FATTY-ACIDS AS BIOMARKERS OF PLANKTONIC INPUTS IN THE STRATIFIED ESTUARY OF THE KRKA RIVER, ADRIATIC SEA - RELATIONSHIP WITH PIGMENTS

Fatty acids (FA) associated with particles were quantitatively determined at several stations in the Krka Estuary in March 1987 and May 1988. Particles were sampled from riverine stations (E0 and E2) down to the open sea (C2). Two intermediate stations (E3 and E4a) were sampled on both sides of the freshwater-seawater interface (FSI) and as close as possible to the interfacial layer. In winter (March 1987), total FA concentrations were highest at station E4a, located off the city of Sibenik (75.9-mu-g l-1 at the surface). Total FA concentrations found at stations E(o) (7.0-mu-g l-1) and C2 (2.6-mu-g l-1) were low and of the same range as usually encountered in non-productive waters. A net decrease of terrigenous inputs was observed between riverine station E(o) and marine station C2 (FA > C24 = 9.7% and 0.3% of total FA, respectively). In spring (May 1988), the total FA maxima were found at the FSI at stations E4a and E3 (26.6 and 11.2-mu-g l-1). Polyunsaturated fatty acids (PUFA) were predominant compared with the monounsaturated fatty acids (MUFA) at stations E(o) and E4a (surface and interface), indicating freshly biosynthesized planktonic material. The terrigenous fatty acids (FA > C24) were totally absent at this period. The microbially derived fatty acid contribution is still low, except at station E4a interface (9.2% of total FA). In May 1988, FA and pigment taxonomic information was qualitatively in good agreement, and in some cases a significant linear correlation was observed, e.g. the ratio of sum C16/sum C18, known to be very much higher in diatoms than in other algae, was correlated with the fucoxanthin percentage of chlorophyll a. The concentration of 18:4-omega-3 was also positively correlated with the alloxanthin concentration and 18:3-omega-3 with chlorophyll b. These results are in good agreement with a phytoplanktonic production generally dominated by diatoms, with ubiquitous Cryptophyceae and Chlorophyceae present to a smaller extent throughout the estuary. In contrast, in winter conditions (March 1987) FA and pigments do not follow the same general trends.