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 t hroughout the water column (50-3000 m) in a northeastern Atlantic area wher e deep Mediterranean waters outflowing through the Strait of Gibraltar are incorporated at mid-depth into Atlantic waters. Particles collected by wate r filtration (0.7 pin pore size) and by vertical hauls of a plankton net (5 0 mum mesh size) were analysed for their organic carbon and lipid compositi on, namely fatty acids, hydrocarbons, aliphatic and alicyclic alcohols and ketones, and pigments. Small-size particles exhibited POC concentrations tw o to three orders of magnitude higher (5-50 mug/l) than large particles (0. 01-0.32 mug/l). Strong spatial and temporal variations were also observed. Surface small particles collected in January accounted for 15-50 mug/l of P OC, whereas in August, these represented only 5-15 mug/l following the seas onal 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 compositi on (mainly of haptophytes and moderately of diatoms, prasinophytes, chrysop hytes and dinoflagellates) with a bacterial contribution. The vertical prof iles of the different lipid classes showed a general decrease with depth, p articularly significant in the upper 200 m, consistently with the POC conte nts. Compositional changes were more evident in small particles and include d the loss of unsaturated compounds and the increase of diagenetic and bact erial markers. Unusual increases were observed at mid-depths indicating add itional particle inputs, either by in situ formation or by advective transp ort from the Mediterranean. The latter was recognized because small particl es in Mediterranean waters entrained large quantities of continental detrit ic 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 it s signature in the raid-depth northeastern Atlantic waters. (C) 2001 Elsevi er Science B.V. All rights reserved.