Abrupt transition of the northwestern Black Sea shelf ecosystem from a eutrophic to an alternative pristine state

Abstract

Long-term (1960-2005) ecological data were used to identify a regime shift in the north- western Black Sea shelf and to assess whether it has recovered from its former eutrophic state. Fol- lowing the collapse of fish stocks and the population explosion of the ctenophore Mnemiopsis leidyi between 1988 and 1991, anthropogenic P-PO4 loads from the River Danube dropped strongly in 1992-1993. This decline in P-PO4 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, mesozoo- plankton 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.