Short term variability of dissolved lipid classes during summer to autumn transition in the Ligurian sea (NW Mediterranean)

Changes in concentration and composition of dissolved lipid classes (Iatroscan TLC/FID) were examined at daily to month scale, in relation to the hydrological and biological situation at a central site of the Ligurian sea, NW Mediterranean during the PECHE-DYNAPROC 2 experiment (14 September to 17 October). Dissolved lipid concen- 5 trations (TLd) and lipid to DOC ratios varied in the range 5.3–48.5 µg l − 1 and 0.01 to 0.08 respectively, along the 0–1000 m water column. The highest TLd concentration values were found in the 0–150 m surface layer coinciding with phytoplankton biomass. Lipid class composition provided valuable information on the origin of DOM, and the changes that occurred during the period investigated. The significant 10 correlations ( p< 0 . 01, n = 87) between glycolipids from chloroplast membrane (MGDG) (38.7 ± 8.5% of TLd, n = 166), and various phytoplankton pigments (chlorophyll cs-170, violaxanthin, diadinoxanthin, zeaxanthin, and lutein), suggested that picoeucaryotes were the major source of dissolved lipids. Lipid metabolites (37.6 ± 11.1%, n = 166), the second most important compounds in TLd, showed a greater degree of degradation 15 of lipids in this transition period than previously observed earlier in the year. A contribution of lipids to DOM in the mesopelagic zone was observed before the winter mixing: At mid time of the cruise (4–6 October), zooplankton wax esters biomarkers (WE, 5.5–13.6 µg L − 1 ) appeared in the 0–150 m surface layer. WE were observed later and deeper in the mesopelagic layer (6 to 11 October), accompanied by re-increases 20 of hydrocarbons (6–8 October) and phospholipids concentrations (12 October) in the 400–1000 m depth layer. Zooplankton migration and/or fecal pellets egestion, followed by DOM release from POM, were likely responsible for the appearance of these lipid signatures in the mesopelagic layer, which occurred during the period of low wind ( < 15 knots) (28 September–12 October). The low salinity water lenses that appeared 25 twice during the cruise in the 40–80 m surface layer had little e ff ect on total biogenic lipid concentrations. Lower concentrations in phosphoglycerides and hydrocarbons than the nearby sea water suggested di ff erent microbial assemblages and di ff erent level of HC contamination in this less-salted water. The analytical TLC/FID technic on an apparatus allows detection of low concentrations of lipids without prior split- 25 ting of the extract, opens interesting by removing a obstacle to the study of neutral and polar lipid classes in the environment. This technique involves a qualitative separation of the lipid extract by thin-layer chromatography on chromarods coupled to a quantification of separated compounds by flame ionization