Long-term evolution and trends of the hydrological and hydrochemical parameters in Bulgarian Black Sea waters during the period 1992–2000

The dynamics of chemical parameters in the Bulgarian Black Sea during the 1990s reflects the complex relations in the ecosystem itself and the influence of the Danube water discharge, which is a major climatic and anthropogenic factor for the Western Black Sea. Analyses of hydrological (temperatures, salinity) and hydrochemical (dissolved oxygen, oxygen saturation, nitrite nitrogen, nitrate nitrogen) data collected during the period 1992–2000 in the 30-miles zone offshore along the Bulgarian Black Sea coast were carried out in the framework of the “DANUBS” project. In the period 1995–2000 gradually the winters were becoming warmer, the springs colder and the summers were short and hot. The long-term averages show spatially a minimum of salinity in front of the Cape Galata at 10 miles offshore, whereas in front of the Cape Emine the salinity increases gradually from the coast towards the 30-miles zone offshore. In the late 1990s very low summer values or even complete absence of inorganic nitrogen in the Bulgarian Black Sea were registered. Seasonally the oxygen varied in broad terms, however on average the surface waters were saturated or slightly oversaturated with oxygen. There was a regular decrease in oxygen concentrations with depth.