Distribution and transportation pathways of marine particulate matter off Northwest Africa

Abstract

This study focusses on the vertical distribution and transportation pathways of marine particles off NW-Africa with aid of optical methodologies.A profiling camera system and a remotely operated vehicle (ROV) was used for in-situ observations on the distribution, transport processes and sinking behaviour of marine particulate matter. Between the regional areas of investigation Cape Blanc, Dakhla and Cape Bojador significant differences could be observed with respect to distribution and transportation patterns. These differences are primarily related to the different primary production conditions between the investigation areas. Primary production is the most important factor for the abundance of particulate matter, where highest particle concentrations in the entire water column were seen off Cape Blanc. Next to primary production, water depth, currents and density gradients are factors influencing the particle distribution patterns. With respect to the prevailing transport processes, the Cape Blanc region is characterised by vertically orientated transport patterns. In addition to the continuous supply of large, relatively fast sinking particle aggregates a sinking event could be documented for the first time in-situ in the water column. These events deliver huge amounts of particulate matter from the ocean surface to the seafloor within several days. The observed transport processes off Cape Blanc emphasize a very close coupling between surface and deep-sea. Although vertical transport is important off Dakhla and Cape Bojador as well, it is not as pronounced and intense compared to the Cape Blanc region. Off Cape Bojador, large amounts of material from the shelf or upper slope areas are laterally advected towards the open ocean. This advection process could be observed in-situ with aid of the particle camera.