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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 3, 1088, doi:10.1029/2003GB002031, 2003

Climatic warming and accompanying changes in the ecological regime of the Black Sea during 1990s

Temel Oguz

Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey

Tulay Cokacar

Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey

Paola Malanotte-Rizzoli

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Hugh W. Ducklow

Virginia Institute of Marine Sciences, College of William and Mary, Gloucester Point, Virginia, USA

Abstract

The Black Sea ecosystem is shown to experience abrupt shifts in its all trophic levels from primary producers to apex predators in 1995–1996. It arises as a manifestation of concurrent changes in its physical climate introduced by intensive warming of its surface waters as well as abrupt increases in the mean sea level and the net annual mean fresh water flux. The warming is evident in the annual-mean sea surface temperature (SST) data by a continuous rise at a rate of ∼0.25°C per year, following a strong cooling phase in 1991–1993. The most intense warming event with ∼2°C increase in the SST took place during winters of the 1994–1996 period. It also coincides with 4 cm yr⁻¹ net sea level rise in the basin, and substantial change in the annual mean net fresh water flux from 150 km³ yr⁻¹ in 1993 to 420 km³ yr⁻¹ in 1997. The subsurface signature of warming is marked by a gradual depletion of the Cold Intermediate Layer (characterized by T < 8°C) throughout the basin during the same period. Winters of the warming phase are characterized by weaker vertical turbulent mixing and upwelling velocity, stronger stratification and, subsequently, reduced upward nutrient supply from the nutricline. From 1996 onward, the major late winter-early spring peak of the classical annual phytoplankton biomass structure observed prior to mid-90s was, therefore, either weakened or disappeared altogether depending on local meteorological and oceanographic conditions during each of these years. The effect of bottom-up limited unfavorable phytoplankton growth is reflected at higher trophic levels (e.g., mesozooplankton, gelatinous macrozooplankton, and pelagic fishes) in the form of their reduced stocks after 1995.

Received 1 January 2003; accepted 31 July 2003; published 27 September 2003.

Index Terms: 1635 Global Change: Oceans (4203); 4215 Oceanography: General: Climate and interannual variability (3309); 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics.

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Citation: Oguz, T., T. Cokacar, P. Malanotte-Rizzoli, and H. W. Ducklow (2003), Climatic warming and accompanying changes in the ecological regime of the Black Sea during 1990s, Global Biogeochem. Cycles, 17(3), 1088, doi:10.1029/2003GB002031.

Copyright 2003 by the American Geophysical Union.