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Research article - Peer-reviewed, 2021

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

Bradshaw, Clare; Jakobsson, Martin; Brüchert, Volker; Bonaglia, Stefano; Mörth, Carl-Magnus; Muchowski, Julia; Stranne, Christian; Sköld, Mattias

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
L1), 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.

Keywords

otter trawl; sediment suspension; turbidity; biogeochemistry; disturbance; nutrients; oxygen; multibeam echo-sounding

Published in

Frontiers in marine science
2021, volume: 8, article number: 683331

Authors' information

Bradshaw, Clare
Stockholm University
Jakobsson, Martin
Stockholm University
Brüchert, Volker
Stockholm University
Bonaglia, Stefano
Stockholm University
Mörth, Carl-Magnus
Stockholm University
Muchowski, Julia
Stockholm University
Stranne, Christian
Stockholm University
Swedish University of Agricultural Sciences, Department of Aquatic Resources

Associated SLU-program

Coastal and sea areas

Sustainable Development Goals

SDG14 Life below water

UKÄ Subject classification

Oceanography, Hydrology, Water Resources

Publication Identifiers

DOI: https://doi.org/10.3389/fmars.2021.683331

URI (permanent link to this page)

https://res.slu.se/id/publ/114067