4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Autonomous hydrographic profiling: a tool for assessing the nepheloid structure of trawled submarine canyons
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Pere Puig, Albert Palanques, Mikhail Emelianov, Ruth Durán, Martin White
Autonomous hydrographic profiling: a tool for assessing the nepheloid structure of trawled submarine canyons
Autonomous hydrographic profilers have been recently used to conduct continuous measurements of water column properties over large depth ranges at high temporal and vertical resolution. In the frame of the FORMED and the ABIDES Projects, the Aqualog profiling carrier, equipped with a CTD and a turbidimeter, was successfully tested in two submarine canyons (Foix and La Fonera) from the Catalan continental margin (NW Mediterranean), with the objective to study the temporal evolution of the nepheloid structure and to assess the contribution of trawling resuspension plumes. From April to June 2014, the mooring line was deployed in the Foix Canyon axis at 870 m depth, and the instruments were programmed to collect hydrographic profiles once per day from 800 to 200 m water depth. A similar mooring line was deployed in La Fonera Canyon axis at 929 m depth, from February to April 2017, increasing the profiling frequency, from 750 to 150 m water depth, twice a day. At this location, closed spaced CTD transects across the canyon were also conducted during three oceanographic cruises in June and October 2017, and in March 2018.
Daily hydrographic profiles collected throughout both field studies illustrated a well-defined water turbidity structure consisting in intermediate nepheloid layers (INLs), developed mostly above the canyon rims, and near-bottom nepheloid layers (BNLs) confined within the canyon. Using fishing vessels’ positioning from Vessel Monitoring System (VMS) and Automated Information System (AIS) data, the temporal and spatial distribution of the local trawling fleets over both canyons at the time of the deployments suggests that trawling is an important cause increasing turbidity in the water column. Nepheloid layers were absent when there was no fishing activity next to the mooring locations, whereas both INLs and BNLs were observed with trawling activity on the nearby fishing grounds. Nonetheless, natural processes also contribute to the advection and/or retention of resuspended particles, playing a major role in their transport along and across margin via nepheloid layers. These field experiments describe, for the first time, the temporal evolution of water column turbidity using an autonomous hydrographic profiler deployed within two trawled submarine canyons on the NW Mediterranean region. Although our data only provide a snapshot of a section of the water column, autonomous profiling observatories of the moored type have great potential in terms of marine environmental monitoring.
Session 2: New ways to study submarine canyons: integrated programmes, new technologies and coordinated monitoring efforts