Short scale (6 h) temporal variation of sinking fluxes of planktonic and terrigeneous lipids at 200 m in the NW Mediterranean Sea

Abstract. Drifting sediment trap experiments were carried out at high temporal frequency in the northwestern Mediterranean in the course of the DYNAPROC2 campaign, every 6 h at 200 m depth. Molecular biomarkers were analyzed in selected subsets of consecutive samples. Fluxes of n-alkanes, long-chain alkenones, sterols and steroid ketones show high variability between consecutive 6-h' samples, comparable in range to seasonnal variability. n-Alkane export ranges from 1.4 to 29.7 μg m⁻² d⁻¹, fluxes of C₃₇ alkenones varies from 0 to 14.2 μg m⁻² d⁻¹. Fluxes of sterols, steroid ketones and C₃₀ alkane diol, respectively range from 31 to 377, 2.2 to 46 and 0.3 to 9.3 μg m⁻² d⁻¹. The Biomarker composition is consistent with reworked algal and zooplanktonic organic matter with a remarkable refractory character. After a rain event ensuing the intrusion of coastal water at the study site, the relative signature of higher plant increases and corresponds to higher export fluxes of long-chain odd n-alkanes. Most phytoplanktonic biomarkers show concurrent variability in fluxes. Linear correlations between fluxes of distinct biomarkers and between fluxes of biomarkers and flux of total carbon suggest that the short term temporal variability of export fluxes depends primarily on physical constrains exerted by carrier particle dynamics. Linear correlation of their carbon-normalized concentrations explained a lower part of the variance, indicating that short-term variability in particle composition is a secondary driver of flux timing. At the end of summer stratification, export fluxes account for ca. 1% of the primary productivity. In this studied situation, biomarkers have a long residence time in the water column before they are exported at 200 m. Biomarkers exported at 200 m may thus record processes averaged over a larger period than the sampling frequency. For instance, phytoplanktonic biomarker composition of sinking particles fails to reflect the community changes occurring over the 4 weeks of study. At higher time resolution, the diel variability in primary productivity is not recorded by biomarker fluxes either. The coupling between primary productivity and biomarker export shows significant changes on time scales of days and even of 6 h.