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

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.

Evaluating the historical sedimentation patterns in two different Mediterranean deep environments (Sardinia and Sicily Channels)

The sediment accumulation rate in the Sardinia and Sicily channels in the central part of the Mediterranean Sea was studied by using short-lived radionuclides (210Pb and 137Cs) in two deep sediment cores. Different sedimentation regimes were identified indicating substantial differences in accumulation rates and historical patterns. The 210Pb-derived mean accumulation rate found in the Strait of Sardinia was 0.05 g.cm-2.y-1, lower than that in Sicily Channel (0.1 g.cm-2.y-1) suggesting an inverse correlation with water depth. Excess 210Pb inventories were 24 ± 1 and 6.0 ± 0.4 kBq.m-2, while the fluxes to the sediment were 745 ± 31 and 188 ± 11 Bq.m-2.y-1 in Sicily and Sardinia channels, respectively. 137Cs failed to use for the validation of the established chronologies, while its inventories found 450 Bq.m-2 and 355 Bq.m-2 in the Sicily and Sardinia channel, respectively.

Effects of the arrival of fresh organic matter on eroded and nutrient-depleted trawling grounds (Gulf of Castellammare, SW Mediterranean)

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.

Sediment geochronology and spatio-temporal and vertical distributions of radionuclides in the Upper Bonny Estuary (South Nigeria)

Surface deposits and sediment cores were collected from the Upper Bonny Estuary, located in Southwest Nigeria, and analyzed to determine spatio-temporal and vertical distributions of radio-nuclide activities expressed in Bq·kg−1 dry weight. The results of activities of naturally occurring radionuclides of 226Ra (15 ± 2–34 ± 3 Bq·kg−1), 228Ra (32 ± 5–48 ± 6 Bq·kg−1), 40K (264 ± 29–462 ± 36 Bq·kg−1) were found to be all within the range of typical values reported for coastal regions. Ratios of 226Ra to 228Ra suggested accretion for all samples with low sediment accumulation registered during rainy months. In addition, vertical distributions at the three sampling sites were also studied with the initial aim of establishing chronologies from the decay of excess 210Pb. In two cores, excess 210Pb, estimated by subtracting 226Ra from total 210Pb on a layer-by-layer basis, exhibit relatively constant activity with discrete minima and maxima. Therefore, these cores were excluded from radiometric dating. Only the third core could be dated by the constant rate of supply model, and 137Cs was utilized to validate the 210Pb chronology.