Remote climate forcing of decadal-scale regime shifts in Northwest Atlantic shelf ecosystems

Decadal-scale regime shifts in Northwest Atlantic shelf ecosystems can be remotely forced by climateassociated atmosphere–ocean interactions in the North Atlantic and Arctic Ocean Basins. This remote climate forcing is mediated primarily by basinand hemispheric-scale changes in ocean circulation. We review and synthesize results from process-oriented field studies and retrospective analyses of time-series data to document the linkages between climate, ocean circulation, and ecosystem dynamics. Bottom-up forcing associated with climate plays a prominent role in the dynamics of these ecosystems, comparable in importance to that of topdown forcing associated with commercial fishing. A broad perspective, one encompassing the effects of basinand hemispheric-scale climate processes on marine ecosystems, will be critical to the sustainable management of marine living resources in the Northwest Atlantic. An ecosystem regime shift occurs when some perturbation leads to an abrupt restructuring of the ecosystem from one relatively persistent, quasi-equilibrium state to another (Scheffer et al. 2001; Bakun 2005; DeYoung et al. 2008). In large marine ecosystems, regime shifts have often been associated with bottom-up forcing by climate, top-down forcing by commercial fishing, or some combination of the two (DeYoung et al. 2004; Steele 2004). Determining the relative roles of climate and fishing in forcing such regime shifts can be challenging because of our limited understanding of marine ecosystem dynamics and their responses to both natural and anthropogenic perturbations. Here, we focus our attention on the role of climate in forcing decadal-scale regime shifts in Northwest Atlantic shelf ecosystems. Specifically, we explore the processes linking the dynamics of these ecosystems to two sources of highlatitude climate variability and change, one associated with climate forcing within the North Atlantic Basin and the other associated with climate forcing from the Arctic. We conclude by discussing the implications of our findings for the management of marine living resources within these ecosystems during the coming decades. Table 1 lists acronyms used in the present paper. Shelf ecosystem responses to climate forcing within the North Atlantic Basin In the North Atlantic Ocean, the major basin-scale mode of interannual to inter-decadal climate variability is the North Atlantic Oscillation (NAO) (Hurrell et al. 2003; Hurrell and Deser 2010). As quantified by the station-based NAO index (Table 2) (Hurrell 1995), this mode is * Corresponding author: [email protected] 1 Present address: Marine Institute, University of Plymouth, Plymouth, United Kingdom, Marine Biological Association of the United Kingdom, Plymouth, United Kingdom Limnol. Oceanogr., 58(3), 2013, 803–816 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.3.0803