Lipid class and fatty acid distributions in particulate and dissolved fractions in the north Adriatic sea

Abstract

Lipid classes (Iatroscan TLC/FID method) and fatty acids (GC/MS) of settling particles and dissolved fraction were investigated in the north Adriatic sea in June 1995. The differences in composition and concentrations were examined according to distance from Isonzo river Estuary, depth and fraction (particulate versus dissolved). Total particulate lipids showed large variations, from 32.6 to 139.5 μg/l. The highest concentration was observed as for POC and total individual fatty acids in surface waters close to the river mouth. The ratio particulate lipids/POC ranged from 0.14 to 0.40 (average 0.27), which expressed different biosynthetic pathways by planktonic communities, as previously shown when nitrogen limitation can favor a switch from protein to lipid metabolism. The highest value was found in surface waters in the central part of the Gulf of Trieste, suggesting the existence of different stress conditions such as nutrient limitation, from those existing near the Isonzo river mouth. Total dissolved lipids did not show large variations (49–88 μg/l). The ratio dissolved lipids/DOC also varied in a narrow range (0.03–0.04), but with much lower values than those found in the particulate fraction. The comparison between particulate and dissolved lipid classes and fatty acids indicated a decoupling between the two reservoirs. Inputs from living organisms and freshly synthesized organic matter were clearly revealed in the dissolved fraction by phospholipids and polyunsaturated fatty acids. Degradation indicators such as free fatty acids and monoacylglycerols were significantly more represented in the particulate fraction than in the dissolved one near the river mouth, whereas the contrary was observed in the central part of the Gulf. Complementary informations were obtained from fatty acid distribution patterns such as for terrestrial originating inputs using the long-chain series. The importance of bacteria was observed using branched fatty acids and 18:1 ω7. The two bacterial signatures were found in both fractions, with highest imprints near the river mouth. Some enrichment of the branched fatty acids in the dissolved fraction in the central part of the Gulf could be related with the high contribution of polar lipids, suggesting a marked contribution of bacteria to the polar lipid fraction.