Abstract
Increases of water turbidity and suspended sediment transport in submarine canyons have been associated with high-energy events such as storms, river floods and dense shelf water cascading (DSWC), and occasionally with bottom trawling along canyon flanks and rims. To assess the variations on the water column turbidity and sediment transport in the Palamós Canyon linked to both natural and trawling-induced processes, an autonomous hydrographic profiler, as well as a near-bottom current meter and a turbidimeter were deployed in the canyon axis (929 m depth) from February to June 2017, covering a trawling closure (February) and trawling activities (March-June). Periods of enhanced water turbidity during the trawling closure were mostly associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such events, the water column displayed low suspended sediment concentrations (~ 0.3 mg L− 1) until the trawling season began, when particulate matter detachments, ranging between > 1 mg L− 1 and 3.8 mg L− 1, were observed at the water depths where the trawling grounds are found. During the trawling closure, high near-bottom suspended sediment fluxes (35–44 g m− 2 s− 1) were sporadically registered at ~ 920 m depth associated with a major storm and DSWC event. Smaller but more frequent increases of near-bottom suspended sediment fluxes (0.1–1.4 g m− 2 s− 1) were recorded during trawling activities. Despite these smaller trawling-induced suspended sediment fluxes, 30 days of continuous bottom trawling activity transported a total amount of 40 kg m− 2, of similar magnitude to that generated by a major DSWC event (50 kg m− 2). Since bottom trawling in Palamós Canyon is practiced on a daily basis throughout the year, a much larger contribution of anthropogenically derived water turbidity and suspended sediment transport can be expected.
Highlights
Submarine canyons are widespread morphological features incising continental margins (Shepard and Dill, 1966; Shepard, 1972; Harris and Whiteway, 2011) considered to be preferential pathways for the transference of water and sediments between nearshore areas and deep-sea environments (e.g., Garcia et al, 2008; Allen and Durrieu de Madron, 2009; Puig et al, 2014; Porter et al, 2016)
Time-series observations on otter trawl boards (OTB) vessel positions, based on fishing effort, revealed no trawling activity occurred around the mooring site during the trawling closure at the flanks of the canyon, while during the trawling season, the fishing effort increased on both canyon flanks (Fig. 2)
The results presented in this new study contribute to refine the sediment dynamics associated to dense shelf water cascading (DSWC) events in the Palamós Canyon providing additional information throughout the water column, as well as near the bottom at a deeper canyon axis location (Figs. 7, 8, 9, 10 and 11)
Summary
Submarine canyons are widespread morphological features incising continental margins (Shepard and Dill, 1966; Shepard, 1972; Harris and Whiteway, 2011) considered to be preferential pathways for the transference of water and sediments between nearshore areas and deep-sea environments (e.g., Garcia et al, 2008; Allen and Durrieu de Madron, 2009; Puig et al, 2014; Porter et al, 2016) For this reason, several studies have focused on studying the shelf-slope exchanges of downward particle fluxes and water turbidity by means of moored oceanographic instruments (see review by Puig et al., 2014). Cold and windy winters, DSWC can be exceptionally intense, and dense shelf waters can affect the entire continental slope and even reach the basin floor (Canals et al, 2006; Palanques et al, 2012; Durrieu de Madron et al, 2013; Palanques and Puig, 2018)
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