Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor

Bottom trawling is known to affect benthic faunal communities but its effects onsediment suspension and seabed biogeochemistry are less well described. In addition,few studies have been carried out in the Baltic Sea, despite decades of trawling inthis unique brackish environment and the frequent occurrence of trawling in areaswhere hypoxia and low and variable salinity already act as ecosystem stressors. Wemeasured the physical and biogeochemical impacts of an otter trawl on a muddyBaltic seabed. Multibeam bathymetry revealed a 36 m-wide trawl track, comprisingparallel furrows and sediment piles caused by the trawl doors and shallower groovesfrom the groundgear, that displaced 1,000 m3 (500 t) sediment and suspended 9.5 tsediment per km of track. The trawl doors had less effect than the rest of the gearin terms of total sediment mass but per m2 the doors had 5× the displacement and2× the suspension effect, due to their greater penetration and hydrodynamic drag.The suspended sediment spread >1 km away over the following 3–4 days, creatinga 5–10 m thick layer of turbid bottom water. Turbidity reached 4.3 NTU (7 mgDWL−1), 550 m from the track, 20 h post-trawling. Particulate Al, Ti, Fe, P, and Mnwere correlated with the spatio-temporal pattern of suspension. There was a pulse ofdissolved N, P, and Mn to a height of 10 m above the seabed within a few hundredmeters of the track, 2 h post-trawling. Dissolved methane concentrations were elevatedin the water for at least 20 h. Sediment biogeochemistry in the door track was stillperturbed after 48 h, with a decreased oxygen penetration depth and nutrient andoxygen fluxes across the sediment-water interface. These results clearly show thephysical effects of bottom trawling, both on seabed topography (on the scale of kmand years) and on sediment and particle suspension (on the scale of km and days-weeks). Alterations to biogeochemical processes suggest that, where bottom trawling isfrequent, sediment biogeochemistry may not have time to recover between disturbanceevents and elevated turbidity may persist, even outside the trawled area.