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.
Highlights
Bottom trawling is used worldwide as a method of catching benthic or demersal fish and shellfish and even several decades ago was estimated to impact an area equivalent to 75% of the world’s continental shelf (Kaiser et al, 2002)
The parallel furrows left by trawl door tracks on soft sediments are easy to detect with acoustic mapping methods, e.g., side-scan sonar and multibeam, and such tracks can remain for many years (Krost et al, 1990; Oberle et al, 2018; Bunke et al, 2019)
9.5 t sediment, including tens to hundreds of kg of associated particulate elements such as Al, Fe, P, and Mn, were suspended per km of track; the sediment plume in the near-bottom water was transported more than 1 km away over the following 3–4 days
Summary
Bottom trawling is used worldwide as a method of catching benthic or demersal fish and shellfish and even several decades ago was estimated to impact an area equivalent to 75% of the world’s continental shelf (Kaiser et al, 2002). While effects on benthic community structure are welldescribed (Jennings and Kaiser, 1998; Kaiser et al, 2006; Hiddink et al, 2017; Rijnsdorp et al, 2018), there are fewer studies on the physical or biogeochemical impacts on the seabed, or on sediment suspension. Physical disturbance of the seabed can either lead to increased heterogeneity/roughness, e.g., in areas of low fishing intensity and on flat sedimentary bottoms, or decreased roughness, e.g., in areas of high fishing intensity and/or where seafloor structures are leveled (Martín et al, 2014). The parallel furrows left by trawl door tracks on soft sediments are easy to detect with acoustic mapping methods, e.g., side-scan sonar and multibeam, and such tracks can remain for many years (Krost et al, 1990; Oberle et al, 2018; Bunke et al, 2019)
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