Ryan R. Rykaczewski

Influence of ocean winds on the pelagic ecosystem in upwelling regions

Upwelling of nutrient-rich, subsurface water sustains high productivity in the oceans eastern boundary currents. These ecosystems support a rate of fish harvest nearly 100 times the global mean and account for >20% of the worlds marine fish catch. Environmental variability is thought to be the major cause of the decadal-scale biomass fluctuations characteristic of fish populations in these regions, but the mechanisms relating atmospheric physics to fish production remain unexplained. Two atmospheric conditions induce different types of upwelling in these ecosystems: coastal, alongshore wind stress, resulting in rapid upwelling (with high vertical velocity, w); and wind-stress curl, resulting in slower upwelling (low w). We show that the level of wind-stress curl has increased and that production of Pacific sardine (Sardinops sagax) varies with wind-stress curl over the past six decades. The extent of isopycnal shoaling, nutricline depth, and chlorophyll concentration in the upper ocean also correlate positively with wind-stress curl. The size structure of plankton assemblages is related to the rate of wind-forced upwelling, and sardine feed efficiently on small plankters generated by slow upwelling. Upwelling rate is a fundamental determinant of the biological structure and production in coastal pelagic ecosystems, and future changes in the magnitude and spatial gradient of wind stress may have important and differing effects on these ecosystems. Understanding of the biological mechanisms relating fisheries production to environmental variability is essential for wise management of marine resources under a changing climate.

Climate change and wind intensification in coastal upwelling ecosystems

Strong winds, upwelling, and teeming shores Climate warming has produced stronger winds along some coasts, a result of growing differences in temperature and pressure between land and sea. These winds cause cold nutrient-rich seawater to rise to the surface, affecting climate and fueling marine productivity. Sydeman et al. examined data from the five major world regions where upwelling is occurring. Particularly in the California, Humboldt, and Benguela upwelling systems, winds have become stronger over the past 60 years. These regions represent up to a fifth of wild marine fish catches and are hot spots of biodiversity. Science, this issue p. 77 Increasing greenhouse gas concentrations have caused windier conditions in most major coastal upwelling regions. In 1990, Andrew Bakun proposed that increasing greenhouse gas concentrations would force intensification of upwelling-favorable winds in eastern boundary current systems that contribute substantial services to society. Because there is considerable disagreement about whether contemporary wind trends support Bakun’s hypothesis, we performed a meta-analysis of the literature on upwelling-favorable wind intensification. The preponderance of published analyses suggests that winds have intensified in the California, Benguela, and Humboldt upwelling systems and weakened in the Iberian system over time scales ranging up to 60 years; wind change is equivocal in the Canary system. Stronger intensification signals are observed at higher latitudes, consistent with the warming pattern associated with climate change. Overall, reported changes in coastal winds, although subtle and spatially variable, support Bakun’s hypothesis of upwelling intensification in eastern boundary current systems.

Anticipated Effects of Climate Change on Coastal Upwelling Ecosystems

Ecosystem productivity in coastal ocean upwelling systems is threatened by climate change. Increases in spring and summer upwelling intensity, and associated increases in the rate of offshore advection, are expected. While this could counter effects of habitat warming, it could also lead to more frequent hypoxic events and lower densities of suitable-sized food particles for fish larvae. With upwelling intensification, ocean acidity will rise, affecting organisms with carbonate structures. Regardless of changes in upwelling, near-surface stratification, turbulent diffusion rates, source water origins, and perhaps thermocline depths associated with large-scale climate episodes (ENSO) maybe affected. Major impacts on pelagic fish resources appear unlikely unless couples with overfishing, although changes toward more subtropical community composition are likely. Marine mammals and seabirds that are tied to sparsely distributed nesting or resting grounds could experience difficulties in obtaining prey resources, or adaptively respond by moving to more favorable biogeographic provinces.

A measured look at ocean chlorophyll trends

Arising from D. G. Boyce, M. R. Lewis & B. Worm 466, 591–596 (2010)10.1038/nature09268; Boyce et al. replyIdentifying major changes in global ecosystem properties is essential to improve our understanding of biological responses to climate forcing and exploitation. Recently, Boyce et al. reported an alarming, century-long decline in marine phytoplankton biomass of 1% per year, which would imply major changes in ocean circulation, ecosystem processes and biogeochemical cycling over the period and have significant implications for management of marine fisheries. Closer examination reveals that time-dependent changes in sampling methodology combined with a consistent bias in the relationship between in situ and transparency-derived chlorophyll (Chl) measurements generate a spurious trend in the synthesis of phytoplankton estimates used by Boyce et al.. Our results indicate that much, if not all, of the century-long decline reported by Boyce et al. is attributable to this temporal sampling bias and not to a global decrease in phytoplankton biomass.

Poleward displacement of coastal upwelling‐favorable winds in the ocean's eastern boundary currents through the 21st century

Upwelling is critical to the biological production, acidification, and deoxygenation of the oceans major eastern boundary current ecosystems. A leading conceptual hypothesis projects that the winds that induce coastal upwelling will intensify in response to increased land-sea temperature differences associated with anthropogenic global warming. We examine this hypothesis using an ensemble of coupled, ocean-atmosphere models and find limited evidence for intensification of upwelling-favorable winds or atmospheric pressure gradients in response to increasing land-sea temperature differences. However, our analyses reveal consistent latitudinal and seasonal dependencies of projected changes in wind intensity associated with poleward migration of major atmospheric high-pressure cells. Summertime winds near poleward boundaries of climatological upwelling zones are projected to intensify, while winds near equatorward boundaries are projected to weaken. Developing a better understanding of future changes in upwelling winds is essential to identifying portions of the oceans susceptible to increased hypoxia, ocean acidification, and eutrophication under climate change.

Six centuries of variability and extremes in a coupled marine-terrestrial ecosystem

Rings of ocean upwelling Coastal upwelling along the coast of California has become more variable than during nearly any period in the past 600 years. Black et al. used a 576-year tree ring record to construct a record of wintertime climate along the California coast. Because wintertime climate and coastal upwelling are so closely linked there, they were able to determine that upwelling variability has increased more over the past 60 years than for all but two intervals during that time. The apparent causes of the recent trend appear to be unique, resulting in reduced marine productivity and negative impacts on fish, seabirds, and mammals. Science, this issue p. 1498 Winter upwelling along the Pacific coast of North America became unusually variable during the 20th century. Reported trends in the mean and variability of coastal upwelling in eastern boundary currents have raised concerns about the future of these highly productive and biodiverse marine ecosystems. However, the instrumental records on which these estimates are based are insufficiently long to determine whether such trends exceed preindustrial limits. In the California Current, a 576-year reconstruction of climate variables associated with winter upwelling indicates that variability increased over the latter 20th century to levels equaled only twice during the past 600 years. This modern trend in variance may be unique, because it appears to be driven by an unprecedented succession of extreme, downwelling-favorable, winter climate conditions that profoundly reduce productivity for marine predators of commercial and conservation interest.

Modeling Social—Ecological Scenarios in Marine Systems

Human activities have substantial impacts on marine ecosystems+ including rapid regime shifts with large consequences for human well-being. We highlight the use of model-based scenarios as a scientific tool for adaptive stewardship in the face of such consequences. The natural sciences have a long history of developing scenarios but rarely with an in-depth understanding of factors influencing human actions. Social scientists have traditionally investigated human behavior, but scholars often argue that behavior is too complex to be represented by broad generalizations useful for models and scenarios. We address this scientific divide with a framework for integrated marine social-ecological scenarios, combining quantitative process-based models from the biogeochemical and ecological disciplines with qualitative studies on governance and social change. The aim is to develop policy-relevant scenarios based on an in-depth empirical understanding from both the natural and the social sciences, thereby contributing to adaptive stewardship of marine social-ecological systems.

Reconciling fisheries catch and ocean productivity

Significance Phytoplankton provide the energy that sustains marine fish populations. The relationship between phytoplankton productivity and fisheries catch, however, is complicated by uncertainty in catch estimates, fishing effort, and marine food web dynamics. We enlist global data sources and a high-resolution earth system model to address these uncertainties. Results show that cross-ecosystem fisheries catch differences far exceeding differences in phytoplankton production can be reconciled with fishing effort and variations in marine food web structure and energy transfer efficiency. Food web variations explaining contemporary fisheries catch act to amplify projected catch trends under climate change, suggesting catch changes that may exceed a factor of 2 for some regions. Failing to account for this would hinder adaptation to climate change. Photosynthesis fuels marine food webs, yet differences in fish catch across globally distributed marine ecosystems far exceed differences in net primary production (NPP). We consider the hypothesis that ecosystem-level variations in pelagic and benthic energy flows from phytoplankton to fish, trophic transfer efficiencies, and fishing effort can quantitatively reconcile this contrast in an energetically consistent manner. To test this hypothesis, we enlist global fish catch data that include previously neglected contributions from small-scale fisheries, a synthesis of global fishing effort, and plankton food web energy flux estimates from a prototype high-resolution global earth system model (ESM). After removing a small number of lightly fished ecosystems, stark interregional differences in fish catch per unit area can be explained (r = 0.79) with an energy-based model that (i) considers dynamic interregional differences in benthic and pelagic energy pathways connecting phytoplankton and fish, (ii) depresses trophic transfer efficiencies in the tropics and, less critically, (iii) associates elevated trophic transfer efficiencies with benthic-predominant systems. Model catch estimates are generally within a factor of 2 of values spanning two orders of magnitude. Climate change projections show that the same macroecological patterns explaining dramatic regional catch differences in the contemporary ocean amplify catch trends, producing changes that may exceed 50% in some regions by the end of the 21st century under high-emissions scenarios. Models failing to resolve these trophodynamic patterns may significantly underestimate regional fisheries catch trends and hinder adaptation to climate change.

Integrated Assessment of Wind Effects on Central California’s Pelagic Ecosystem

Ecosystem-based management requires integrated physical studies on biological functions. In this study, we hypothesized that seasonal variation in upwelling-favorable winds has differential influences on species of the central California Current pelagic ecosystem. To test this hypothesis, we developed multivariate indicators of upwelling and species’ responses using wind and sea surface temperature (SST) data from buoys and growth and reproductive data for 11 species of fish and seabirds. From previous work, we predicted that winds and SST could be decomposed into winter and spring/summer ‘modes’ of variability, but only a single mode of “winter/spring” environmental variability was observed. We attribute this difference from expectations to the local and shorter-term measurements of winds and SST used in this study. Most species responded to winds and SST variability similarly, but SST was a better predictor of most biological responses. Both SST and wind were better predictors than the traditional upwelling index. Notably, Pacific sardine (Sardinops sajax) was disassociated with the other biotic measurements and showed no relationships with coastal upwelling. The multivariate indicators developed here are particularly appropriate for integrated ecosystem assessments of climatic influences on marine life because they reflect both structure and processes (upwelling and timing/growth/productivity) known to determine functions in marine ecosystems.

Climate, Anchovy, and Sardine

Anchovy and sardine populated productive ocean regions over hundreds of thousands of years under a naturally varying climate, and are now subject to climate change of equal or greater magnitude occurring over decades to centuries. We hypothesize that anchovy and sardine populations are limited in size by the supply of nitrogen from outside their habitats originating from upwelling, mixing, and rivers. Projections of the responses of anchovy and sardine to climate change rely on a range of model types and consideration of the effects of climate on lower trophic levels, the effects of fishing on higher trophic levels, and the traits of these two types of fish. Distribution, phenology, nutrient supply, plankton composition and production, habitat compression, fishing, and acclimation and adaptation may be affected by ocean warming, acidification, deoxygenation, and altered hydrology. Observations of populations and evaluation of model skill are essential to resolve the effects of climate change on these fish.