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

Bradshaw, Clare and Jakobsson, Martin and Brüchert, Volker and Bonaglia, Stefano and Mörth, Carl-Magnus and Muchowski, Julia and Stranne, Christian and Sköld, Mattias (2021). Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor. Frontiers in marine science. 8 , 683331
[Research article]

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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.

Authors/Creators:Bradshaw, Clare and Jakobsson, Martin and Brüchert, Volker and Bonaglia, Stefano and Mörth, Carl-Magnus and Muchowski, Julia and Stranne, Christian and Sköld, Mattias
Title:Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor
Series Name/Journal:Frontiers in marine science
Year of publishing :2021
Volume:8
Article number:683331
Number of Pages:20
Associated Programs and Other Stakeholders:SLU - Environmental assessment > Programme Coastal and sea areas
ISSN:2296-7745
Language:English
Publication Type:Research article
Article category:Scientific peer reviewed
Version:Published version
Copyright:Creative Commons: Attribution 4.0
Full Text Status:Public
Subjects:(A) Swedish standard research categories 2011 > 1 Natural sciences > 105 Earth and Related Environmental Sciences > Oceanography, Hydrology, Water Resources
Keywords:otter trawl, sediment suspension, turbidity, biogeochemistry, disturbance, nutrients, oxygen, multibeam echo-sounding
URN:NBN:urn:nbn:se:slu:epsilon-p-114067
Permanent URL:
http://urn.kb.se/resolve?urn=urn:nbn:se:slu:epsilon-p-114067
Additional ID:
Type of IDID
DOI10.3389/fmars.2021.683331
Web of Science (WoS)000729031300001
ID Code:26676
Faculty:NJ - Fakulteten för naturresurser och jordbruksvetenskap
Department:(NL, NJ) > Department of Aquatic Resources
Deposited By: SLUpub Connector
Deposited On:10 Jan 2022 15:40
Metadata Last Modified:10 Jan 2022 15:41

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