Planktonic response to main oceanographic changes in the Alboran Sea (Western Mediterranean) as documented in sediment traps and surface sediments

In order to determine the influence of seasonal oceanographic changes to the export of particulates and the flux of organisms as well as its corresponding integrated signal left in the bottom sediments of the Alboran Sea (Western Mediterranean), two mooring lines, ALB-1F and ALB-1D, were deployed from July 1997 to May 1998 on the continental slope south of Malaga at 36degrees01' N/04degrees16' W and 36degrees14' N/04degrees28' W, and a water depth of 1004 and 958 m, respectively. The mooring locations are under the influence of the incoming Atlantic Surface Water (ASW), at the northern and productive edge of the Western Alboran Gyre (WAG), and of the wind-induced upwelling offshore the Spanish coast. Additionally, a total of five core tops located in the same area were studied. The results are compared to information based on SeaWIFS images of chlorophyll-a concentration, SST, meteorological data on wind direction and intensity, and fluvial discharges from the main rivers of the Alboran Basin. The temporal oceanographic evolution of the Alboran Sea during the studied period was divided into five episodes: E1 (July 1997), E2 (August-October 1997), E3 (November to mid-December, 1997), E4 (December March 1998) and E5 (April-May 1998). The episodes represent different oceanographic conditions in the Alboran Sea during the studied period. Detailed analyses of the recorded pattern of bulk components, calcareous nannoplankton, planktonic foraminifera, diatoms and phytoliths reflect seasonal changes in the main hydrographic regimes in the western Alboran Sea. All biogenic bulk components, as well as the annual evolution of the floral and faunal assemblages follow a tri-modal pattern, with maximum fluxes recorded during E1, E3 and E5, representing periods of higher productivity and wind-induced upwelling. During these periods, species such as the Small Gephyrocapsa Group, Gephyrocapsa muellerae and Calcidiscus leptoporus, the foraminifer Globigerina bulloides and the diatom Chaetoceros resting spores (RS) were dominant. Gephirocapsa oceanica and Globorotalia inflata were coupled to the evolution of the Atlantic Surface Water (ASW) or related to surface water stability (E2 and E4). The diatom Leptocylindrus danicus responded to upwelling relaxation, and water stratification (E1, E2 and E4) and benthic diatoms were related to eddy intensity at bottom. Phytolith fluxes were directly related to wind intensities and the prevailing westerly winds (E3 and E5). The distribution of microfossils in surface sediments differs from the biogenic assemblages recorded in the sediment traps. In this sense, during 1997-1998 the strongest El Nino Event in the last century took place, which could have forced an unusual seasonality in the Alboran Sea, reflecting an exceptional micropaleontological signal not observed in the surface sediments. (C) 2004 Elsevier B.V. All rights reserved.