engleski

Seasonal and spatial distribution of phytoplankton in the highly stratified estuary (Zrmanja, Adriatic Sea)

Small karstic Zrmanja river discharges into the eastern Adriatic Sea forming highly stratified estuary. Sharp halocline is the boundary between upper brackish and lower marine layer. Seasonal and spatial variations of phytoplankton were investigated microscopically in the period 1998-2004. Distribution of phytoplankton was regulated by riverine water inflow and nutrient concentrations. Significant correlations were found among salinity, nitrates, silicates and phosphates. One spring phytoplankton maximum and low concentration of nutrients only indicated oligotrophy of the estuary. Higher concentrations of phosphorous and ammonia, below the halocline, in the upper estuary, indicated high microbial regeneration. Low concentration of orthophosphates (average 0.06 µ ; ; ; ; mol L-1) limited phytoplankton development. Diatoms dominated in the phytoplankton community in February to April, while dinoflagelates and the nanoplankton dominated in July and October. Cluster and CCA analysis defined four phytoplankton communities which developed under various thermohaline conditions and nutrient concentrations. In the lower segment of the upper estuary, and in the middle estuary the environmental (salinity) conditions were stabile, providing development of abundant microphytoplankton below the halocline. The head of the estuary was characterized by unstable environmental condition and exclusive development of nanoplankton.

phytoplankton; highly stratified estuary; Zrmanja; Adriatic Sea

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