Variations of zooplankton metabolism and feeding in the frontal area of the Alboran Sea (western Mediterranean) in winter

Respiration, excretion (NH 4 and PO 4) and feeding of zooplankton from the upper 200 m of the Almeria–Oran frontal region (western Mediterranean), were studied during winter 1997. Eight sites belonging to the three main hydrodynamic structures of the area (Mediterranean water, geostrophic Atlantic jet and associated anticyclonic gyre) were considered. Lowest values of respiration, PO 4 excretion, O/P atomic ratio and particle ingestion were found in Mediterranean waters. At the frontal limit, respiration and ammonium excretion were enhanced compared to the adjacent structures (Mediterranean water and Atlantic jet). In the gyre water, lowest O/N and highest N/P values were observed and maximum faecal pellet production and reproduction activity (inferred from the proportion of nauplii in plankton) were recorded. Selectivity was observed for feeding with respect to particular spectra. At the front and in the jet, large particles (> 20 μm), were preferred, while medium- and small-sized particles were mainly used for food at the gyre and in Mediterranean water. As the bulk of zooplankton components was the same at all the stations, physiological variations were interpreted as adaptive processes to qualitative and quantitative changes in the food resource, i.e. Mediterranean scarcity and Atlantic water richness, “new production” of diatoms at the front limit, enhancement of the microbial food chain at the gyre and in Mediterranean waters. The maximum proportion of copepod nauplii observed at the gyre could result, either from an isopycnal transport from the front, of larvae issued from eggs produced at the frontal limit, or from a local enhancement of copepod reproductive activity induced by the favourable food conditions prevailing at the gyre, as suggested by the increase of faecal production. La respiration, l'excrétion (NH 4 et PO 4) et la nutrition du zooplancton des deux cents premiers mètres de la région du front Alméria–Oran (Méditerranée occidentale) ont été étudiées pendant l'hiver 1997. Huit sites appartenant aux trois principales structures hydrodynamiques de la région (eau méditerranéenne, jet géostrophique atlantique et tourbillon anticyclonique associé) ont été considérés. Les plus basses valeurs de respiration, d'excrétion de PO 4, du rapport atomique O/N et d'ingestion de particules ont été trouvées dans les eaux méditerranéennes. À la limite frontale, les taux de respiration et d'excrétion d'ammonium sont plus élevés que dans les structures adj