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Human-induced Trophic Cascades and Ecological Regime Shifts in the Baltic Sea

Abstract

The ecosystems of coastal and enclosed seas are under increasing anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and the Black and Baltic Seas as well known examples. We use an ecosystem model (Ecopath with Ecosim, EwE) to show that reduced top-down control (seal predation) and increased bottom-up forcing (eutrophication) can largely explain the historical dynamics of the main fish stocks (cod, herring and sprat) in the Baltic Sea between 1900 and 1980. Based on these results and the historical fish stock development we identify two major ecological transitions. A shift from seal to cod domination was caused by a virtual elimination of marine mammals followed by a shift from an oligotrophic to a eutrophic state. A third shift from cod to clupeid domination in the late 1980s has previously been explained by overfishing of cod and climatic changes. We propose that the shift from an oligotrophic to a eutrophic state represents a true regime shift with a stabilizing mechanism for a hysteresis phenomenon. There are also mechanisms that could stabilize the shift from a cod to clupeid dominated ecosystem, but there are no indications that the ecosystem has been pushed that far yet. We argue that the shifts in the Baltic Sea are a consequence of human impacts, although variations in climate may have influenced their timing, magnitude and persistence.

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Acknowledgments

Drs S. Carpenter, D. Karl and two anonymous referees Provided valuable comments on the manuscript. We thank A. Bignert for assistance with Figure 1, T. J. Härkönen and K. Harding for access to seal data and comments on a previous draft, C. Harvey for providing the original EwE model and support, a wide range of marine institutes for contributing oxygen data (for example, the Finnish Institute of Marine Research; Estonian Marine Institute; Institute of Aquatic Ecology, University of Latvia; Lithuanian Maritime Safety Administration; Inst. of Meteorology and Water Management, Poland; Sea Fisheries Institute, Poland; Bundesamt für Seeschifffahrt und Hydrographie, Germany; National Environmental Research Institute; Denmark; the Swedish Meteorological and Hydrological Institute, and many others), the Swedish EPA (marine monitoring), and the Swedish Foundation for Strategic Environmental Research, Mistra for funding. SH also received funding from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, FORMAS, OH from the EU project FRAP (http://www.frap-project.net), and RE from the Swedish Research Council, VR.

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Correspondence to Henrik Österblom.

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Österblom, H., Hansson, S., Larsson, U. et al. Human-induced Trophic Cascades and Ecological Regime Shifts in the Baltic Sea. Ecosystems 10, 877–889 (2007). https://doi.org/10.1007/s10021-007-9069-0

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Keywords

  • eutrophication
  • fishing
  • marine mammal
  • predation
  • regime shift
  • trophic cascade