Large-scale transitions between alternative states in ecosystems are known as regime shifts. Once described as healthy and dominated by various marine predators, the Black Sea ecosystem by the late 20th century had experienced anthropogenic impacts such as heavy fishing, cultural eutrophication, and invasions by alien species. We studied changes related to these ''natural experiments'' to reveal the mechanisms of regime shifts. Two major shifts were detected, the first related to a depletion of marine predators and the second to an outburst of the alien comb jelly Mnemiopsis leidyi; both shifts were triggered by intense fishing resulting in system-wide trophic cascades. The complex nature of ecosystem responses to human activities calls for more elaborate approaches than currently provided by traditional environmental and fisheries management. This implies challenging existing practices and implementing explanatory models of ecosystem interactions that can better reconcile conservation and ecosystem management ideals.Black Sea ͉ disturbance ͉ foodweb control ͉ invasive species ͉ top predators

During recent decades, environmental conditions have deteriorated in the Black Sea. Population explosions of phytoplankton and jellyfish have become frequent and several fish stocks have collapsed. In this study, literature sources and long-term data are explored in order to find empirical evidence for ecosystem effects of fishing. Inverse trends of decreasing predators, increasing planktivorous fish, decreasing zooplankton and increasing phytoplankton biomass are revealed. Increased phytoplankton biomass provoked decreasing transparency and nutrient content in surface water. A massive development of jellyfish during the 1970s and 1980s had a great impact on consumption and consequent decrease in zooplankton. The turning point for these changes occurred in the early 1970s, when industrial fishing started and stocks of pelagic predators (bonito, mackerel, bluefish, dolphins) became severely depleted. A 'trophic cascade' is invoked as a mechanism to explain observed changes. According to this hypothesis, reduction in apex predators decreases consumer control and leads to higher abundance of planktivorous fish. The increased consumption by planktivorous fish causes a consequent decline in zooplankton biomass, which reduces grazing pressure on phytoplankton and allows its standing crop to increase. The effects of fishing and eutrophication are explored using a dynamic mass-balance model. A balanced model is built using 15 ecological groups including bacteria, phytoplankton, zooplankton, protozoa, ctenophores, medusae, chaetognaths, fishes and dolphins. Ecosystem dynamics are simulated over 30 yr, assuming alternative scenarios of increasing fishing pressure and eutrophication. The changes in simulated biomass are similar in direction and magnitude to observed data from long-term monitoring. The cascade pattern is explained by the removal of predators and its effect on trophic interactions, while the inclusion of eutrophication effects leads to biomass increase in all groups. The present study demonstrates that the combination of uncontrolled fisheries and eutrophication can cause important alterations in the structure and dynamics of a large marine ecosystem. These findings may provide insights for ecosystem management, suggesting that conserving and restoring natural stocks of fish and marine mammals can contribute greatly to sustaining viable marine ecosystems.

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It is well known that human activities, such as harvesting, have had major direct effects on marine ecosystems. However, it is far less acknowledged that human activities in the surroundings might have important effects on marine systems. There is growing evidence suggesting that major reorganization (i.e., a regime shift) is a common feature in the temporal evolution of a marine system. Here we show, and quantify, the interaction of human activities (nutrient upload) with a favourable climate (run-off) and its contribution to the eutrophication of the Black Sea in the 1980s. Based on virtual analysis of the bottom-up (eutrophication) vs. top-down (trophic cascades) effects, we found that an earlier onset of eutrophication could have counteracted the restructuring of the trophic regulation at the base of the food web that resulted from the depletion of top predators in the 1970s. These enhanced bottom-up effects would, however, not propagate upwards in the food web beyond the zooplankton level. Our simulations identified the removal of apex predators as a key element in terms of loss of resilience that inevitably leads to a reorganization. Once the food web has been truncated, the type and magnitude of interventions on the group replacing the apex predator as the new upper trophic level have no effect in preventing the trophic cascade. By characterizing the tipping point at which increased bottom-up forcing exactly counteracts the top-down cascading effects, our results emphasize the importance of a comprehensive analysis that take into account all structuring forces at play (including those beyond the marine system) at a given time.