The level of hydrodynamic drag as a main component in explaining concentrations of nutrients and particle loads at different heights of a fisheries trawl plume

Shelf sea ecosystems are amongst the most productive on Earth. A large proportion of the nutrients required to sustain this productivity is supplied via the remineralization of organic material in the underlying sediments. Industrial fisheries trawling at the seafloor, the most intense form of anthropogenic disturbance in shelf seas, has the potential to influence the release of sediment and nutrients from the seafloor and hence shelf-scale primary production. However, the absence of data on the concentrations and composition of the materials resuspended by benthic trawling hinders our ability to robustly assess the wider ecosystem effects of this industrial activity. We addressed this key knowledge gap by conducting the first in-situ experiments to quantify the concentrations of particles and nutrients released by different benthic trawling gears.

Our results demonstrate that the composition of resuspended particles and nutrients are both influenced by sampling height above the seafloor and the amount of drag exerted by the trawl gear, although the relative importance of these factors differs between the two response variables examined. These differences likely reflect that sediment particles adhere to physical laws of a solid in a fluid with associated weights, while dissolved nutrients follow the physical laws of fluids and dilution.  

Our findings demonstrate that trawl gear design strongly influences the amount of dissolved and particulate material resuspended. This suggests that gear design could be modified to reduce the impacts on shelf ecosystems. Future work will upscale our observations to assess the potential effects of benthic trawling activities on primary production at the shelf-scale.