Long-term (1960Long-term ( -2005 ecological data were used to identify a regime shift in the northwestern Black Sea shelf and to assess whether it has recovered from its former eutrophic state. Following the collapse of fish stocks and the population explosion of the ctenophore Mnemiopsis leidyi between 1988 and 1991, anthropogenic P-PO 4 loads from the River Danube dropped strongly in 1992-1993. This decline in P-PO 4 levels was caused by reductions in fertilizer use and emissions from land-based point sources during the economic recession of former Eastern Bloc countries, as well as by the low discharge rate of the River Danube. Commencing in 1993, the phosphate limitation apparently maintained a low-energy, inefficient food web dominated by the dinoflagellate Noctiluca scintillans and jellyfish, and relatively low levels of phytoplankton, bacterioplankton, mesozooplankton and fish. This 'post-eutrophication' regime was markedly different from the classical phytoplankton-mesozooplankton-fish chain of the similarly low nutrient 'pre-eutrophication' regime prior to 1970. Therefore it appears that the food web can attain 2 alternative regimes during periods of low productivity of the ecosystem. The post-eutrophication state cannot be considered as a major improvement or restoration of the northwestern coastal ecosystem.
Time series of phytoplankton density, biomass and species composition in the Bulgarian Black Sea (specifically Varna Bay) are analysed. The main patterns of year-to-year fluctuations in the phytoplankton community controlled by natural and man-induced factors are discussed. Many indications lead to the conclusion that phytoplankton blooms have not become less frequent in comparison with the period of progressive eutrophication (1975-85), and there is a trend towards an increase in the number of blooming species involved. The community maintains the capacity to produce a lot of biomass. Areas of high productivity occur mainly in coastal waters. In open sea areas the level of algal biomass is higher than in the 80s. Irrespective of low silicate concentrations, in general the 90s are the period of diatom predomination.
Since the early 1990s the persistent reports about the irreversibly degrading Black Sea have started to disappear. A large set of data, reflecting the temporal and spatial variability of microalgae and hydrochemical parameters in Western Black Sea waters (WBS), collected in 2001-2002 (EU 5th FP project "daNubs"), allows us to compare the present-day ecosystem functioning to previous ones and to certainly conclude on positive signs of relaxation. The years 2001 and 2002 were without ample, negative biological and hydrochemical events in the WBS. Nutrient general trend was a decreasing one since the late 1980s, and inorganic phosphorus and nitrogen concentrations were frequently below the method detection limits during summer autumn periods of 2001-2002. The shelf waters were properly oxygenated. The phytoplankton community was producing biomass in a decreasing tendency, especially obviously since 1995. Simultaneously, there was an increase in Si:P and Si:N molar ratios and concurrent increase in the diversity of mass algal species (mainly diatoms) but only a few of them generated exceptional blooms. Reduction of the ratios between phytoplankton and zooplankton biomasses became evident. The observed ecological status in 2001-2002 confirms that the Black Sea is no longer a reference point for progressive water quality deterioration.
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