Organic matter contents and degradation in a highly trawled area during fresh particle inputs (Gulf of Castellammare, southwestern Mediterranean)

Abstract

Abstract. Bottom trawling in the deep sea is one of the main drivers of sediment resuspension, eroding the seafloor and altering the content and composition of sedimentary organic matter (OM). The physical and biogeochemical impacts of bottom trawling were studied on the continental slope of the Gulf of Castellammare, Sicily (southwestern Mediterranean), through the analysis of two triplicate sediment cores collected at 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 siltier sediment with low excess 210Pb concentrations, possibly resulting from the resuspension, posterior advection, and eventual deposition of coarser and older sediment from adjacent trawling grounds. In contrast, the trawled site was eroded and presented compacted century-old sediment highly depleted in OM components, which were between 20 % and 60 % lower than those in the untrawled site. However, the upper 2 cm of the trawled site consisted of recently accumulated sediments enriched in excess 234Th, excess 210Pb, and phytopigments, while OM contents were similar to those from the untrawled core. This fresh sediment supported protein turnover rates of 0.025 d−1, which doubled those quantified in surface sediments of the untrawled site. The enhancement of remineralization rates in surface sediment of the trawled site was associated with the arrival of fresh particles on a chronically trawled deep-sea region that is generally deprived of OM. 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 science-based management strategies to mitigate the impacts of bottom trawling.

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

“…observed decreases in sedimentary chl a in the upper 1 cm immediately after trawling, of 41 and 83 % for pulse and tickler gears respectively. Also, OM depletion as a result of long term fishing has been reported, even at water depths beyond 500 m (Martín et al, 2014;Paradis et al, 2019), where comparisons between trawled and untrawled sites yielded a difference in OC between 20 and 60 % (Paradis et al, 2019). Our results contrast with studies that found enhanced OC concentrations in trawled areas (Palanques et al, 2014;Pusceddu et al, 2005;Sciberras et al, 2016), possibly due to differing hydrodynamic and morphological conditions of the North Sea compared to other areas.…”

“…For this, the site-specific conditions such as rates of biogeochemical recovery and sedimentation rates would need to be investigated to fine-tune management (Paradis et al, 2019). Alternatively, areas with high denitrification rates, crucial for eutrophication mitigation can be closed to trawling completely, as done in other regions of the world (Ferguson et al, 2020).…”

“…Consistent fishing pressure favors organisms with shorter life spans and/or increasing resistance to trawling, while communities become depleted of species with key functional roles (Kaiser et al, 2006;Hiddink et al, 2017;Sciberras et al, 2018). Both fining (Trimmer et al, 2005) and coarsening (Palanques et al, 2014;Mengual et al, 2016) of the sediment has been attributed to trawling, as well as chronic organic matter depletion (Pusceddu et al, 2014;Paradis et al, 2019). The effects of bottom trawling on biogeochemical dynamics, however, remain relatively understudied.…”

Evidences of human impact on megabenthic assemblages of bathyal sediments in the Alboran Sea (western Mediterranean)

“…Impacts associated to bottom trawling include removal or damage to epibenthic fauna, reduction of habitat complexity and alteration of sedimentary and geochemical processes (e.g. De Leo et al, 2017;Hiddink et al, 2017;Paradis et al, 2019). With bottom trawling progressively expanding to deeper environments, VMEs associated to sedimentary bottoms have undergone a vast decimation (Sardà et al, 2004;Gerovasileiou et al, 2019;Pusceddu et al, 2014), restricting their distribution to isolated regions or areas inaccessible to trawlers, such J o u r n a l P r e -p r o o f as sub-vertical walls or regions surrounded by rocky outcrops (Bo et al, 2015;Pierdomenico et al, 2018).…”

Organic carbon in surface sediments of the North Sea and Skagerrak

“…Several studies show lower OC contents in surface sediments of trawled areas (Bhagirathan et al, 2010;Martín et al, 2014b;Paradis et al, 2019;Pusceddu et al, 2014), while others report higher OC contents, presumably due to fertilization brought about by resuspension or uplifting of OC from deeper layers caused by trawling (Palanques et al, 2014;Pusceddu et al, 2005). In the short term, demersal fishing-induced sediment disturbance stimulates OC mineralisation in cohesive sediments, likely due to the enhanced decomposition of previously buried refractory OC (van de Velde et al, 2018).…”