Distribution of pigments and fatty acid biomarkers in particulate matter from the frontal structure of the Alboran Sea (SW Mediterranean Sea)

Abstract

The distribution of pigments and fatty acid biogeochemistry of suspended and sinking particles collected in a frontal zone in the western Mediterranean (Almeria-Oran Front) was investigated to determine the sources and fate of the organic matter in relation to the physical forcing of the geostrophic front. Three hydrodynamic structures, comprising the Atlantic jet, an anticyclonic gyre and the surrounding Mediterranean waters, were explored. Fatty acid and pigment distributions differed markedly between the two classes of particles. Whereas branched fatty acids and flagellate pigments were present at higher percentages in suspended particles, the polyunsaturated fatty acids (PUFA) and pigments indicative of degradation and diatoms, were dominant in sinking particles. Fatty acid distribution and compound specific isotope analysis of δ 13C suggested that the acids were totally marine in origin. Bacterial biomass, indicated by odd/branched chain fatty acids, was minimal and mainly associated with the planktonic biomarkers. Diatom biomarkers predominated in the frontal zone and gyre downstream. Flagellate biomarkers were mainly associated with the suspended particles of the Mediterranean site and the gyre upstream. Prokaryote pigments were more abundant in Mediterranean waters. The relatively high levels of fucoxanthin, polyunsaturated fatty acids and degradation pigments (phaeophorbides) in all sediment traps demonstrated that both diatom aggregates and fecal pellets contributed substantially to the sedimentation processes. Based on biomarker indices, high bacterial abundance with relatively degraded phytoplankton material and an enhanced grazing or zooplankton biomass were observed at the gyre upstream. Phytoplankton, showing little evidence of degradation or grazing by zooplankton, predominated at the gyre downstream. The high accumulation of partially degraded plant material with relatively higher bacterial signatures and low zooplankton grazing occurred in the Mediterranean waters influenced by the proximity of the Atlantic jet.