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 measurements were carried out at high temporal frequency in the northwestern Mediterranean Sea in the course of the DYNAPROC 2 campaign, during the transition between late summer to autumn conditions. Molecular biomarkers were analyzed in selected subsets of consecutive samples collected for 6 h at 200 m depth. Fluxes of n-alkanes, long-chain alkenones, sterols and steroid ketones show high variability between consecutive 6-h samples, comparable in range to seasonal variability. n-Alkane export ranges from 1.4 to 29.7μg 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. Biomarker flux ranges are coherent with the relatively low primary production and the low export ratio encountered during the study. Molecular characteristics of biomarker composition is consistent with reworked algal and zooplanktonic organic matter. Lipid biomarker composition points to the dominance of haptophytes over dinoflagellates in the exported material, and to a minor contribution of diatoms and eustigmatophytes. The intrusion of coastal water at the study site was recorded by an enhanced imprint of higher plant n-alkanes.
Two processes can be responsible for the pulses in biomarker flux: changes in the magnitude of particle flux (dry weight mass flux) and changes in the concentration of biomarkers in the particles.
Concurrent variability in fluxes, significant Pearson correlations between fluxes of biomarkers of haptophytes, dinoflagellates, eustigmatophytes, zooplankton activity and higher plants (alkenones, dinosterol, alkyl diols, dehydrocholesterol, steroid ketones, p<0.01, n=31, suggest that the short term temporal variability of biomarker export fluxes depends primarily on the intensity of particle dry weight flux. Biomarker concentrations vary on a relatively narrower range than biomarker fluxes, indicating that changes in particle composition (due to degradation or change in source apportionment) has a weaker influence on flux variability. Thus, particle aggregation and sinking are key processes to explain the dynamic of biomarker export at a temporal scale of 6 h. None the less, abrupt changes of OC-normalized concentrations of biomarkers, in the time scale of 6 h, indicate that the exported particles composition also show short-time variability. Highest biomarker flux values recorded by the trap samples from 27 to 29 September correspond to highest values of zooplankton biomass integrated between the surface and 200 m. The decrease in primary production after the October wind events is echoed by minima in biomarker flux values. Despite these correspondences, the coupling between primary production and biomarker export is complex.