Stocks of sardine and anchovy inhabiting eastern boundary upwelling systems of the world’s oceans have exhibited large fluctuations in population size, often with peaks in biomass of one taxon ...alternating with that of the other over multidecadal periods. One hypothesis offered to explain such variability attributes changes in population growth to distinctions in the optimal size of the fishes’ planktonic prey. However, the factors affecting size structure in mesozooplankton assemblages are poorly understood. Here, plankter sizes and concentrations were measured for samples collected across a trophic gradient in the California Current Ecosystem with coincident measures of nutrient concentrations. There was a clear distinction between mesozooplankter sizes in samples from oligotrophic and eutrophic waters, with the relative abundances of large individuals being greater in areas where upwelling conditions enhanced nutrient availability and increased abundances of large phytoplankters. The relative contributions of small zooplankters were greater in oligotrophic waters. In light of the observed variability in the biomasses and size structures of phytoplankton and zooplankton assemblages, the potential growth rates of sardine and anchovy are estimated using previously established models of ingestion, absorption, excretion, and respiration. These bioenergetic models suggest that the potential for anchovy growth is limited to nearshore, eutrophic waters where large zooplankters are abundant. In contrast, growth of sardine is possible under more oligotrophic conditions and is influenced by oceanographic conditions in the offshore region of the ecosystem.
Climate, Anchovy, and Sardine Checkley, David M; Asch, Rebecca G; Rykaczewski, Ryan R
Annual review of marine science,
01/2017, Letnik:
9, Številka:
1
Journal Article
Recenzirano
Odprti dostop
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.
Upwelling is critical to the biological production, acidification, and deoxygenation of the ocean's 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.
Key Points
Comprehensive assessment of pressures, temperatures, and coastal upwelling winds in CMIP5 models
Poleward shift in distribution of coastal upwelling‐favorable winds projected with climate change
Changes due to displacement of high‐pressure systems, not land‐sea surface air temperature contrasts
Transfer efficiency is the proportion of energy passed between nodes in food webs. It is an emergent, unitless property that is difficult to measure, and responds dynamically to environmental and ...ecosystem changes. Because the consequences of changes in transfer efficiency compound through ecosystems, slight variations can have large effects on food availability for top predators. Here, we review the processes controlling transfer efficiency, approaches to estimate it, and known variations across ocean biomes. Both process-level analysis and observed macroscale variations suggest that ecosystem-scale transfer efficiency is highly variable, impacted by fishing, and will decline with climate change. It is important that we more fully resolve the processes controlling transfer efficiency in models to effectively anticipate changes in marine ecosystems and fisheries resources.
Transfer efficiency is a key parameter describing ecosystem structure and function and is used to estimate fisheries production; however, it is also one of the most uncertain parameters.Questions remain about how habitats, food resources, fishing pressure, spatiotemporal scales, as well as temperature, primary production, and other climate drivers impact transfer efficiency.Direct measurements of transfer efficiency are difficult, but observations of marine population abundances, diets, productivity, stable isotope analysis, and models integrating these constraints can provide transfer efficiency estimates.Recent estimates suggest that transfer efficiency is more variable than previously thought, compounding uncertainties in marine ecosystem predictions and projections.Increased understanding of factors contributing to variation in transfer efficiency will improve projections of fishing and climate change impacts on marine ecosystems.
Common approaches for summarizing multivariate environmental or community data assume that relationships among variables are stationary over time, and this assumption is often not tested. Here we ...test the hypothesis that relationships among environmental and community time series are nonstationary in the Gulf of Alaska ecosystem (North Pacific Ocean) over multidecadal time scales. Dynamic factor analysis (DFA) is applied to environmental and community data from before and after 1988/1989, corresponding to the timing of an abrupt decline in temporal variance of the Aleutian Low atmospheric pattern, a leading driver of Gulf of Alaska climate. Results show that covariance among local atmosphere and ocean environmental variables weakened simultaneous to the decline in Aleutian Low variance. At the same time, community-wide responses of 14 fish and crustacean populations to physical forcing weakened, as indicated by nonstationary environment–biology regression coefficients. In line with theoretical predictions, this loss of a shared response to environmental variability was accompanied by weakening community covariance. Individual populations also showed nonstationary relationships with shared trends of community variability. We conclude that assumptions of fixed environmental and community relationships are likely to produce mistaken inference in this ecosystem. Similar concerns may apply in other ecosystems subject to changing climate patterns.
Studies of climate effects on ecology often account for non-stationarity in individual physical and biological variables, but rarely allow for non-stationary relationships among variables. Here, we ...show that non-stationary relationships among physical and biological variables are central to understanding climate effects on salmon (
spp.) in the Gulf of Alaska during 1965-2012. The relative importance of two leading patterns in North Pacific climate, the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), changed around 1988/1989 as reflected by changing correlations with leading axes of sea surface temperature variability. Simultaneously, relationships between the PDO and Gulf of Alaska environmental variables weakened, and long-standing temperature-salmon and PDO-salmon covariance declined to zero. We propose a mechanistic explanation for changing climate-salmon relationships in terms of non-stationary atmosphere-ocean interactions coinciding with changing PDO-NPGO relative importance. We also show that regression models assuming stationary climate-salmon relationships are inappropriate over the multidecadal time scale we consider. Relaxing assumptions of stationary relationships markedly improved modelling of climate effects on salmon catches and productivity. Attempts to understand the implications of changing climate patterns in other ecosystems might also be aided by the application of models that allow associations among environmental and biological variables to change over time.
Upwelling of nutrient-rich, subsurface water sustains high productivity in the ocean's eastern boundary currents. These ecosystems support a rate of fish harvest nearly 100 times the global mean and ...account for >20% of the world's 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.
A leading hypothesis relating productivity with climate variability in the California Current Ecosystem (CCE) describes an alternation between warmer, well‐stratified periods of low productivity and ...cooler periods of high productivity. This empirical relationship suggests that productivity will decline with global warming. Here, we explore the response of productivity to future climate change in the CCE using an earth system model. This model projects increases in nitrate supply and productivity in the CCE during the 21st century despite increases in stratification and limited change in wind‐driven upwelling. We attribute the increased nitrate supply to enrichment of deep source waters entering the CCE resulting from decreased ventilation of the North Pacific. Decreases in dissolved‐oxygen concentration and increasing acidification accompany projected increases in nitrate. This analysis illustrates that anthropogenic climate change may be unlike past variability; empirical relationships based on historical observations may be inappropriate for projecting ecosystem responses to future climate change.
Given the importance of coastal upwelling systems to ocean productivity, fisheries, and biogeochemical cycles, their response to climate change is of great interest. However, there is no consensus on ...future productivity changes in these systems, which may be controlled by multiple drivers including wind‐driven and geostrophic transport, stratification, and source water properties. Here we use an ensemble of regional ocean projections and recently developed upwelling indices for the California Current System to disentangle these sometimes‐competing influences. Some changes are consistent among models (e.g., decreased mixed layer depth), while for others there is a lack of agreement even on the direction of future change (e.g., nitrate concentration in upwelled waters). Despite models' diverging projections of productivity changes, they agree that those changes are predominantly driven by subsurface nitrate concentrations, not by upwelling strength. Our results highlight the need for more attention to processes governing subsurface nutrient changes, not just upwelling strength.
Plain Language Summary
The California Current System is one of the world's eastern boundary upwelling systems—some of the most productive regions in the global ocean. These regions support a wide range of human activities, such as fisheries and tourism, motivating extensive research on how they might evolve under future climate change. A number of hypotheses have been offered to describe future physical and chemical change in these systems, and in terms of their impacts on primary production (which forms the base of the marine food web), these mechanisms may reinforce or oppose each other. Enhanced nutrient concentrations in upwelling source waters would support higher productivity, increased stratification would limit nutrient supply and productivity, and increased upwelling could enhance productivity to a point but limit productivity if it is too strong. There is no consensus on which mechanism(s) will predominantly drive future productivity changes. Here we provide a detailed analysis of projected physical and biogeochemical changes and how they relate to productivity changes. Even though different models project different futures, we find that in all of them the primary control on productivity is the nitrate concentration of subsurface waters, not the strength of upwelling, which has received more attention to date.
Key Points
Future changes in the California Current System are evaluated using an ensemble of downscaled ocean projections
We evaluate changes in Ekman and geostrophic transports, water column structure, and subsurface nitrate concentrations
Across models, phytoplankton biomass changes are more closely tied to subsurface nitrate concentration than upwelling strength
PDF
