Lateral transport of suspended particulate matter in nepheloid layers along the Irish continental margin - a case study of the Whittard Canyon, North-East Atlantic Ocean

Abstract

Nepheloid layers, defined by their increased concentration of suspended particulate material (SPM), are an important transport mechanism in the pelagic to benthic coupling of material, including rich organic matter. Fortuitously located at the edge of the Celtic Sea shelf along the NE Atlantic margin; an area of high energy and primary production–the Whittard Canyon is recognised as a refuge for benthic and suspension feeding fauna. The formation and composition of material in benthic (BNL) and intermediate nepheloid layers (INLs) in the Whittard Canyon were investigated over the course of three research surveys between 2011 and 2013, in order to investigate the extent and significance of this transport pathway. BNLs were detected in the four surveyed branches, to water depths greater than 2500 m, with INLs occurring as extensions from the benthic source and stretching distances of 25 km off the slope. Hotspots for nepheloid layer generation were identified at depths of critical and supercritical conditions for semidiurnal internal tide reflection and at the boundaries of the permanent thermocline and Mediterranean Outflow Water. Seasonal variations in primary productivity and, temporal variations induced by the combined effects of (seasonal) stratification and storm activity, influenced nepheloid layer generation and the distribution patterns of SPM. Recently, bottom trawling activity has also become a recognised and legitimate mechanism for sediment transport, feeding thick or enhanced nepheloid layers (ENLs). ENLs, with concentrations of SPM typically an order of magnitude higher than normal nepheloid layers, were detected during the survey in June 2013. High spatial and temporal coverage of bottom trawlers, identified using Vessel Monitoring System data, coincided with the occurrence of these events. Material collected from (normal) BNLs and INLs in 2013 showed enrichment of fresh particulate organic material (molar C/N, pigments, SEMs, lipid biomarkers). BNLs in the upper reaches of the canyon (650 – 750 m) had high concentrations of labile lipids and showed high contributions of chlorophyll a and other compounds derived from primary production in the surface waters. Considerable compositional heterogeneity was also observed in the nepheloid layers, indicative of the inherent natural, spatial and temporal variance of settling organic and resuspended material that is influenced by different processes in the different branches. Localised variations in energy fluxes in the different canyon branches partly explain the frequency, location and level of turbidity of the nepheloid layers. However the differing degree of trawling activity adjacent to the canyon branches is also likely to have an influence, particularly on the compositional components. Qualitative analysis (lipid biomarkers) from benthic nepheloid layers (1300 – 1400 m) showed an apparent eastern and western differentiation which is likely associated with the alteration of material by trawling activity. In terms of sediment transport rates, the magnitude of the fishing activity adjacent to the Whittard Canyon is shown to have impacts on human rather than geological timescales. Furthermore, a unique assemblage of limid bivalves and deep-sea oysters was found in association with nepheloid layers in the canyon. Changes to the distribution and delivery of rich organic matter by nepheloid layers are likely to affect faunal feeding, distribution patterns and, the functioning of these canyon ecosystems.