Temporal Variability of Nepheloid Layer Structures on Continental Slope Environments: Natural and Anthropogenic Processes

Abstract

This Thesis aims to investigate the contribution of the deep-sea trawling activities, besides that of natural processes, to the sediment dynamics in different continental slope areas, particularly in the NW Mediterranean and the NE Pacific basins.

The temporal and spatial variability of the water column turbidity structure in the Foix Canyon (NW Mediterranean) are studied by analysing two months of hydrographic profiles collected by an autonomous hydrographic profiler. The water column turbidity structure consists in intermediate nepheloid layers (INLS) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels positioning during the deployment period reveals that the presence and depths of the observed INLs and BNLs depend on the operating depths of bottom trawlers and on the specific fishing grounds being exploited by the local trawling fleet. This data strongly suggests that bottom trawling activities in the study area are the main driver of the turbidity increases observed in the water column.

The spatial and temporal variations on the water column turbidity structure and near- bottom suspended sediment transport associated to both natural processes (storms, river floods, and dense shelf water cascading, DSWC) and bottom trawling activities are also analysed for three months in the Palamós Canyon (NW Mediterranean) using an autonomous hydrographic profiler and near-bottom instrumentation installed in a mooring, covering a trawling closure period, and an active trawling period. Periods of enhanced water turbidity during the trawling closure period are associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such energetic events, the water column remains unchanged, displaying the lowest suspended sediment concentrations of the recording period, until the trawling season began. INLs (250 and 350 m depth) and BNLs (> 500 m depth) are found at the water depths where trawling grounds from the Palamós Canyon are located. High near-bottom suspended sediment fluxes are sporadically registered during the trawling closure associated with a major wet eastern storm and a DSWC event, whereas smaller but more frequent increases in near-bottom suspended sediment fluxes are recorded during trawling activities. However, the continued bottom trawling activity, at least for 30 days of monitoring, produces an accumulated suspended sediment transport similar to that generated by a major DSWC event. Considering that bottom trawling in Palamós Canyon is practised on a daily basis throughout the year, except for the 2-month trawling closure period, a much larger contribution of anthropogenically derived suspended sediment transport can be expected.

The temporal evolution of the hydrographic and nepheloid layer structure in the upper slope of Vancouver Island (British Columbia, Cascadia margin, NE Canadian Pacific) is

also assessed by analysing four months of sensor data from the NEPTUNE observatory. The water column turbidity structure during the study period consists in surface nepheloid layers (SNLs) (<100 m depth) associated to primary and secondary productivity, INLs between 150 and 300 m depth at the shelf-break and upper slope domain, and BNLs developed at deeper continental slope regions (~400 m depth). Strong fall storms occurring during the monitored months seem to generate the more intense INLs found at shelf-break depths. However, not all the recorded INLs coincide with the more intense storms, indicating that other sediment resuspension mechanisms might be modulating these INL detachments. Results from this study 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 transference along the margin as nepheloid layers.

Aquesta Tesi té com a objectiu principal investigar la contribució que exerceixen les activitats de pesca d’arrossegament a aigües profundes, a més dels processos naturals, a la dinàmica sedimentària en diferents àrees del talús continental, particularment a la Mediterrània nord- occidental i al Pacífic nord-est. Les tècniques experimentals utilitzades consisteixen en mesures contínues de durada variable de les propietats de la columna d’aigua, realitzades amb diferents sensors oceanogràfics allotjats en perfiladors verticals autònoms instal·lats a línies de fondeig instrumentades, o bé a un observatori submarí cablejat, totes elles obtingudes a profunditats del talús continental. Els resultats als canyons submarins del Foix i de Palamós (marge Català, Mediterrània nord- occidental) il·lustren una estructura de la columna d’aigua similar què consisteix en capes nefeloids intermèdies (INLs) i pròximes al fons (BNLs) desenvolupades a les profunditats on es troben els caladors de pesca explotats per les corresponents flotes locals de pesca d’arrossegament. Per altra banda, les condicions hidrodinàmiques a l’interior dels canyons, així com els processos naturals energètics (tempestes, riuades, o esdeveniments de cascading) i les corrents locals marines també contribueixen a incrementar la terbolesa de l’aigua, i a la retenció i/o advecció de partícules de sediment al llarg del marge, transportant-les en forma de capes nefeloids. Els resultats al talús superior de l’Illa de Vancouver (marge Canadenc, Pacífic nord-est) mostren una estructura de la terbolesa de la columna d’aigua que consisteix en capes nefeloids superficials (SNLs) associades a productivitat primària i secundària. , INLs a profunditats de la plataforma continental i el talús superior i BNLs a les zones més profundes del talús. Les tempestes que es produeixen durant el període d’estudi semblen generar les INLs més intenses, tots i que no totes coincideixen amb les tempestes de més intensitat, indicant que altres mecanismes de resuspensió podrien estar modulant la generació d’aquestes capes. En combinació amb les corrents regionals, la presència de pesca d’arrossegament contínua al llarg del marge també contribueix a l’advecció de partícules de sediment en suspensió al llarg del marge. Aquesta regió presenta una gran complexitat oceanogràfica, dificultant la determinació dels processos que intervenen en la formació de capes nefeloids.