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

Abstract

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

Quantifying the Physical Impact of Bottom Trawling Based on High-Resolution Bathymetric Data

“…The incision depth of trawl marks in the Fehmarn Belt is in line with other studies conducted in the Baltic Sea [31] or in comparable settings (e.g., [32]). The calculation of sediment displacement in small (10 m × 10 m) tiles as conducted in this study does not require knowledge of individual trawl marks and thus can be rapidly applied over larger areas of interest.…”

“…Figure 6 represents the cumulative physical impact of trawling over several years. Trawl marks in cohesive sediment below the wave base and in areas of low sedimentation rates may be preserved for several years [32]. Therefore, the sediment displacement within the trawl marks can be thought of as a long-term impact that-in the Baltic Sea basins-will disappear only over longer time periods.…”

Linking Species Functional Traits to Specific Biogeochemical Processes under Trawling Pressure

“…Sediment resuspension and sediment-water nutrient exchanges may also be strongly influenced by the mechanical effects of trawling [ 15 , 16 , 17 , 18 , 19 , 20 ]. The effects of trawling on biogeochemistry depend on sediment characteristics, with stronger impacts occurring on muddy sediments compared to sand [ 17 , 21 ].…”

Biogenic silica cycling in the Skagerrak

“…The unusually intense and deep mixing rates required to simulate the data at MUC1 and MUC3 compared to other sites could be attributed to bottom trawling, which is frequent in the MUC1 region at water depths<200 m (Sköld et al, 2018). Piles of sediment alongside furrows created by dragging trawl doors through the sediment can reach 1 to 2 dm in height (Bradshaw et al, 2021) and it may be that sediment cores taken at stations MUC1 and MUC3 penetrated such sediment piles during sampling. We find this explanation more convincing than deep bioturbation since no visual evidence of macrofauna was observed below ca.…”