Large size suspended matter: analyses of some organic compounds, associated concentrations and evolution in the water column

Abstract

The vertical input of recent biogenic organic material to the sediment has been evaluated by the study of large size (>50μm) suspended particles from superficial and deep water layers. These large size rapidly, sinking particles represent up to 90% of the vertical flux of organic carbon, but they are dispersed in the water column among large amounts of small particles and thus, their sampling is difficult. Vertical tows of plankton nets (50 μm mesh) equipped with metallic collectors were performed. The material collected in a few hours, was divided into several fractions for microscopic examination and chemical analysis. Hydrocarbons (n-alkanes and isoprenoids), fatty acids and sterols were analysed using CGC and computerized CGC/MS. Previous observations have enhanced the chemical identity of suspended material according to its size. Distributions of biochemical markers show that large size particles sampled below 1000 m have a composition close to that of plankton collected between 200 m and the surface. This confirms microscopic observations, that this material has been rapidly transported from the surface by non-living particles such as fecal pellets, aggregates and dead organisms. However, evidence for a beginning of bacterial degradation is observed. In deep layers, n-alkane concentrations (0.002–0.004 ng 1 −1 in the Arabian Sea and 0.04–0.5 ng 1 −1 in the Mediterranean Sea) represent only a few percent of those in surface waters (0.08–0.16 ng 1 −1 in the Arabian Sea and 0.46–1.20 ng 1 −1 in the Mediterranean Sea). This degradation is confirmed by analysis of fatty acids in the Arabian Sea (1.6–5.3 ng 1 −1 in deep water and 20–116 ng 1 −1 in surface water). Absolute concentrations and a value for the settling velocity of 100 m day −1 lead to estimations of vertical fluxes throughout the deep ocean of ∼ 100 mg m −2 y −1 for fatty acids and ⋍ 2.5 mg m −2 y −1 for sterols. These fluxes are significant for the benthic activity due to the high metabolic value of biochemical molecules, such as unsaturated fatty acids and sterols.