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

Abstract

Distribution of carbohydrates (CHO) and photosynthetic pigments were studied in the Gulf of Trieste, northern Adriatic Sea, during the period of summer stratification with a special emphasis on determining the impact of the taxonomic composition and concentration of phytoplankton biomass on the carbohydrate levels in the water column. Dissolved total carbohydrates (DTCHO), dissolved monosaccharides (DMCHO) as well as particulate carbohydrates (PTCHO) were determined using the colorimetric MBTH-method, while pigment biomarkers of the phytoplankton biomass were determined by reversed-phase HPLC. Concentrations of the total CHO (dissolved+particulate) varied in a wide range from 173 μg C1−1 to 1552 μg C 1−1. The percentage of PTCHO in the total CHO concentration was relatively low (4–25%), indicating that the main pool of CHO was in the dissolved phase. The contribution of DTCHO to the total dissolved organic carbon (DOC) in late summer was highly variable (10–65%) with an average value of 20 ± 14%, while in early summer this percentage was somewhat lower and less variable (range 11–23%; average 17 ± 3%). Analyses of biomarker pigments revealed a very high diversity and a rather heterogenous vertical and spatial distribution of the phytoplankton biomass during the period of summer stratification. In September 1994, the predominant taxonomic groups of phytoplankton were prymnesiophytes, diatoms, silicoflagellates, cyanobacteria and, especially in the bottom layer, dinoflagellates. A relatively good correlation (r 2 = 0.51) found between DTCHO and chl a suggested that DTCHO were mainly of phytoplankton origin. Furthermore, a concomitant increase of DTCHO with peridinin and fucoxanthin indicated that dinoflagellates and diatoms had a decisive impact on CHO levels in the water column. By contrast, early summer phytoplankton (June), which was dominated by prymnesiophytes, exhibited a comparatively lower impact on the CHO distribution.