Oguz¹, Temel, Tulay Cokacar¹, Paola Malanotte-Rizzoli² and Hugh W. Ducklow³

¹Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey, Tel: 90 324 521 2150, E-mail: oguz@ims.metu.edu.tr, ²Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA and ³Virginia Institute of Marine Sciences, The College of William and Mary, Gloucester Point, VA, USA

Climate-induced decadal warming and accompanying changes in the ecological regime of the Black Sea

The Black Sea ecosystem is shown to experience abrupt shifts in its all trophic levels from primary producers to apex predators in 1996 as a manifestation of concurrent changes in its physical climate. The warming is evident in the annual-mean sea surface temperature (SST) data in the form of a gradual and continuous rise at a rate of ~0.25^oC per year, following an intensive cooling phase in 1991-1993. The most intense warming event took place during winters of 1994-1996 period. It led to an almost 2^oC rise in the winter-mean SST, and triggered the changes in the ecosystem The subsurface signature of warming is marked by a gradual depletion of the Cold Intermediate Layer (characterized by T<8^oC) throughout the basin during the same period. The warming particularly dominated the eastern basin where the winter mean temperatures exceed those of the colder western part by about 3^oC.

The mild winters during this a decade-long warming event, particularly its 1994-1994 intense warming phase, are characterized by weaker vertical turbulent mixing, stronger stratification, and reduced upward nutrient supply from the nutricline. From 1996 onwards, the major late winter-early spring peak of the classical annual phytoplankton biomass structure observed prior to mid-90s was then either weakened or disappeared all together, depending on local meteorological and oceanographic conditions during each of these years. The new annual phytoplankton structure possesses a gradual decrease of biomass from a peak in November towards summer and winter-spring months on its both side. The autumn bloom, albeit weak, thus lasts longer as compared to its counterparts before the mid-90s. It is mainly supported by regenerated production fueled by organic matter supplied as a by-product of the seasonally enhanced gelatinous carnivore activity from summer to early winter. These bottom-up limited type unfavorable growth conditions introduced similar marked changes in abundances and seasonal distributions of mesozooplankton, gelatinous macrozooplankton and pelagic fish groups. Many higher trophic level species then acquired reduced stocks after 1995.