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Phosphatase enzymes are capable of releasing phosphate through cleavage of phosphoester bonds. The seasonal importance of this process was examined by using a model substrate paranitrophenylphosphate and the Michaelis-Menten equation to estimate the release rate of PO4-P from phosphomonoesters. The seasonal occurrence of phosphomonoesters and acid phosphatase activity was used to estimate the velocity of phosphate release from these compounds. Filter fractionation of phosphatase activity demonstrated most activity (> 60 %) was in size fractions less than 0.45 μm. The release rates were highest in May and June (15 to 25 nmol L-1 min -1) during the Gonyostomum semen (Raphidophyta) bloom and decreased to less than 2 nmol L-1 min-1 in two weeks and remained low throughout the summer and the fall. Fractionation of 32P-H3PO4 labelled dissolved organic phosphorus showed this fraction to vary considerably through the year. Potential phosphate release declined through the summer and into the fall. Significance of the co-occurrence of phosphomonoesters and acid phosphatase activity maxima and Gonyostomum semen (Raphidophyta) bloom is discussed.

The abundance of phytoplankton, thermohaline conditions and concentration of nutrients were determined in the highly stratified Zrmanja Estuary, eastern Adriatic coast, Croatia, in the period from June 1998 to October 2000.

Periphyton assemblages (algal biofilms) can be important factor for majority of production in shallow, unshaded, coastal aquatic habitats. The karstic river Zrmanja (69 km) discharges into the eastern–central Adriatic Sea, and forms shallow, highly stratified estuary. The distribution of attached diatoms on artificial substrates and those in plankton was analyzed in the upper reach of the estuary in July 2000. The sharp halocline was detected in the 1–1.5 m layer. The brackish layer above the halocline (14 PSU), and marine layer (30-34 PSU) bellow the halocline was detected. Thermic conditions (19 to 24 şC) were characterized by subsurface temperature maximum in the halocline. The extremely low concentrations of nutrients (orthophosphates 0.02-0.06 mmol L-1, nitrates 5.3-7.6 mmol L-1), and high oxygen saturation (80-90%) indicated oligothrophic character of the estuary. The euphotic layer extended to the bottom (4 m deep). The maximum abundance of penatae diatoms (9.6 x 107 cells cm–2) was registered after 16 days of colonization in the upper reach of the estuary. The similar abundance was found after 28 days of colonization. The attached penatae diatom Cocconeis scutellum reached the abundance of 105 cells cm-2 after 16 days, while 3.8 x 105 cells cm-2 after 28 days of colonization, in the brackish layer. The local anthropogenic input of nutrients resulted in the 11% and 15% greater abundance of total attached pennatae diatoms, and C. scutellum, respectively. In plankton, C. scutellum was mostly distributed in the 1-3 m layer (400-6400 cells L-1). The outflowing current above the halocline detached cells from the substrate and transported them down the estuary. The population successively sank and accumulated around the halocline, died in the layer of strong gradient of salinity and continued to sink to the bottom. Due to transport by currents, suspended C. scutellum cells were 30% more abundant at the 6 km downstream station. The population was not detectable in the middle reach of the estuary where salinity increased to 34.51 PSU.