engleski

Distribution of Cyanobacteria in highly stratified estuary (The Zrmanja estuary, Croatia)

Cyanobacteria, nano- and microphytoplankton abundance, and pigment analysis were determined in relation to hydrographic conditions in the highly stratified, karstic Zrmanja Estuary, eastern Adriatic coast. Water samples were collected at seven stations along the thermohaline gradient during June 2002. Cyanobacteria were counted using epifluorescence microscope, while nano- and microphytoplankton cells were counted using inverted microscope. Pigment concentrations were determined by HPLC method. Salinity and temperature were determined using a conductivity, temperature and depth profiler, while chemical parameters were determined using standard oceanographic methods. Maximum abundance and biomass of cyanobacteria were detected in the area where marine influences decrease. The observations performed by epifluorescence microscopy mostly showed the presence of class Chroococcales which are distributed in plankton community in both environments, freshwater and marine. During this case study, the maximum cyanobacteria abundance (1.8 x 104 cells mL-1) was detected in the brackish environment of the upper estuary. Cyanobacteria abundances were much lower in the lower estuary, where salinity was increased. In the middle, wider part of the estuary the green flagellates were abundant (2.5 x 105 cells L-1). Phytoplankton pigments are increasingly used as markers of phytoplankton groups. Cyanobacteria are characterized by pigment zeaxanthin although chlorophytes and prasinophytes also contain a small amount of this pigment. Lutein is another pigment which is typical for chlorophytes. Using HPLC methodology is difficult to separate difficult carotenoid pairs such as lutein and zeaxanthin. In this study, increased zeaxanthin/lutein concentrations were attributed to cyanobacteria but not to green flagellates in the estuary. Chlorophyll a concentrations well agree with microphytoplankton and nanophytoplankton abundance pattern.

highly stratified estuary; cyanobacteria; Zrmanja

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