Multi-level oscillating trophodynamic control causes regime shifts in large marine ecosystem

Background/Question/Methods

Changes in trophodynamic control from predominantly bottom-up (resource-driven) to top-down (consumer-driven) have been suggested as a mechanism causing sudden changes in the structure and functioning of foodwebs, so called ecosystem regime shifts. Trophic cascades represent top-down controls and conspicuous indirect effects over two or more links distant from the initial one. They are the most pronounced phenomenon related to changes in trophodynamic control and have been shown for multiple marine ecosystems. Fewer studies show oscillations between bottom-up and top-down control and do usually not consider interactions between all trophic levels. We studied long-term variability in trophodynamic control in the North Sea, one of the most productive ecosystems in the world ocean. For this purpose we used a unique multi-trophic level data set covering > 4 decades (1963 – 2007) and four trophic levels, i.e. phyto-, zooplankton, planktivorous and piscivorous fish. We analysed the data set using moving correlation analyses and Generalized Addtitive Modelling (GAM).

Results/Conclusions

Our analyses provide evidence for oscillating trophodynamic controls, i.e. alternating changes in the dominance of either bottom-up or top-down control between trophic levels. These oscillations strikingly coincide with major ecosystem regime shifts that have been documented for the North Sea during the late 1970s and late 1980s. We furthermore demonstrate using Generalized Additive Modelling that oscillations in controls and hence ecosystem regime shifts are caused by complex interactions between climatic changes and unsustainable exploitation, i.e. fishing levels.