The timing and strength of wind-driven coastal upwelling along the eastern margins of major ocean basins regulate the productivity of critical fisheries and marine ecosystems by bringing deep and nutrient-rich waters to the sunlit surface, where photosynthesis can occur. How coastal upwelling regimes might change in a warming climate is therefore a question of vital importance. Although enhanced land-ocean differential heating due to greenhouse warming has been proposed to intensify coastal upwelling by strengthening alongshore winds, analyses of observations and previous climate models have provided little consensus on historical and projected trends in coastal upwelling. Here we show that there are strong and consistent changes in the timing, intensity and spatial heterogeneity of coastal upwelling in response to future warming in most Eastern Boundary Upwelling Systems (EBUSs). An ensemble of climate models shows that by the end of the twenty-first century the upwelling season will start earlier, end later and become more intense at high but not low latitudes. This projected increase in upwelling intensity and duration at high latitudes will result in a substantial reduction of the existing latitudinal variation in coastal upwelling. These patterns are consistent across three of the four EBUSs (Canary, Benguela and Humboldt, but not California). The lack of upwelling intensification and greater uncertainty associated with the California EBUS may reflect regional controls associated with the atmospheric response to climate change. Given the strong linkages between upwelling and marine ecosystems, the projected changes in the intensity, timing and spatial structure of coastal upwelling may influence the geographical distribution of marine biodiversity.

The near-term progression of ocean acidification (OA) is projected to bring about sharp changes in the chemistry of coastal upwelling ecosystems. The distribution of OA exposure across these early-impact systems, however, is highly uncertain and limits our understanding of whether and how spatial management actions can be deployed to ameliorate future impacts. Through a novel coastal OA observing network, we have uncovered a remarkably persistent spatial mosaic in the penetration of acidified waters into ecologically-important nearshore habitats across 1,000 km of the California Current Large Marine Ecosystem. In the most severe exposure hotspots, suboptimal conditions for calcifying organisms encompassed up to 56% of the summer season, and were accompanied by some of the lowest and most variable pH environments known for the surface ocean. Persistent refuge areas were also found, highlighting new opportunities for local adaptation to address the global challenge of OA in productive coastal systems.

8Ecosystems are shaped by processes occurring and interacting over multiple and temporal spatial 9 scales. Theory suggests such complexity can be simplified by focusing on processes sharing the 10 same scale as the pattern of interest. This scale-dependent approach to studying communities has 11 been challenged by multi-scale meta-ecosystem theory, which recognizes that systems are 12 interconnected by the movement of "ecological subsidies" and suggests that cross-scale 13 feedbacks between local and regional processes can be equally important for understanding 14 community structure. We reconcile these two perspectives by developing and testing a 15 hierarchical meta-ecosystem model. The model predicts local community responses to 16 connectivity over multiple oceanographic spatial scales, defined as macro-(100s km), meso-17 (10s km) and local-scales (100s m). It assumes that local communities occur in distinct regions 18 and that connectivity effects are strongest among local sites. Predictions are that if macro-scale 19 processes dominate, then regardless of meso-scale differences, (1) local communities will be 20 similar, and (2) even more so with increased connectivity. With dominance of meso-scale (i.e., 21 regional) processes (3) local structure will be similar within but distinct between regions, and (4) 22 with increased connectivity similar both within and among regions. With dominance of local-23 scale processes (5) local communities will differ both within and among regions, and (6) with 24 increased connectivity be similar within but not between regions. We tested the model by 25 evaluating rocky intertidal community structure patterns to variation in ecological subsidies and 26 environmental conditions at 13 sites spanning 725 km of the northern California Current System. 27External factors operating at meso-and local-scales had strong effects, explaining 52% and 27% 28 of the variance, respectively, in community structure. Sessile invertebrate and predator 29 dominance was associated with weaker upwelling, higher phytoplankton abundance and higher 30 Rocky intertidal meta-ecosystem ecology 3 recruitment and the opposite was true for macrophyte dominance. Overall, our results support the 31 theory that meta-ecosystems are organized hierarchically, with environmental processes 32 dominating at meso-to macro-scales and ecological processes playing a more important role at 33 local scales, but with important bidirectional cross-scale interactions. 34

Synchrony has fundamental but conflicting implications for the persistence and stability of food webs at local and regional scales. In a constant environment, compensatory dynamics between species can maintain food web stability, but factors that synchronize population fluctuations within and between communities are expected to be destabilizing. We studied the dynamics of a food web in a metacommunity to determine how environmental variability and dispersal affect stability by altering compensatory dynamics and average species abundance. When dispersal rate is high, weak correlated environmental fluctuations promote food web stability by reducing the amplitude of compensatory dynamics. However, when dispersal rate is low, weak environmental fluctuations reduce food web stability by inducing intraspecific synchrony across communities. Irrespective of dispersal rate, strong environmental fluctuations disrupt compensatory dynamics and decrease stability by inducing intermittent correlated fluctuations between consumers in local food webs, which reduce both total consumer abundance and predator abundance. Strong correlated environmental fluctuations lead to (i) spatially asynchronous and highly correlated local consumer dynamics when dispersal is low and (ii) spatially synchronous but intermediate local consumer correlation when dispersal is high. By controlling intraspecific synchrony, dispersal mediates the capacity of strong environmental fluctuations to disrupt compensatory dynamics at both local and metacommunity scales.

Determining the relative importance of local and regional processes for the distribution of population abundance is a fundamental but contentious issue in ecology. In marine systems, classical theory holds that the influence of demographic processes and dispersal is confined to local populations whereas the environment controls regional patterns of abundance. Here, we use spatial synchrony to compare the distribution of population abundance of the dominant mussel Mytilus californianus observed along the West Coast of the United States to that predicted by dynamical models undergoing different dispersal and environmental treatments to infer the relative influence of local and regional processes. We reveal synchronized fluctuations in the abundance of mussel populations across a whole continent despite limited larval dispersal and strong environmental forcing. We show that dispersal among neighboring populations interacts with local demographic processes to generate characteristic patterns of spatial synchrony that can govern the dynamic distribution of mussel abundance over 1,800 km of coastline. Our study emphasizes the importance of dispersal and local dynamics for the distribution of abundance at the continental scale. It further highlights potential limits to the use of "climate envelope" models for predicting the response of large-scale ecosystems to global climate change.dispersal | environmental variability | metapopulation | synchrony | cross-scale interactions S ynchronized fluctuations in abundance among spatially segregated populations are common in nature and can be used to quantify and understand the distribution of abundance in space and time (1). Synchrony can be induced by local intrinsic processes such as dispersal among populations and strong interactions with mobile predators or regional extrinsic processes such as spatially correlated environmental variability (1). Although these processes are well known, identifying their relative contribution to patterns of synchrony remains a challenge (1). Recent work has shown that when the processes that contribute to synchrony can be studied in isolation, be it via natural barriers to dispersal among populations (2, 3) or experimental manipulation (4), synchrony patterns can be ascribed to their underlying cause. However, when intrinsic and extrinsic causes of synchrony co-occur, as is the case in most systems, assigning synchrony patterns to any specific causal process becomes onerous (1). Here, we show that in marine populations experiencing both intrinsic and extrinsic sources of synchrony, the shape of spatial synchrony patterns can be used to infer the cause of synchrony and explain the regional distribution of abundance.Marine population theory has relied mostly on the environment to explain the regional (>1,000 km) dynamics of populations. This focus is motivated by the lengthy pelagic larval stage commonly found in marine organisms, during which the larvae can be transported over large distances by strong nearshore currents (5). The potential fo...