Evaluating bottom trawling effects on North Sea productivity

Bottom trawling impacts benthic-pelagic coupling and biogeochemical cycling by resuspending particulate organic matter (POM) through the action of bottom-contacting gear. Despite widespread trawling in many shelf seas, its effects on benthic-pelagic exchanges are often overlooked in ecosystem analyses and simulations. This study evaluates the impact of bottom trawling-induced sediment resuspension (BTISR) on North Sea primary and secondary productivity for 2000–2005 with the coupled 3D physical-biogeochemical model SCHISM-ECOSMO. The first simulation includes only natural resuspension by currents, while the second and third additionally consider BTISR. Since the trawling effect on resuspension of biogenic sedimentary carbon is poorly understood, we propose two different parameterization methods. For both, daily BTISR rates were based on fishing activity data, including vessel position, size, and engine power, combined with resuspension rates for fishing gears and sediment types. We found the lower trophic level response to BTISR strongly depends on parameterization. One method (bulk mass resuspension) suggests an 8.0% NPP increase; the other (swept area ratio) suggests a −0.04% change. Both methods showed spatial heterogeneity in NPP impact and a gradient between the deep, trawled Norwegian Trench and Skagerrak, and the shallow southern North Sea. Large-scale resuspension reduced production in the shallow south due to lower light, while NPP increased in the Norwegian Trench and Skagerrak due to enhanced nutrients. Spatial pattern differences at Dogger Bank stemmed from frontal dynamics, advection, and phenology. Our results show the swept area ratio method aligns better with observations than the bulk mass approach, supporting its use in future applications. Resuspension by bottom trawling can influence ecosystem productivity in heavily trawled areas, but quantifying resuspended POM remains highly uncertain.