Physical and electrical disturbance experiments uncover potential bottom fishing impacts on benthic ecosystem functioning

Tiano, Justin C.; Borger, Emil De; O'Flynn, Sarah; Cheng, Chiu H.; Oevelen, Dick van; Soetaert, Karline


Both physical and electrical impacts have been linked to North Sea fisheries activity. This study evaluates how these effects can influence marine ecological functioning by assessing their consequences on benthic pelagic coupling. Experiments were conducted on sediment microcosms taken from 9 North Sea and 2 Eastern Scheldt locations. Samples were subjected to physical disturbances by mechanically stirring the sediment surface or electrical stimulation with exposure to high frequency pulsed bipolar or direct currents. Electrical exposure times of 3 and 120-s were used to simulate in situ exposure times related to sole (Solea solea) and razor clam (Ensis spp.) electric fisheries respectively. Water column oxygen rapidly declined after sediment resuspension, inducing an immediate uptake ranging from 0.55 to 22 mmol oxygen per m−2 of sediment disturbed. Mechanical disturbances released the equivalent of up to 94 and 101 h of natural ammonium and silicate effluxes respectively. Fresh organic material significantly predicted the magnitude of mechanical-induced oxygen, ammonium, phosphate and silicate changes. No biogeochemical effects from bipolar (3 s or 120 s) or 3-s direct current exposures were detected. However, significant changes were induced by 120-s exposures to direct currents due to electrolysis and ionic drift. This lowered the water column pH by 1–1.3 units and caused the appearance of iron oxides on the sediment surface, resulting in the equivalent of 25–28 h of sedimentary phosphate removal. Our findings demonstrate that prolonged (+1 min) exposure to high frequency pulsed direct currents can cause electrochemical effects in the marine environment, with implications for phosphorus cycling. Nevertheless, bi-directional pulsed currents used in flatfish pulse trawling and AC waveforms featured in Ensis electrofishing, seem to severely limit these effects. Mechanical disturbance, on the other hand, causes a much greater effect on benthic pelagic coupling, the extent of which depends on sediment grain size, organic matter content, and the time of the year when the impact occurs.