Northern Adriatic mesocosm experiment Rovinj 2003: oceanographic conditions at the sampling station

Background and purpose: A mesocosm experiment with nutrient enriched seawater was performed to verify if important production of particles, considered as mucilage precursors occur in the northern Adriatic in May, when semi-enclosed circulation is well developed and plankton growth with phosphorus limited. In this paper, the oceanographic and biological characteristics of the sampled area were described and evaluated by comparison with historical data. Materials and Methods: Geostrophic currents, temperature, salinity, density, oxygen saturation, nutrients, Chla, POC, DOC, microphytoplankton, and heterotrophic bacteria and nanoflagellates were determined by methods widely used in oceanography. Surface active (SAP) and gel particle (GeP) concentrations were measured electrochemically. Results and conclusions: An anticyclonic gyre was observed in May 2003 in the open northern Adriatic, including the sampling station, in which freshened waters were retained. In these conditions nutrient, chlorophyll a concentrations, as well as phytoplankton and bacterial abundances did not differ essentially from long term averages, in spite of extremely low Po River discharge rates. In contrast, the content of SAP, and probably of GeP as well, was exceptionally high. All identified microphytoplankton species are often present in May at the sampling station. The diatom Dactyliosolen fragilissimus highly dominated the microphytoplankton community in the sea