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.
Highlights
Bottom trawling is among the most extensive forms of anthropogenic activities affecting marine ecosystems (Amoroso et al, 2018; Eigaard et al, 2017), and it is one of the most harmful in terms of fish stock overexploitation (Pauly et al, 2002), destruction of habitats (Kaiser et al, 2002; Simpson and Watling, 2006), and the physical impact it exerts on the sediments (Martín et al, 2014a; Oberle et al, 2018; Puig et al, 2012)
This study aims to reveal whether erosion prevails in bottom trawling grounds and what the consequent alterations on sedimentary organic matter are, by comparing sediment cores collected at a trawled and untrawled site in the Gulf of Castellammare
The results revealed that the biochemical composition of sedimentary organic matter (OM) differed between trawled and untrawled sites, but that these differences varied depending on the depth in the core
Summary
Bottom trawling is among the most extensive forms of anthropogenic activities affecting marine ecosystems (Amoroso et al, 2018; Eigaard et al, 2017), and it is one of the most harmful in terms of fish stock overexploitation (Pauly et al, 2002), destruction of habitats (Kaiser et al, 2002; Simpson and Watling, 2006), and the physical impact it exerts on the sediments (Martín et al, 2014a; Oberle et al, 2018; Puig et al, 2012). Trawling on cohesive sediments can increase superficial concentrations of sedimentary OM (Palanques et al, 2014; Pusceddu et al, 2005a; Sciberras et al, 2016; Polymenakou et al, 2005), whereas trawling on coarse non-cohesive sediments can exert null or minimal effects on OM contents and benthic community metabolism (Hale et al, 2017; Tiano et al, 2019; Trimmer et al, 2005) Most of these impacts have been documented in shallow environments, where sediment and OM fluxes are generally high and sediment resuspension and OM remineralization induced by bottom trawling can be comparable to those induced by natural high-energy events such as storms (Buscail et al, 1990; Dellapenna et al, 2006; Ferré et al, 2008; Pusceddu et al, 2005b). Since these natural physical disturbances are persistent, shallow benthic communities generally present higher resilience to the impacts of bottom trawling than communities that live in less disturbed areas, such as the deep sea (Kaiser, 1998)
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