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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on February 1, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(3):302-310; doi:10.1093/icesjms/fsm197
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© 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feedback loops in a simple ecosystem

Christian Möllmann1, Bärbel Müller-Karulis2, Georgs Kornilovs3 and Michael A. St John1

1 Institute for Hydrobiology and Fisheries Science, University of Hamburg, Grosse Elbstr. 133, Hamburg D-22767, Germany
2 Latvian Institute of Aquatic Ecology, Daugavgrivas Street 8, LV-1048 Riga, Latvia
3 Latvian Fish Resources Agency, Daugavgrivas Street 8, LV-1007 Riga, Latvia

Correspondence to C. Möllmann: tel: +49 40 42838 6621; fax: +49 40 42838 6618; e-mail: christian.moellmann{at}uni-hamburg.de

Möllmann, C., Müller-Karulis, B., Kornilovs, G., and St John, M. A. 2008. Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feeback loops in a simple ecosystem. – ICES Journal of Marine Science, 65: 302–310.

The Central Baltic Sea is the largest brackish waterbody in the world ocean, containing a highly productive but low-diversity ecosystem. Climate-induced changes in hydrography recently caused an ecosystem regime shift with changes at all trophic levels. The most pronounced changes in the ecosystem occurred at the zooplankton and fish trophic levels. In the zooplankton, dominance changed between the copepods Pseudocalanus acuspes and Acartia spp., a result of reduced salinities and increased temperatures. The change in hydrography also affected the reproductive success of the major fish species, resulting in a change in dominance from the piscivorous cod (Gadus morhua) to the planktivorous sprat (Sprattus sprattus). First, we investigate statistically the occurrence of regime shifts in time-series of key hydrographic variables and the biomass time-series of key species. Second, we demonstrate a three-level trophic cascade involving zooplankton. Finally, we model the ecosystem effects of the abiotic and biotic changes on copepod biomass and recruitment of fish stocks. Our results demonstrate the linkage between climate-induced zooplankton and fish regime changes, and how overfishing amplified the climate-induced changes at both trophic levels. Hence, our study demonstrates (i) the multiple pathways along which climatic and anthropogenic pressures can propagate through the foodweb; (ii) how both effects act synergistically to cause and stabilize regime changes; and (iii) the crucial role of zooplankton in mediating these ecosystem changes.

Keywords: Acartia spp., Baltic Sea, climate, cod, overfishing, prey-to-predator loops, Pseudocalanus acuspes, regime shifts, sprat, trophic cascade

Received 8 July 2007; accepted 10 November 2007; advance access publication 1 February 2008.


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