Impact of circulation on high phytoplankton blooms and fish catch in the northern Adriatic (1990–2004)

We analyzed oceanographic, hydrologic and atmospheric conditions in the northern Adriatic (section between the Po River delta and Rovinj) from 1990 to 2004 in which extreme phytoplankton blooms appeared. The largest blooms occurred in February, March, July and October, and were higher in surface layers and in the western, more eutrophic part of the region. They often appeared when the Istrian Coastal Countercurrent (ICCC; strong southward current indicating a “closure” of the northern Adriatic circulation system) was pronounced. They seem to be related to the intensities of surface fluxes and Po River discharge rates. The blooms can float over the northern Adriatic for a long period (up to 30 days; typical for March) and can appear simultaneously in separate circulation systems with a different species distribution. Finally, we present a hypothesis based on comparisons of geostrophic currents and fish stock that specific February northern Adriatic oceanographic conditions play the key role in the Adriatic anchovy stock.     

Dense water generation on a shelf: the case of the Adriatic Sea

The paper overviews recent and past studies of preconditioning, generation and spreading of North Adriatic Dense Water (NAdDW), by analysing both transient episodes and climatological data. The importance of wind stress, heat and water fluxes, and particularly river discharges during the preconditioning and generative period is emphasized, as well as the advection of saline levantine intermediate water from the southeast. After the generation, NAdDW affects deep and bottom layers of the middle and south Adriatic Sea, flowing as a dense current and mixing with the adjacent waters; it can be traced even in the Otranto Strait, contributing to the formation of deep water in the Eastern Mediterranean. Objective shortcomings and the projections in NAdDW investigations are also discussed, due to their high importance in the circulation of the Adriatic Sea, which may relate other similar basins around the world.     

Dense water characteristics in the northern Adriatic in the 1967–2000 interval with respect to surface fluxes and Po river discharge rates

Winter thermohaline properties of the northern Adriatic are analysed here with the aim of getting a better insight into dense water formation on the shelf. The hydrographic parameters collected in February in the 1967–2000 interval at two stations, the first located close to the eastern shore (station 1), and the second positioned near the Po river mouth (station 2), are compared. Two types of winter hydrographic conditions are distinguished: type A when bottom salinity and density are higher at station 1 than at station 2 and type B when these parameters are higher at station 2 than at station 1. Type A is more likely to occur in warmer and type B in colder winters. Both A and B distribution types can occur in periods when the Adriatic is under the influence of very saline waters of Mediterranean origin. Interannual changes in density are, at both stations, more dependant on haline than on thermal variations. At both stations temperature was somewhat higher in the early seventies than during the eighties and nineties, while salinity and density were lower in the early seventies and early nineties than in other years of the analysed period. By comparing the 1967–2000 changes in hydrographic conditions in February to monthly values of northern Adriatic surface fluxes and Po river discharge rates, it is shown that winter thermohaline characteristics in the region depend on processes which occur much earlier, i.e. during the previous autumn and late in spring of the preceding year, and even during the previous winter, 12 months before.