, high-trawled areas generally exhibited higher concentrations of TOC (total organic carbon) in their sediments as well as higher DIC (dissolved inorganic carbon) and nutrients in their porewaters. The higher porewater concentrations are likely a result of reduced advective transport and macrofaunal-related mixing, as a consequence of lower bioturbation and bioirrigation, whereby mineralization end-products (NH₄⁺, PO₄^3- and DIC) accumulate in porewaters due to slow upward diffusive transport to the sediment surface in comparison to low-trawled localities. Chlorophyll concentrations at the sediment surface from recently settled organic material from the spring phytoplankton bloom were higher at high-trawled localities and likely associated with the decreased macrofauna biomass and grazing activity. It is concluded that biogeochemical ecosystem functions in once-trawled sediment biotopes are only partially reconstituted after 4 years, with lasting differences in macrofaunal benthic community structure and bioturbation potential.