Influence of Natural Processes and Bottom Trawling in the Nepheloid Layer Structure Off Vancouver Island (British Columbia, Canada, NE Pacific)

20 pages, 8 figures, 2 tables, supplementary material https://www.frontiersin.org/articles/10.3389/fmars.2021.770280/full#supplementary-material.-- Data Availability Statement: The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation. Data from all the instruments are archived and publicly available online in near-real time via ONC’s Oceans 2.0 Data Management System (https://data.oceannetworks.ca/DataSearch)

Continental margins are transitional areas between the land and the deep ocean where large amounts of particulate matter are supplied, transported, and ultimately deposited in the deep sea. High-energy hydrodynamic processes such as storms, ocean currents, or internal waves and tides, as well as bottom trawling activities contribute to the resuspension and remobilization of these particles. These mechanisms favor their transference from the continental shelf to the continental slope and maintain high concentrations of particulate matter in the water column forming nepheloid layers. The temporal evolution of the hydrographic and nepheloid structure in the upper slope off Vancouver Island (British Columbia, Canada, NE Pacific) was assessed by analyzing 4 months (from mid-August to late-November 2018) of sensor data from Ocean Networks Canada (ONC) NEPTUNE cabled seafloor observatory. The distribution of particulate matter during the study period illustrated a well-defined turbidity structure consisting of surface nepheloid layers (SNLs) at <100 m depth associated to primary and secondary productivity, intermediate nepheloid layers (INLs) between 150 and 300 m depth at the shelf-break and upper slope domain, and bottom nepheloid layers (BNLs) developed at >400 m depth. Moderate storm events occurred during fall when the more intense INLs were recorded at shelf-break depths. However, not all the INLs recorded during this period occurred in coincidence with these storms, indicating that these INL detachments were modulated by a different sediment resuspension mechanism. Analyses of fishing vessel activity during the study period revealed that trawlers operated over the same depth range as these INLs. Our results suggest that, in combination with the regional currents, the presence of continuous fishing along the continental slope off Vancouver Island contributes to the advection of suspended sediment particles, playing a major role in their transfer as nepheloid layers

This work was supported by the ABIDES (CTM2015-65142) Spanish Project and by the Autonomous Government of Catalonia Grants 2017 SGR 863 and 2017 SGR 1241, as well as by the TrawledSeas Project receiving funding from the European Union’s Horizon 2020 Research and Innovation programme under a MSCA grant agreement (No. 867471). MA-C was supported by a predoctoral F. P. I. grant (BES-2016-076983) from the Spanish Ministry of Economy, Industry and Competitiveness. This work is contributing to ICM’s “Center of Excellence” Severo Ochoa (CEX2019-000928-S). Ocean Networks Canada was funded through Canada Foundation for Innovation-Major Science Initiative (CFI-MSI) fund 30199

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