The Effect of Different Types of Otter Trawl Ground Rope on Benthic Nutrient Releases and Sediment Biogeochemistry
Costas G. Dounas, Ian M. Davies, Peter J. Hayes, Christos D. Arvanitidis, and Panayota T. Koulouri
Abstract. The effects of seabed disturbance by otter trawling on the rate of nutrient regeneration from the sediment to the overlying water column and on the biogeochemical sediment zonation have never been studied directly. These effects may have important implications for nutrient supply to the pelagic ecosystem, on primary production over the continental shelf where most trawling activity is concentrated, and, consequently, on fish production and fisheries management. The labile fractions of sedimentary organic matter, the main source of energy to benthic environments, are usually concentrated on or near the sediment surface. Consequently, the most important impacts of trawling on sediment biogeochemistry arise from the artificial disturbance of the sediment surface layer. The trawl ground rope contributes most to this process and accounts for more than 90% of the total contact area of the trawl with the seabed in some cases. Therefore, a range of ground ropes was selected for trawling simulation experiments. The study site was the continental shelf of Heraklion Bay (Eastern Mediterranean, Cretan Sea). Nutrient releases induced by ground ropes were measured via a sampling schedule that included six deployments of a towed trawl simulator sledge. Five different ground ropes, ranging from 6.5 to 9.5 cm in diameter and 2–6 kg/m weight (in water), were used in order to estimate potential quantitative differences in their effects on nutrient releases arising from the contact of the ground rope with the muddy seabed. Average net releases of particulate organic carbon and total solids disturbed by the two heavier ground ropes were significantly greater than those from the other three, lighter ground ropes. Releases of phosphate, inorganic nitrogen compounds, chlorophyll a, and chloroplastic pigment equivalent showed no significant differences between ground ropes. The results indicate that almost all the biologically active compounds at the sediment surface are resuspended by a single passage of the simulating gear. These observations also imply that the upper, extremely thin layer of sediments contains a considerable reservoir of dissolved and particulate nutrients in concentrations that are much higher than in the immediately underlying surface sediment layers.