A coupled model of lower trophic levels and anchovy Engraulis encrasicolus ponticus population dynamics was developed to analyze the mechanisms controlling sharp anchovy and gelatinous zooplankton biomass transitions from the 1960s to the 1980s in the Black Sea. An increase in anchovy stocks from estimated low (similar to 300 kt) to moderate (similar to 700 kt) in the late 1960s was related to weakening piscivore predation pressure, slight nutrient enrichment of the basin during an early eutrophication phase, and competitive exclusion of gelatinous carnivores. The transition to high stocks (similar to 1500 kt) from 1979 to 1980 was caused by additional nutrient enrichment. With enhanced enrichment, gelatinous carnivores started to coexist with anchovy at low biomass levels (<1.0 gC m(-2)), but they did not yet exert a strong control on anchovy because of their competitive disadvantage of consuming prey at low carrying capacity. The third transition (1989-1990) returned the anchovy stock to the low regime and increased the biomass of the alien gelatinous species Mnemiopsis leidyi (hereafter Mnemiopsis) to 3.0 gC m(-2). The anchovy-Mnemiopsis shift was pre-conditioned by nutrient accumulation in the subsurface layer and triggered by their more effective transport into the productive surface layer following the switch of regional climate into a severe winter phase during 1985-1987. The resulting enhanced resource carrying capacity, together with decreasing adult anchovy stocks, led to a competitive advantage of Mnemiopsis in food exploitation relative to anchovy, growth and reproductive advantages relative to the native gelatinous species Aurelia aurita, and stronger predation on anchovy eggs and larvae. The anchovy stock depletion was caused by increasing fishing pressure and by competition with and predation by Mnemiopsis. While nonlinear coupling of these 2 independent processes amplified the anchovy collapse, neither would be able to individually impose such a severe anchovy stock change under the observed environmental conditions of the Black Sea.