LAUSR

Abstract. Bottom trawling in the deep sea is one of the main drivers of sediment resuspension, eroding the deep seafloor and altering the content and composition of sedimentary organic matter (OM). The physical and biogeochemical impacts of bottom trawling on the seafloor were studied in the continental slope of the Gulf of Castellammare, Sicily (Southwestern Mediterranean) through the analysis of two triplicate sediment cores collected in trawled and untrawled sites (~ 550 m water depth) during the summer of 2016. Geochemical and sedimentological parameters (excess 210Pb, excess 234Th, 137Cs, dry bulk density, and grain size), elemental (organic carbon and nitrogen) and biochemical composition of sedimentary OM (proteins, carbohydrates, lipids), as well as its freshness (phytopigments) and degradation rates were determined in both coring locations. The untrawled site had a sedimentation rate of 0.15 cm yr−1 and presented a 6-cm thick surface mixed layer that contained coarser sediment with low excess 210Pb concentrations, possibly resulting from the resuspension, posterior advection, and eventual deposition of siltier and older sediment from adjacent trawling grounds. In contrast, the trawled site was characterized by highly eroded and compacted century-old sediment, as shown by the lack of excess 210Pb and high dry bulk densities. The continuous erosion in the trawled site has led to the depletion of OM, which were between 20 % and 60 % lower than those in the untrawled site, as well as to statistically significant differences in the biochemical composition of OM. Nevertheless, the upper 2 cm of the trawled site consisted of recently accumulated sediments, enriched in excess 234Th, excess 210Pb, and phytopigments, which had similar OM contents to surface sediments from the untrawled core. The arrival of fresh sediment in a chronically-trawled deep-sea site that is generally deprived of OM was associated with an enhancement of remineralization rates, reflected by protein turnover rates of 0.025 d−1, which doubled the rates quantified in surface sediments of the untrawled site. We conclude that the detrimental effects of bottom trawling can be temporarily and partially abated by the arrival of fresh and nutritionally-rich OM, which stimulate the response of benthic communities. However, these ephemeral deposits are likely to be swiftly eroded due to the high trawling frequency over fishing grounds, highlighting the importance of establishing management strategies to mitigate the impacts of bottom trawling.