engleski

Ecological conditions and indicators of eutrophication in the small karstic estuary (Zrmanja, Adriatic sea)

The small, karstic Zrmanja River forms an shallow, highly stratified estuary. Hydrographic condition in the estuary are influenced by the river inflow. The tides in the area are weak. Distribution of phytoplankton was closely related to river infow, nutrient input, salinity gradients and water dynamics in the estuary. In the upper estuary, increased flow led to low salinities (0-10), phisical instability and low abundance and biomass of phytoplankton, above the halocline. The phytoplankton was much more abundant below the halocline. In the middle estuary, increased salinities and more stable water dynamics resulted in enhanced phytoplankton abundance and biomass in both marine and brackish layers. The phytoplankton was composed mainly of marine diatoms with lower contributions of cryptophytes, dinoflagellates, coccolithophorids and green algae. The surrounding area is without considerable anthropogenic influence, resulting in moderate phytoplankton abundance, low chlorophyll a and phosphorus concentrations, well-oxygenated and transparent water column, as well as low TRIX indexes (<4), indicating natural eutrophication and oligotrophic conditions. Abundant mixotrophic flagellates (cryptophytes, dinoflagellates) and ciliates might appear in late spring-summer, when nutrient concentrations were low, indicating metabolic mutualism which could allow existance of symbiothic community in the oligotrophic environment.

Estuary; eutrophication; phytoplankton; nutrients; chlorophyll a; transparency; TRIX; nanoflagellates; mixotrophic flagellates

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