Biogeochemical evolution of the outflow of the Mediterranean deep-lying particulate organic matter into the northeastern Atlantic

Small- and large-size particles were collected in January and August 1989 throughout the water column (50–3000 m) in a northeastern Atlantic area where deep Mediterranean waters outflowing through the Strait of Gibraltar are incorporated at mid-depth into Atlantic waters. Particles collected by water filtration (0.7 μm pore size) and by vertical hauls of a plankton net (50 μm mesh size) were analysed for their organic carbon and lipid composition, namely fatty acids, hydrocarbons, aliphatic and alicyclic alcohols and ketones, and pigments. Small-size particles exhibited POC concentrations two to three orders of magnitude higher (5–50 μg/l) than large particles (0.01–0.32 μg/l). Strong spatial and temporal variations were also observed. Surface small particles collected in January accounted for 15–50 μg/l of POC, whereas in August, these represented only 5–15 μg/l following the seasonal variability of primary production. Concurrently, the variety of lipid components was larger in January.The lipid components of large-size particles were dominated by zooplankton markers, whereas small particles showed evidence of a mixed algal composition (mainly of haptophytes and moderately of diatoms, prasinophytes, chrysophytes and dinoflagellates) with a bacterial contribution. The vertical profiles of the different lipid classes showed a general decrease with depth, particularly significant in the upper 200 m, consistently with the POC contents. Compositional changes were more evident in small particles and included the loss of unsaturated compounds and the increase of diagenetic and bacterial markers. Unusual increases were observed at mid-depths indicating additional particle inputs, either by in situ formation or by advective transport from the Mediterranean. The latter was recognized because small particles in Mediterranean waters entrained large quantities of continental detritic materials that were also found in the Mediterranean water lenses in the Atlantic. From these observations, it was inferred that the outflow of the Mediterranean deep-lying particulate organic matter might still preserve its signature in the mid-depth northeastern Atlantic waters.