Nepheloid layer distribution in the Whittard Canyon, NE Atlantic Margin

The dynamics and distribution of nepheloid layers in multiple branches of the Whittard Canyon were observed during three hydrographic surveys carried out between 2011 and 2013. The sources of permanent and temporal variation of the layers were investigated. Seasonal variations in primary production at the surface were reflected in the nepheloid layers extending down in the water column, particularly in the upper reaches of the canyon. Benthic nepheloid layers were observed along the canyon axes to depths greater than 2500 m, a distance of ~ 30 km. Intermediate nepheloid layers were detected as lateral extensions of the benthic nepheloid layer, centred at ~ 550 m, 850 m, 1150 m and 1600 m and stretching ~ 20-25 km off slope. Critical and supercritical conditions for semidiurnal tide reflection were identified coinciding with the occurrence of intermediate nepheloid layers, with other hotspots found at the boundaries of the permanent thermocline and Mediterranean Outflow Water. Weak seasonal stratification during the survey in 2012 appeared to permit storm induced mixing at deeper water depths than expected, altering the distribution of material in the nepheloid layers. The dynamic conditions of the canyon encourage nepheloid layer formation with implications ensued for deposition rates and food supply to the unique communities that are found in the Whittard Canyon and likely other similar canyon systems.