Effects of demersal trawling on ecosystem functioning in the North Sea: a modelling study

Demersal trawling causes chronic and widespread disturbance to the seabed in shallow shelf seas potentially leading to changes in function and trophic structure of benthic communities and with important implications for the processing of primary production and the wider functioning of the marine ecosystem. We used a coupled physical-ecological model (the European Regional Seas Ecosystem Model (ERSEM) with the General Ocean Turbulence Model (GOTM)) to investigate the impact of demersal trawling on the benthic and pelagic ecosystems of generic stratified and unstratified water columns in the central North Sea. Perturbation experiments were used to simulate trawling events using estimates of mortality of benthic fauna caused by different fishing gears in different habitats, derived from a meta-analysis of over 100 trawling disturbance experiments reported in the literature. The results suggest that the biogeochemical impact of demersal trawling is most significant in regions where gear type, trawl frequency and bed type cause high levels of filter feeder mortality. This results in substantially increased oxygen content of the benthic system and significant changes in its biogeochemistry (increased phosphorus absorption, increased nitrification of ammonia, reduced silicate cycling). The impacts of these changes on the overlying pelagic ecosystem are, however, buffered by the physical environment and the ability of phytoplankton to vary their internal cell nutrient contents. Analysis of recovery of the benthic system on complete cessation of demersal trawling suggests that the system will return to its original state within 5 yr, except in extreme cases where the deposit or filter feeder function is effectively removed, when a permanent change in the function of the benthic ecosystem may result.