Interdisciplinary Studies Integrating The Black Sea Biogeochemistry and Circulation Dynamics

The Black Sea is a deep (about 2 km) elliptic basin with zonal and meridional dimensions of approximately 1000 km and approximately 400 km, respectively, located roughly between 28 ° and 42°E longitudes, 41 ° and 46°N latitudes (Figure 1). It has only a narrow opening to the shallow (<75 m deep) Bosphorus Strait; otherwise, it is a completely enclosed marginal sea. Until the early 1970s, the Black Sea consisted of a mosaic of diverse ecosystems that provided a vital habitat for many commercial species. It supported fisheries almost five times richer than those of neighboring Mediterranean. Then, it has been gradually transformed into a severely degraded marine habitat suffering from the introduction of large volumes of nutrients and contaminants from the Danube River and others along the northwestern coast (Mee, 1992). The state of the ecosystem after the mid-1970s reflected severe environmental deterioration, intense eutrophication, dramatic decreases in biodiversity and fish stocks, especially in the northwestern shelf and the Sea of Azov (Zaitsev and Mamaev, 1997). The long-term massive fertilization of the sea by nutrients from domestic, agricultural and industrial sources has caused a substantial modification of the phytoplankton community structure and succession, as well as an increase in the intensity, frequency and expansion of microalgal blooms. The total phytoplankton biomass has increased by an order of magnitude along the western coast, and about 3-4 times within the interior. Individual blooms have been documented as becoming more and more monospecific. From the food web point of view, the eutrophic Black Sea ecosystem has been producing more biomass than it used to three to four decades ago. But its taxonomic composition has changed, and is mostly dominated by opportunistic species and gelatinous carnivores acting as top predators and deadend of the food chain. Total biomass of mesozooplankton species was decreased by a factor of five, while the abundance of opportunistic species (Noctiluca, Pleurobrachia, jelly fish Aurelia aurita, and ctenophore Mnemiopsis leidyi) reached 90% of the total zooplankton wet weight (Vinogradov et al., 1999; Kideys et al., 2000). First, Aurelia during the late 1970s and 1980s, and later Mnemiopsis during the late 1980s consumed almost all zooplankton groups, which were essential components in the marine food web. Because Aurelia and Mnemiopsis had no predators in the Black Sea, their communities quickly dominated the entire ecosystem, and changed its structure and, together with overfishing, contributed to the collapse of the fishery in the late 1980s. The situation has become so severe that it has affected the health, well-being and standard of living of nearly 160 million people in the region. Economic losses exceeded $500 million per year, as estimated by the World Bank. The sharp density stratification, accompanied with weak vertical circulation and mixing as well as limited lateral influxes, inhibit ventilation of sub-pycnocline waters of the Black Sea from the surface. Organic matter, continually sinking and decomposing, has thus led to development of a permanent anoxia and high concentrations of hydrogen sulfide below a depth of 100-150 m during approximately the last 7000 years. The Black Sea vertical biogeochemical structure thus differs significantly from its typical oceanic counterparts. The complex prey-predator interactions between different phytoplankton and zooplankton groups are tightly linked with a very efficient remineralization-