Group-specific phytoplankton biomass/dissolved carbohydrate relationships in the Gulf of Trieste (Northern Adriatic)

Distribution of carbohydrates (CHO) and photosynthetic pigments werestudied in the Gulf of Trieste, northern Adriatic Sea, during the period ofsummer stratification with a special emphasis on determining the impact ofthe taxonomic composition and concentration of phytoplankton biomass on thecarbohydrate levels in the water column. Dissolved total carbohydrates(DTCHO), dissolved monosaccharides (DMCHO) as well as particulatecarbohydrates (PTCHO) were determined using the colorimetric MBTH-method,while pigment biomarkers of the phytoplankton biomass were determined byreversed-phase HPLC. Concentrations of the total CHO (dissolved+particulate) varied in a wide range from 173 µg Cl™1 to 1552 µg C l™1. The percentage ofPTCHO in the total CHO concentration was relatively low(4–25%), indicating that the main pool of CHO was in thedissolved phase. The contribution of DTCHO to the total dissolved organiccarbon (DOC) in late summeontribution r was highly variable(10–65%) with an average value of 20 ± 14%, whilein early summer this percentage was somewhat lower and less variable (range11–23%; average 17± 3%). Analyses of biomarkerpigments revealed a very high diversity and a rather heterogenous verticaland spatial distribution of the phytoplankton biomass during the period ofsummer stratification. In September 1994, the predominant taxonomic groupsof phytoplankton were prymnesiophytes, diatoms, silicoflagellates,cyanobacteria and, especially in the bottom layer, dinoflagellates. Arelatively good correlation (r2 =0.51) found betweenDTCHO and chl a suggested that DTCHO were mainly of phytoplankton origin.Furthermore, a concomitant increase of DTCHO with peridinin and fucoxanthinindicated that dinoflagellates and diatoms had a decisive impact on CHOlevels in the water column. By contrast, early summer phytoplankton (June),which was dominated by prymnesiophytes, exhibited a comparatively lowerimpact on the CHO distribution.