Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea†

Daniel E Duplisea,Stephen J Malcolm,Simon Jennings,Ruth Parker,David B Sivyer

doi:10.1186/1467-4866-2-112

Daniel E Duplisea, Stephen J Malcolm + Show 3 more

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https://doi.org/10.1186/1467-4866-2-112

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Abstract

Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low. In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems, where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance could cause large fluctuations in benthic chemical fluxes and storage.

Highlights

Bottom trawling is a key source of physical disturbance in shallow shelf seas, and trawling is well known to affect the diversity, community structure, size composition and production of benthic invertebrate communities.[1,2,3,4,5,6] little is known of the effects of trawling disturbance on processes in the marine ecosystem, despite the expectation that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling disturbance
Our preliminary assessment of the effects of trawling disturbance on carbon mineralisation and chemical concentrations in a soft sediment system implies that trawling can have profound effects on functional processes in benthic systems
The model has been parameterised for a soft sediment system, which is typical of beam trawled areas in the deeper parts of the central North Sea

Summary

Introduction

Bottom trawling is a key source of physical disturbance in shallow shelf seas, and trawling is well known to affect the diversity, community structure, size composition and production of benthic invertebrate communities.[1,2,3,4,5,6] little is known of the effects of trawling disturbance on processes in the marine ecosystem, despite the expectation that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling disturbance. Biogeochemical effects of trawling disturbance are expected because (i) trawling reduces the abundance of bioturbating macrofauna that play a key role in biogeochemical processes and (ii) because the physical mixing by trawling may be likened to bioturbation by macrofauna, and yet trawling, unlike the macrofauna, does not directly contribute to community metabolism.[7]. Each trawl weighs about 8000 kg (in air) and is towed at about 11 km h21.3 The beam trawls are rigged with various ground gears, usually consisting of heavy chain mats or ‘tickler’ chains, that are designed to exclude rocks from the gear and to disturb and fluidise the upper layers of sediment and drive flatfish from the seabed and into the net.[3]

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