Contrasting particle fluxes and composition in a submarine canyon affected by natural sediment transport events and bottom trawling

Abstract

Submarine canyons are important conduits of sediment and organic matter to deep-sea environments, mainly during high-energy natural events such as storms, river floods, or dense shelf water cascading, but also due to human activities such as bottom trawling. The contributions of natural and trawling-induced sediment and organic matter inputs into Palamós Canyon (NW Mediterranean) were assessed from three instrumented moorings deployed in the axis and northern flank of the canyon covering the trawling closure (February) and the trawling season (March-December) of 2017. During the trawling closure, large sediment fluxes with high contents of labile marine organic matter content were registered in the canyon axis, associated to storm resuspension on the shelf that coincided with dense shelf water cascading and high surface water productivity. Although no major natural sediment transport events occurred during the following spring and summer months, near-daily trawling-induced sediment gravity flows were recorded in the northern flank mooring, placed directly below a fishing ground, which sometimes reached the canyon axis. Compositionally, the organic matter transferred by trawling resuspension was impoverished in the most labile biomarkers (fatty acids, amino acids, and dicarboxylic acids) and had a high degree of degradation, which was similar to surficial sediment from the adjacent fishing ground. Trawling resuspended particles masked the transfer of organic matter enriched in labile biomarkers that naturally occur during the quiescent summer months. Overall, bottom trawling enhances the magnitude of particle fluxes while modifying its organic carbon composition, increasing the re-exposure and transfer of degraded organic carbon and potentially affecting benthic communities that rely on the arrival of fresh organic matter.

Biogeochemical dynamics in a marine storm demonstrates differences between natural and anthropogenic impacts

“…This causes immediate reductions in dissolved oxygen and an elevated presence of degraded OM in suspension. This is supported by a recent study in the Mediterranean which attributes higher levels of degraded OM being resuspended and transported by trawling activities compared to storm events 17 . Nevertheless, the capacity for storms to transport fresh OM and nutrients into a system can trigger longer-term changes to dissolved oxygen levels.…”

“…Sediment resuspension, whether induced by storms or bottom trawling, has been shown to diminish OM quality by accelerating degradation rates through heightened exposure to oxygen 9 , 16 . However, higher observed levels of degraded OM associated with trawl-induced resuspension suggests that trawling may expose OM and nutrients within comparatively deeper layers of sediment 17 . Disturbance from trawling and storms can increase nutrient concentrations in water column can through enhanced mineralization and desorption processes 5 , 6 , 18 .…”

The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling

“…Much more often, turbidity currents are initiated by sediment settling from surface river plumes that are less dense than seawater, which occurs for rivers with a much wider range of sediment concentrations (Hage et al 2019). Turbidity currents can be triggered by human activities such as seabed trawling, although these flows tend to be relatively small (Puig et al 2012, Paradis et al 2022.…”

“…There is an increasing focus on how carbon stocks on continental shelves are remobilized by human activities such as trawling (Atwood et al 2020). Where trawling occurs close to canyons, turbidity currents play a key role in exporting and remineralizing carbon (Puig et al 2012, Payo-Payo et al 2017, Paradis et al 2022. Turbidity currents may also transfer microplastics in the deep sea, potentially explaining why ∼99% of plastic entering the ocean is currently unaccounted for (Kane & Clare 2019).…”

Sources and Fate of Sedimentary Organic Matter in the Western Mediterranean Sea

“…Finally, the bottom trawling activity performed on the margin down to 1,000 m depth is also responsible for grain size sorting and modifies OM composition (Paradis, Goñi, et al., 2021). Such activity triggers sediment gravity flows into submarine canyons (Puig et al., 2012) that may form persistent nepheloid layers (Arjona‐Camas et al., 2021) that also affect the OM signature that is transported toward the deep sea (Paradis et al., 2022).…”