Mediterranean and Adriatic Sea; exchange of water and nutrients on the Palagruza sill (Middle Adriatic)  <B>Mediterranean and Adriatic Sea; exchange of water and nutrients on the Palagruza sill (Middle Adriatic)</B><P>I. Vukadin and L. Stojanoski</B><I><P> Institute of Oceanography and Fisheries, Split, Croatia</I><P> Abstract<P> Owing to its specific topographic properties, the Palagruza Sill acts as a natural barrier to both incoming and outgoing branches of the Adriatic cyclonic circulation. These conditions cause intensive mixing of waters of different origin, like those from the northern Adriatic and those from the southern Adriatic and Ionian Sea. Exchange between the Mediterranean and Adriatic waters is well manifested by the variations in the concentrations of nutrient salts. Bottom Ionian waters are considerably richer in nitrate (occasionally phosphate), thus enriching the Adriatic (McGill, 1963). At the same time, cooler and denser northern Adriatic water, or the Jabuka Pit water, carries large quantities of nutrients into this area.<P> This has strong implications on the chemical and biological properties of the central Adriatic waters. Surface layers of these waters are poor in nutrients, which is indicative of the high primary production, or better to say of an intensive consumption of these salts. Bottom layer concentrations of nutrients are much higher, with reduced oxygen content pointing to regenerative processes of these salts there. This is confirmed by a number of heterotrophic organisms recorded there recently, which are considerably higher than previous recordings. The mentioned factors point to some signs of eutrophication of the open central Adriatic waters.<P>