Abstract
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.
Highlights
Research on functional ecosystem dynamics help link together structural components in marine ecology (Yvon-Durocher and Allen, 2012; Bell, 2019)
A major concern of electrofisheries impacts are the potential biogeochemical effects imposed by these types of fisheries (Soetaert et al, 2015) and how this can affect functional ecosystem dynamics such as the transfer of organic matter, nutrients and energy
Our results regarding the inhibi tion/limitation of biogeochemical impacts when using bipolar currents can be directly applicable to any electro fishing technique
Summary
Research on functional ecosystem dynamics help link together structural components in marine ecology (Yvon-Durocher and Allen, 2012; Bell, 2019). The cycling of nutrients in the seafloor is the sum of biological, chemical and physical processes, the results of which can alter oxygen and primary production in the water column (Fisher et al, 1982; Murphy et al, 2000) thereby affecting the func tioning (transfer of energy, nutrient regeneration/sequestration etc.) of marine ecosystems. The sedimentary release and/or removal of nutrients, such as nitrogen and phosphorus, help control pelagic primary production and influence marine ecosystems through bottom up effects (Slomp et al, 1996; Soetaert and Middelburg, 2009). The removal of nutrients by sediments buffers marine systems against the formation of low oxygen zones, which often result from nutrient overloading com bined with water column stratification (Weston et al, 2008; van der Molen et al, 2013). Bottom disturbance created by fisheries, for example, can change benthic nutrient concentrations (van de Velde et al, 2018; Tiano et al, 2019)
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