Photosynthetic pigments have proven to be useful biomarkers of the abundance, composition and physiological status of phytoplankton biomass as well as the trophic state in the marine environment. Phytoplankton pigments were investigated using High Performance Liquid Chromatography (HPLC). We sampled the water column in the middle of the Gulf of Trieste, at sewage outlet areas and a fish farm area. We also sampled mucilage, a natural but unusual phenomenon in the northern Adriatic. Selected physical and chemical parameters at the investigated areas were also determined. The results were compared with the estimation of trophic state based on the TRIX index and to the Fp ratio, which is based on concentrations of biomarker pigments.
Phytoplankton pigment composition in the water column of the Gulf of Trieste showed season based variability. The main pigment is fucoxanthin (diatoms), while during the spring-summer period 19’-hexanoyloxyfucoxanthin may predominate. Annual biomass peaks, expressed in chlorophyll a concentrations, appear at the end of winter (February) and in the autumn (September-November), mainly due to increased diatom abundance. Before the mucilage events and in the freshly formed mucilage we measured higher concentrations of 19’-hexanoyloxyfucoxanthin (Prymnesiophyceae). Diatoms were the main phytoplankton group in mucilage aggregates. Phytoplankton composition inside the mucilage aggregates was much more stable compared to the water column. The diatom contribution in mucilage was mostly higher than 50 % of the total autotrophic biomass. Based on phytoplankton pigment analysis in different types of mucilage aggregates and the surrounding water, we anticipate diatoms are not the main phytoplankton group responsible for mucilage events. Due to the prevalence of Prymnesiophyceae in the freshly formed mucilage aggregates as well as in the water column just before the event starts, their role in mucilage appearance should be investigated. The diatom contribution in mucilage normally increases in aged mucilage aggregates due to favourable conditions for their growth and reproduction inside the aggregates. According to our results, the Fp ratio might be useful for predicting mucilage events. In the period of April-May before the mucilage appearances the values of the Fp ratio were significant lower as in years, when we didn’t observed the mucilage phenomenon. With the enrichment experiments we determined the eutrophic effect of sewage water and nutrient enriched water from the fish farm area. In winter and spring when the water column was mixed, the Fp ratio and TRIX values were higher at the sewage outlet areas compared to the control site. In the period of stratified water column those values were higher in the surface layer of the sewage outlet areas. Under natural conditions during summer the Fp ratio and TRIX values were higher at the bottom layer and lower at the surface layer of the water column. The Fp ratio and TRIX are complementary. When knowledge of the natural dynamics of the phytoplankton community at a study area is good, then it is meaningful to calculate both.