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Regime shifts are characterized by sudden, substantial and temporally persistent 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 zooplankton 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 propose 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.

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Impermanence is an ecological principle 1 involving changes that can sometimes occur non-linearly as Abrupt Community Shifts (ACSs) to transform ecosystem states and the goods and services they provide 2. Here, we present a model based on niche theory 3 to explain and predict ACSs at the global scale. We test our model using 14 multi-decadal time series of marine metazoans from zooplankton to fish, spanning all latitudes and the shelf to the open ocean. Predicted and observed fluctuations correspond, with both identifying ACSs at the end of the 1980s 4-7 and 1990s 5,8. We show that these ACSs coincide with changes in climate that alter local thermal regimes, which in turn interact with the thermal niche of species to trigger long-term and sometimes abrupt shifts at the community level. A large-scale ACS is predicted after 2014-unprecedented in magnitude and extent-coinciding with a strong El Niño event and major shifts in Northern Hemisphere climate. Our results underline the sensitivity of the Arctic Ocean, where unprecedented melting may reorganize biological communities 5,9 and suggest an increase in the size and consequences of ACS events in a warming world. Main text The processes that cause long-term changes and Abrupt Community Shifts (ACSs) in ecosystems are poorly understood despite decades of research 2,4,10-12. We define an ACS as a stepwise shift in community structure 12 , a definition that does not necessarily imply the existence of stable states 2,10 ,

Little is known concerning environmental factors that may control the distribution of virioplankton on large spatial scales. In previous studies workers reported high viral levels in eutrophic systems and suggested that the trophic state is a possible driving force controlling the spatial distribution of viruses. In order to test this hypothesis, we determined the distribution of viral abundance and bacterial abundance and the virus-tobacterium ratio in a wide area covering the entire Adriatic basin (Mediterranean Sea). To gather additional information on factors controlling viral distribution on a large scale, functional microbial parameters (exoenzymatic activities, bacterial production and turnover) were related to trophic gradients. At large spatial scales, viral distribution was independent of autotrophic biomass and all other environmental parameters. We concluded that in contrast to what was previously hypothesized, changing trophic conditions do not directly affect virioplankton distribution. Since virus distribution was coupled with bacterial turnover times, our results suggest that viral abundance depends on bacterial activity and on host cell abundance.Viruses are the most abundant dynamic component among the microorganisms in the surface waters of the world's oceans. The distribution of virus abundance has been examined in many locations and habitats worldwide. Viral counts from all sorts of environments (coastal, offshore, deep sea, and tropical to polar latitudes) have been found to range from 10 4 particles ml Ϫ1 in oligotrophic systems to over 10 8 particles ml Ϫ1 in eutrophic systems (30).Over the past decade, much effort has been devoted to improving virus quantification and to obtaining a better understanding of the ecological role of this component in the biogeochemical cycling of carbon (10, 18) and nutrients in marine systems (28). However, most studies have been focused on phage-host cell interactions and have been confined to experimental studies or small-scale field investigations. Information on viral and bacterial distribution on a large (i.e., basin or regional) scale based on synoptical samples is practically nonexistent.The problem of the spatial scale is crucial for understanding which environmental factors influence viral distribution (6). The available data indicate that viral abundance decreases with water depth (2, 6, 13), and there is increasing evidence that physical and chemical changes (water temperature or salinity gradients) can also play a role in viral abundance and distribution (27). As a consequence, information on the relationship between hydrological features and viral distribution is needed for a predictive understanding of viral development in response to environmental changes. A reexamination of bacterial abundance data and viral abundance data collected synoptically over small spatial scales indicated that these two components are significantly correlated (30). However, studies examining larger data sets (e.g., studies based on regression analysis of reported va...