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

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, bidirectional 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.

Sediment resuspension enhances nutrient exchange in intertidal mudflats

“…In contrast, sediment erosion decreased the concentration of phosphate in the water column of ES sites, therefore highlighting sediment resuspension as a possibly important process for PO 4 3− removal in this system. A decrease in phosphate with erosion has also been observed in North Sea sediments (Tiano et al, 2021), the Baltic Sea (Tengberg et al, 2003;Niemistö et al, 2018) and Great Bay estuary, US (Percuoco et al, 2015).…”

“…This exchange is generally caused by passive diffusion, bioturbation and bioirrigation (Aller, 1982;Braeckman et al, 2010;De Borger et al, 2020), or sediment resuspension from wave and tidal action (Cabrita et al, 1999;Tengberg et al, 2003;Kalnejais et al, 2010). It can also be triggered by human activities such as dredging and bottom trawling (Morin and Morse, 1999;Dounas et al, 2007;Tiano et al, 2021).…”

“…During a resuspension event, nutrients and dissolved OM in the water column can absorb onto resuspended sediment particles and decrease their concentration in the water column (Tengberg et al, 2003;Tiano et al, 2019). Alternatively, pore water nutrients, whose concentration in the sediment is typically higher than in the water column, may be released into the overlying water at the equivalent of several days of undisturbed fluxes (sum of diffusive and fauna-induced fluxes) (Tiano et al, 2021). An erosion event can also release OM previously stored in sediments resulting in an increase in bacterial mineralization in the water column and higher utilization of dissolved oxygen (DO) (Tengberg et al, 2003).…”

Biogeochemical consequences of a changing Arctic shelf seafloor ecosystem

“…In low sedimentation rate areas, trawling might have less of an effect on long-term carbon storage. Similar considerations can be made for nutrient recycling to the water column by the mechanical disturbance of sediments (Duplisea et al 2001 ): if the disturbance reaches anaerobic layers where nutrient concentrations are significantly higher than in the overlying waters, the resulting enhanced nutrient fluxes can fuel additional pelagic primary production (Dounas et al 2007 ; van der Velde et al 2018 ; Tiano et al 2021 ). Finally, the persistence of any trawling-induced disturbance in the Barents Sea would depend on type and frequency of trawling as well as primary productivity and sedimentation rates, but literature-based estimates range from several year to several decades (Buhl-Mortensen et al 2016 ; Paradis et al 2021 ).…”

The impact of mobile demersal fishing on carbon storage in seabed sediments

“…As mobile demersal fishing generally exposes or suspends fine material, this may reduce overall OC storage through loss of OC–mineral interactions and remineralization (Figure 1c ) (Arnarson & Keil, 2007 ; Estes et al, 2019 ). However, if disturbed OC associated with fine sediments is largely refractory in nature and is not remineralized but reallocated to surface sediments in situ or transported laterally to different locations, this disturbance may increase seabed surface OC in certain environmental settings (Figure 1c ) (LaRowe et al, 2020 ; Oberle, Swarzenski, et al, 2016 ; Palanques et al, 2014 ; Tiano et al, 2021 ).…”