Lipids and their oxidation products as biomarkers for carbon cycling in the northwestern Mediterranean Sea: results from a sediment trap study

Classical lipid biomarkers (sterols, fatty acids and pigments) and their photo-oxidation and autoxidation products were measured in sediment trap samples collected during the summer of 1998 at the DYFAMED time-series station in the northwestern Mediterranean Sea to investigate changes in phytoplanktonic composition and alteration of particulate organic matter (POM). The study period covered a transitional situation from bloom event to oligotrophic conditions. At the beginning of the study, the phytoplankton community appeared to be mainly composed of diatoms, prymnesiophytes, pelagophytes and dinoflagellates. During the transition to the oligotrophic period, the contribution of small cells (pico- and nanoplankton) increased. Quantification of a “pool” of oxidation products resulting from photo-oxidation and autoxidation of phytoplanktonic sterols, monounsaturated fatty acids and chlorophyll phytyl side-chain provided very useful indications concerning the alteration state of POM during the investigated period. This “pool,” composed of some oxidation products which are very sensitive (but labile) indicators and of others which are less sensitive (but more refractory), allowed us to obtain a large range of diagnostic information about organic matter behaviour. Thanks to these indicators, visible light-induced and autoxidative degradative processes, which have been virtually ignored to date owing to the lack of suitable tracers, may be easily distinguished and appear to act intensively in the northwestern Mediterranean Sea.