A pronounced, widespread and persistent regime shift among marine ecosystems is observable on temperate rocky reefs as a result of sea urchin overgrazing. Here, we empirically define regime-shift dynamics for this grazing system which transitions between productive macroalgal beds and impoverished urchin barrens. Catastrophic in nature, urchin overgrazing in a well-studied Australian system demonstrates a discontinuous regime shift, which is of particular management concern as recovery of desirable macroalgal beds requires reducing grazers to well below the initial threshold of overgrazing. Generality of this regime-shift dynamic is explored across 13 rocky reef systems (spanning 11 different regions from both hemispheres) by compiling available survey data (totalling 10 901 quadrats surveyed in situ ) plus experimental regime-shift responses (observed during a total of 57 in situ manipulations). The emergent and globally coherent pattern shows urchin grazing to cause a discontinuous ‘catastrophic’ regime shift, with hysteresis effect of approximately one order of magnitude in urchin biomass between critical thresholds of overgrazing and recovery. Different life-history traits appear to create asymmetry in the pace of overgrazing versus recovery. Once shifted, strong feedback mechanisms provide resilience for each alternative state thus defining the catastrophic nature of this regime shift. Importantly, human-derived stressors can act to erode resilience of desirable macroalgal beds while strengthening resilience of urchin barrens, thus exacerbating the risk, spatial extent and irreversibility of an unwanted regime shift for marine ecosystems.

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The NansClim project (2010–2013) represented a regional collaboration to assess the effects of climate on Benguela dynamics. Based on in situ (since the 1960s in Namibia and South Africa and 1985 in Angola) and satellite (since the 1980s) observations, the project focussed on four subsystems, namely the Angola subtropical, northern Benguela upwelling, southern Benguela upwelling and Agulhas Bank. This contribution summarizes the findings for selected key questions, ranging from changes in the physico‐chemical habitats, plankton, pelagic and demersal fish communities, to cross‐cutting evaluation at subsystem and regional scales. The results underline the overriding importance to of considering the combined effects of climate and fishing as drivers of the dynamics of the ecosystem components. Each subsystem currently continues to function largely as a separate entity as described in earlier reviews. However, some changes have been observed across several subsystems, e.g., a coherent shift from one relatively stable period to another occurred in the northern and southern Benguela in the mid‐1990s. Future climate change could weaken the boundaries between the four subystems. The findings underline the need for continued regional research collaboration and regional surveys focussed at ecosystem, rather than resource, assessment. Our conclusions include implications for ecosystem‐based fisheries management, and recommendations for future regional research.

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Pfaff, MC, et al. 2019. A synthesis of three decades of socio-ecological change in False Bay, South Africa: setting the scene for multidisciplinary research and management. Elem Sci Anth, 7: 32.

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As part of the southern Benguela upwelling ecosystem, South Africa's west coast represents a very productive marine habitat. Decadal‐scale environmental and ecosystem regime shifts have been documented based on wind and ocean temperature data, as well as biota. This study analyses the variability of oxygen‐depleted (<2 mL L−1) bottom waters on South Africa's west coast shelf using data from St Helena Bay, the most productive embayment in the ecosystem, from 1957, data from demersal and pelagic fisheries surveys since the 1980s, and monthly observations from a fixed transect off St Helena Bay from 2000 to 2011. Oxygen‐depleted waters are generally restricted to bottom depths <150 m, and occur regularly during austral summer and autumn, albeit with variable severity. The high spatial variability makes it impossible to define any single station as an indicator station for the extent of oxygen depletion. We provide estimates of the shelf area, 30–33°S, covered by oxygen‐depleted bottom water in austral summer and autumn, and show a parabolic relationship between oxygen depletion and upwelling strength. The results suggest that the cumulative upwelling divergence from the onset of the upwelling season (September) to the establishment of oxygen depletion (January) can be used as a proxy indicator for oxygen depletion in summer. Periods of better bottom ventilation are indicated for the second half of the 1960s to the early 1980s; more extensive oxygen depletion at the beginning of the 1950s, and the mid‐1990s to the mid‐2000s. Implications for environmental monitoring and marine living resources are discussed.