Edwards, M.

Regime shifts are characterized by sudden, substantial and temporally persist- ent changes in the state of an ecosystem. They involve major biological modifications and often have important implications for exploited living resources. In this study, we examine whether regime shifts observed in 11 marine systems from two oceans and three regional seas in the Northern Hemisphere (NH) are synchronous, applying the same methodology to all. We primarily infer marine pelagic regime shifts from abrupt shifts in zooplank- ton assemblages, with the exception of the East Pacific where ecosystem changes are inferred from fish. Our analyses provide evidence for quasi-synchronicity of marine pelagic regime shifts both within and between ocean basins, although these shifts lie embedded within considerable regional variability at both year-to-year and lower-frequency time scales. In particular, a regime shift was detected in the late 1980s in many studied marine regions, although the exact year of the observed shift varied somewhat from one basin to another. Another regime shift was also identified in the mid- to late 1970s but concerned less marine regions. We subsequently analyse the main biological signals in relation to changes in NH temperature and pressure anomalies. The results suggest that the main factor synchronizing regime shifts on large scales is NH temperature; however, changes in atmospheric circulation also appear important. We pro- pose that this quasi-synchronous shift could represent the variably lagged biological response in each ecosystem to a large-scale, NH change of the climatic system, involving both an increase in NH temperature and a strongly positive phase of the Arctic Oscillation. Further investigation is needed to determine the relative roles of changes in temperature and atmospheric pressure patterns and their resultant teleconnections in synchronizing regime shifts at large scales.

The global increase in atmospheric carbon dioxide concentration is potentially threatening marine biodiversity in two ways. First, carbon dioxide and other greenhouse gases accumulating in the atmosphere are causing global warming. Second, carbon dioxide is altering sea water chemistry, making the ocean more acidic. Although temperature has a cardinal influence on all biological processes from the molecular to the ecosystem level, acidification might impair the process of calcification or exacerbate dissolution of calcifying organisms. Here, we show however that North Atlantic calcifying plankton primarily responded to climate-induced changes in temperatures during the period 1960–2009, overriding the signal from the effects of ocean acidification. We provide evidence that foraminifers, coccolithophores, both pteropod and non-pteropod molluscs and echinoderms exhibited an abrupt shift circa 1996 at a time of a substantial increase in temperature and that some taxa exhibited a poleward movement in agreement with expected biogeographical changes under sea temperature warming. Although acidification may become a serious threat to marine calcifying organisms, our results suggest that over the study period the primary driver of North Atlantic calcifying plankton was oceanic temperature.