Increasing variance in North Pacific climate relates to unprecedented ecosystem variability off California

Changes in variance are infrequently examined in climate change ecology. We tested the hypothesis that recent high variability in demographic attributes of salmon and seabirds off California is related to increasing variability in remote, large‐scale forcing in the North Pacific operating through changes in local food webs. Linear, indirect numerical responses between krill (primarily Thysanoessa spinifera) and juvenile rockfish abundance (catch per unit effort (CPUE)) explained >80% of the recent variability in the demography of these pelagic predators. We found no relationships between krill and regional upwelling, though a strong connection to the North Pacific Gyre Oscillation (NPGO) index was established. Variance in NPGO and related central Pacific warming index increased after 1985, whereas variance in the canonical ENSO and Pacific Decadal Oscillation did not change. Anthropogenic global warming or natural climate variability may explain recent intensification of the NPGO and its increasing ecological significance. Assessing non‐stationarity in atmospheric‐environmental interactions and placing greater emphasis on documenting changes in variance of bio‐physical systems will enable insight into complex climate‐marine ecosystem dynamics.     

Environmental drivers of the anchovy/sardine complex in the Eastern Mediterranean

The anchovy/sardine complex is an important fishery resource in some of the largest upwelling systems in the world. Synchronous, but out of phase, fluctuations of the two species in distant parts of the oceans have prompted a number of studies dedicated to determining the phenomena, atmospheric and oceanic, responsible for the observed synchronicity and the biological mechanisms behind the population changes of the two species. Anchovy and sardine are of high commercial value for the fishing sector in Greece; this study investigates the impact of large-scale climatic indices on the anchovy/sardine complex in the Greek seas using fishery catches as a proxy for fish productivity. Time series of catches for both species were analysed for relationships with teleconnection indices and local environmental variability. The connection between the teleconnection indices and local weather/oceanic variation was also examined in an effort to describe physical mechanisms that link large-scale atmospheric patterns with anchovy and sardine. The West African Summer Monsoon, East Atlantic Jet and Pacific–North American (PNA) pattern exhibit coherent relationships with the catches of the two species. The first two aforementioned patterns are prominent atmospheric modes of variability during the summer months when sardine is spawning and anchovy juveniles are growing. PNA is related with El Niño Southern Oscillation events. Sea Surface Temperature (SST) appears as a significant link between atmospheric and biological variability either because higher temperatures seem to be favouring sardine growth or because lower temperatures, characteristic of productivity-enhancing oceanic features, exert a positive influence on both species. However at a local scale, other parameters such as wind and mesoscale circulation describe air–sea variability affecting the anchovy/sardine complex. These relationships are non-linear and in agreement with results of previous studies stressing the importance of optimal environmental windows. The results also show differences in the response of the two species to environmental forcing and possible interactions between the two species. The nature of these phenomena, e.g., if the species interactions are direct through competition or indirect through the food web, remains to be examined.     

Humpback whale diets respond to variance in ocean climate and ecosystem conditions in the California Current

Large, migratory predators are often cited as sentinel species for ecosystem processes and climate‐related changes, but their utility as indicators is dependent upon an understanding of their response to environmental variability. Documentation of the links between climate variability, ecosystem change and predator dynamics is absent for most top predators. Identifying species that may be useful indicators and elucidating these mechanistic links provides insight into current ecological dynamics and may inform predictions of future ecosystem responses to climatic change. We examine humpback whale response to environmental variability through stable isotope analysis of diet over a dynamic 20‐year period (1993–2012) in the California Current System (CCS). Humpback whale diets captured two major shifts in oceanographic and ecological conditions in the CCS. Isotopic signatures reflect a diet dominated by krill during periods characterized by positive phases of the North Pacific Gyre Oscillation (NPGO), cool sea surface temperature (SST), strong upwelling and high krill biomass. In contrast, humpback whale diets are dominated by schooling fish when the NPGO is negative, SST is warmer, seasonal upwelling is delayed and anchovy and sardine populations display increased biomass and range expansion. These findings demonstrate that humpback whales trophically respond to ecosystem shifts, and as a result, their foraging behavior is a synoptic indicator of oceanographic and ecological conditions across the CCS. Multi‐decadal examination of these sentinel species thus provides insight into biological consequences of interannual climate fluctuations, fundamental to advancing ecosystem predictions related to global climate change.     

Climate‐driven trends and ecological implications of event‐scale upwelling in the California Current System

Eastern boundary current systems are among the most productive and lucrative ecosystems on Earth because they benefit from upwelling currents. Upwelling currents subsidize the base of the coastal food web by bringing deep, cold and nutrient‐rich water to the surface. As upwelling is driven by large‐scale atmospheric patterns, global climate change has the potential to affect a wide range of significant ecological processes through changes in water chemistry, water temperature, and the transport processes that influence species dispersal and recruitment. We examined long‐term trends in the frequency, duration, and strength of continuous upwelling events for the Oregon and California regions of the California Current System in the eastern Pacific Ocean. We then associated event‐scale upwelling with up to 21 years of barnacle and mussel recruitment, and water temperature data measured at rocky intertidal field sites along the Oregon coast. Our analyses suggest that upwelling events are changing in ways that are consistent with climate change predictions: upwelling events are becoming less frequent, stronger, and longer in duration. In addition, upwelling events have a quasi‐instantaneous and cumulative effect on rocky intertidal water temperatures, with longer events leading to colder temperatures. Longer, more persistent upwelling events were negatively associated with barnacle recruitment but positively associated with mussel recruitment. However, since barnacles facilitate mussel recruitment by providing attachment sites, increased upwelling persistence could have indirect negative impacts on mussel populations. Overall, our results indicate that changes in coastal upwelling that are consistent with climate change predictions are altering the tempo and the mode of environmental forcing in near‐shore ecosystems, with potentially severe and discontinuous ramifications for ecosystem structure and functioning.     

Climate‐mediated changes in marine ecosystem regulation during El Niño

The degree to which ecosystems are regulated through bottom‐up, top‐down, or direct physical processes represents a long‐standing issue in ecology, with important consequences for resource management and conservation. In marine ecosystems, the role of bottom‐up and top‐down forcing has been shown to vary over spatio‐temporal scales, often linked to highly variable and heterogeneously distributed environmental conditions. Ecosystem dynamics in the Northeast Pacific have been suggested to be predominately bottom‐up regulated. However, it remains unknown to what extent top‐down regulation occurs, or whether the relative importance of bottom‐up and top‐down forcing may shift in response to climate change. In this study, we investigate the effects and relative importance of bottom‐up, top‐down, and physical forcing during changing climate conditions on ecosystem regulation in the Southern California Current System (SCCS) using a generalized food web model. This statistical approach is based on nonlinear threshold models and a long‐term data set (~60 years) covering multiple trophic levels from phytoplankton to predatory fish. We found bottom‐up control to be the primary mode of ecosystem regulation. However, our results also demonstrate an alternative mode of regulation represented by interacting bottom‐up and top‐down forcing, analogous to wasp‐waist dynamics, but occurring across multiple trophic levels and only during periods of reduced bottom‐up forcing (i.e., weak upwelling, low nutrient concentrations, and primary production). The shifts in ecosystem regulation are caused by changes in ocean‐atmosphere forcing and triggered by highly variable climate conditions associated with El Niño. Furthermore, we show that biota respond differently to major El Niño events during positive or negative phases of the Pacific Decadal Oscillation (PDO), as well as highlight potential concerns for marine and fisheries management by demonstrating increased sensitivity of pelagic fish to exploitation during El Niño.     

Climate change scenarios experiments predict a future reduction in small pelagic fish recruitment in the Humboldt Current system

The Humboldt Current System (HCS) sustains the world′s largest small pelagic fishery. While a cooling of this system has been observed during recent decades, there is debate about the potential impacts of rising atmospheric CO₂ concentrations on upwelling dynamics and productivity. Recent studies suggest that under increased atmospheric CO₂ scenarios the oceanic stratification may strongly increase and upwelling‐favorable winds may remain nearly constant off Peru and increase off Chile. Here we investigate the impact of such climatic conditions on egg and larval dispersal phases, a key stage of small pelagic fish reproduction. We used larval retention rate in a predefined nursery area to provide a proxy for the recruitment level. Numerical experiments are based on hydrodynamics downscaled to the HCS from global simulations forced by pre‐industrial (PI), 2 × CO₂ and 4 × CO₂ scenarios. A biogeochemical model is applied to the PI and 4 × CO₂ scenarios to define a time‐variable nursery area where larval survival is optimum. We test two distinct values of the oxycline depth that limits larval vertical distribution: One corresponding to the present‐day situation and the other corresponding to a shallower oxycline potentially produced by climate change. It appeared that larval retention over the continental shelf increases with enhanced stratification due to regional warming. However, this increase in retention is largely compensated for by a decrease of the nursery area and the shoaling of the oxycline. The underlying dynamics are explained by a combination of stratification effects and mesoscale activity changes. Our results therefore show that future climate change may significantly reduce fish capacity in the HCS with strong ecological, economic and social consequences.     

Ecosystem size structure response to 21st century climate projection: large fish abundance decreases in the central North Pacific and increases in the California Current

Output from an earth system model is paired with a size‐based food web model to investigate the effects of climate change on the abundance of large fish over the 21st century. The earth system model, forced by the Intergovernmental Panel on Climate Change (IPCC) Special report on emission scenario A2, combines a coupled climate model with a biogeochemical model including major nutrients, three phytoplankton functional groups, and zooplankton grazing. The size‐based food web model includes linkages between two size‐structured pelagic communities: primary producers and consumers. Our investigation focuses on seven sites in the North Pacific, each highlighting a specific aspect of projected climate change, and includes top‐down ecosystem depletion through fishing. We project declines in large fish abundance ranging from 0 to 75.8% in the central North Pacific and increases of up to 43.0% in the California Current (CC) region over the 21st century in response to change in phytoplankton size structure and direct physiological effects. We find that fish abundance is especially sensitive to projected changes in large phytoplankton density and our model projects changes in the abundance of large fish being of the same order of magnitude as changes in the abundance of large phytoplankton. Thus, studies that address only climate‐induced impacts to primary production without including changes to phytoplankton size structure may not adequately project ecosystem responses.     

Oxygen: a fundamental property regulating pelagic ecosystem structure in the coastal southeastern tropical Pacific

Background

In the southeastern tropical Pacific anchovy (Engraulis ringens) and sardine (Sardinops sagax) abundance have recently fluctuated on multidecadal scales and food and temperature have been proposed as the key parameters explaining these changes. However, ecological and paleoecological studies, and the fact that anchovies and sardines are favored differently in other regions, raise questions about the role of temperature. Here we investigate the role of oxygen in structuring fish populations in the Peruvian upwelling ecosystem that has evolved over anoxic conditions and is one of the world''s most productive ecosystems in terms of forage fish. This study is particularly relevant given that the distribution of oxygen in the ocean is changing with uncertain consequences.

Methodology/Principal Findings

A comprehensive data set is used to show how oxygen concentration and oxycline depth affect the abundance and distribution of pelagic fish. We show that the effects of oxygen on anchovy and sardine are opposite. Anchovy flourishes under relatively low oxygen conditions while sardine avoid periods/areas with low oxygen concentration and restricted habitat. Oxygen consumption, trophic structure and habitat compression play a fundamental role in fish dynamics in this important ecosystem.

Conclusions/Significance

For the ocean off Peru we suggest that a key process, the need to breathe, has been neglected previously. Inclusion of this missing piece allows the development of a comprehensive conceptual model of pelagic fish populations and change in an ocean ecosystem impacted by low oxygen. Should current trends in oxygen in the ocean continue similar effects may be evident in other coastal upwelling ecosystems.     

Phenological and distributional shifts in ichthyoplankton associated with recent warming in the northeast Pacific Ocean

Understanding changes in the migratory and reproductive phenology of fish stocks in relation to climate change is critical for accurate ecosystem‐based fisheries management. Relocation and changes in timing of reproduction can have dramatic effects upon the success of fish populations and throughout the food web. During anomalously warm conditions (1–4°C above normal) in the northeast Pacific Ocean during 2015–2016, we documented shifts in timing and spawning location of several pelagic fish stocks based on larval fish samples. Total larval concentrations in the northern California Current (NCC) during winter (January–March) 2015 and 2016 were the highest observed since annual collections first occurred in 1998, primarily due to increased abundances of Engraulis mordax (northern anchovy) and Sardinops sagax (Pacific sardine) larvae, which are normally summer spawning species in this region. Sardinops sagax and Merluccius productus (Pacific hake) exhibited an unprecedented early and northward spawning expansion during 2015–16. In addition, spawning duration was greatly increased for E. mordax, as the presence of larvae was observed throughout the majority of 2015–16, indicating prolonged and nearly continuous spawning of adults throughout the warm period. Larvae from all three of these species have never before been collected in the NCC as early in the year. In addition, other southern species were collected in the NCC during this period. This suggests that the spawning phenology and distribution of several ecologically and commercially important fish species dramatically and rapidly changed in response to the warming conditions occurring in 2014–2016, and could be an indication of future conditions under projected climate change. Changes in spawning timing and poleward migration of fish populations due to warmer ocean conditions or global climate change will negatively impact areas that were historically dependent on these fish, and change the food web structure of the areas that the fish move into with unforeseen consequences.     

Greenhouse gas,upwelling‐favorable winds,and the future of coastal ocean upwelling ecosystems

Coastal ocean upwelling ecosystems generally represent the most productive large marine ecosystems of the world's oceans, in terms of both primary production rates and tonnages of exploitable fish produced. The Peruvian upwelling system, in particular, stands out as a major factor in world fish production. The Pacific trade winds have traditionally been considered to be the primary driving force for the upwelling system off Peru, but are projected to weaken as climate change proceeds. This leads to concern that the upwelling process in the Peru system, to which its productivity is linked, may likewise weaken. However, other mechanisms involving greenhouse‐associated intensification of thermal low‐pressure cells over the coastal landmasses of upwelling regions suggest general intensification of wind‐driven ocean upwelling in coastal upwelling regions of the world's oceans. But although certain empirical results have supported this expectation, it has not been consistently corroborated in climate model simulations, possibly because the scale of the coastal intensification may be small relative to the scales that are appropriately reflected in the standard models. Here we summarize available evidence for the intensification mechanism and present a proxy test that uses variations in water vapor, the dominant natural greenhouse gas, to offer multiple‐realization empirical evidence for action of the proposed mechanism in the real world situation. While many potential consequences to the future of marine ecosystems would codepend on climate change‐related changes in the thermocline and nutricline structures, an important subset, involving potential increased propensities for hypoxia, noxious gas eruptions, toxic red tide blooms, and/or jellyfish outbreaks, may depend more directly on changes in the upwelling‐favorable wind itself. A prospective role of fisheries in either mitigating or reinforcing this particular class of effects is suggested.     

Adapting to global environmental change in Patagonia: What role for disturbance ecology?

Research from the Patagonian‐Andean region is used to explore challenges and opportunities related to the integration of research on wildfire activity into a broader earth‐system science framework that views the biosphere and atmosphere as a coupled interacting system for understanding the causes and consequences of future wildfire activity. We examine how research in disturbance ecology can inform land‐use and other policy decisions in the context of probable future increases in wildfire activity driven by climate forcing. Climate research has related recent warming and drying trends in much of Patagonia to an upward trend in the Southern Annular Mode which is the leading pattern of extratropical climate variability in the southern hemisphere. Although still limited in spatial extent, tree‐ring fire history studies are beginning to reveal regional patterns of the top‐down climate influences on temporal and spatial pattern of wildfire occurrence in Patagonia. Knowledge of relationships of fire activity to climate variability in the context of predicted future warming leads to the hypothesis that wildfire activity in Patagonia will increase substantially during the first half of the 21st century. In addition to this anticipated increase in extreme fire events due to climate forcing, we further hypothesize that current land‐use trends will increase the extent and/or severity of fire events through bottom‐up (i.e. land surface) influences on wildfire potential. In particular, policy discussions of how to mitigate impacts of climate warming on fire potential need to consider research results from disturbance ecology on the implications of continued planting of flammable non‐native trees and the role of introduced herbivores in favouring vegetation changes that may enhance landscape flammability.     

Late Holocene variability in pelagic fish scales and dinoflagellate cysts along the west coast of Vancouver Island, NE Pacific Ocean

Fish stocks and dinoflagellates are essential components of the marine food chain. Sediment cores from a predominantly anoxic basin in Effingham Inlet, Vancouver Island, British Columbia, archive a late Holocene (500–5300 years BP) record of paleoproductivity in the North American Coastal Upwelling Domain (CUD). We present evidence that late Holocene changes in the dinoflagellate cyst assemblages, sedimentary record, and fish stocks in the northeastern Pacific Ocean fluctuated, at least partially, in accordance with regional and global climate cycles.Principal components analysis (PCA), and trend, wavelet and spectral analyses were used to identify relationships, cycles and trends in sediment grey-scale values, and the abundances of fish scales and dinoflagellate cysts on centennial to millennial time scales. Most observed cycles fluctuated in intensity over time, particularly following transition of the regional climate to a higher rainfall phase that impacted coastal oceanic dynamics 3400 ± 150 years ago. Correlation of the marine paleoproductivity records observed in Effingham Inlet with solar influenced climate proxy cycles observed in the North Atlantic region indicates that solar forcing at different scales might have influenced the climate in the northeast Pacific as well. In particular an 1100- to 1400-year cycle in regional climate is well represented in the fish productivity proxy and sedimentological record. It was also observed that colder water, high-productivity, Selenopemphix nephroides and anchovy-dominated “Anchovy Regime” ecosystems alternate with warmer water, herring-dominated “Herring Regime” ecosystems at millennial time scales. The fish scale record preserved in Effingham Inlet indicates that the NE Pacific is now in transition from an ‘anchovy-’ to a ‘herring’-dominated regime.     

Expanded spatial extent of the Medieval Climate Anomaly revealed in lake‐sediment records across the boreal region in northwest Ontario

Multi‐decadal to centennial‐scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within‐lake calibration models developed using diatom assemblages collected from surface sediments across a water‐depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near‐shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate‐driven influences, and can provide a sensitive record of past drought. Our lake‐sediment records indicate two periods of synchronous signals, suggesting a common large‐scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900‐1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo‐lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models.     

Orbitally forced climate changes in the Tasman sector during the Middle Eocene

The influence of orbital precession on early Paleogene climate and ocean circulation patterns in the southeast Pacific region is investigated by combining environmental analyses of cyclic Middle Eocene sediments and palynomorph records recovered from ODP Hole 1172A on the East Tasman Plateau with climate model simulations. Integration of results indicates that in the marine realm, direct effects of precessional forcing are not pronounced, although increased precipitation/runoff could have enhanced dinoflagellate cyst production. On the southeast Australian continent, the most pronounced effects of precessional forcing were fluctuations in summer precipitation and temperature on the Antarctic Margin. These fluctuations resulted in vegetational changes, most notably in the distribution of Nothofagus (subgenus Brassospora). The climate model results suggest significant fluctuations in sea ice in the Ross Sea, notably during Austral summers. This is consistent with the influx of Antarctic heterotrophic dinoflagellates in the early part of the studied record. The data demonstrate a strong precessionally driven climate variability and thus support the concept that precessional forcing could have played a role in early Antarctic glaciation via changes in runoff and/or precipitation.     

Pacific sardine (<Emphasis Type="Italic">Sardinops melanostictus</Emphasis>) migrations to the shores of Sakhalin Island in the 20th–early 21st centuries

Historical data for the period 1929?2011 were used to generalize information on the seasonal feeding migrations of Pacific sardine to the shores of Sakhalin Island. In the 20th century, the cyclicity of sardine occurrence in the Tatar Strait and southwestern part of the Sea of Okhotsk appeared to have a similar character. Sardine distribution patterns are considered in connection with the oceanological characteristics on these areas. A close correlated relationship was revealed between the SST anomalies and northern boundary of distribution of this subtropical fish. Besides the water temperature, Pacific sardine northern migrations have been formed under the influence of many other factors (by origin and impact), including fluctuations in abundance, index of stock biomass, food supply, state of marine ecosystems, and climatic conditions.     

Climate change alters stability and species potential interactions in a large marine ecosystem

We have little empirical evidence of how large‐scale overlaps between large numbers of marine species may have altered in response to human impacts. Here, we synthesized all available distribution data (>1 million records) since 1992 for 61 species of the East Australian marine ecosystem, a global hot spot of ocean warming and continuing fisheries exploitation. Using a novel approach, we constructed networks of the annual changes in geographical overlaps between species. Using indices of changes in species overlap, we quantified changes in the ecosystem stability, species robustness, species sensitivity and structural keystone species. We then compared the species overlap indices with environmental and fisheries data to identify potential factors leading to the changes in distributional overlaps between species. We found that the structure of the ecosystem has changed with a decrease in asymmetrical geographical overlaps between species. This suggests that the ecosystem has become less stable and potentially more susceptible to environmental perturbations. Most species have shown a decrease in overlaps with other species. The greatest decrease in species overlap robustness and sensitivity to the loss of other species has occurred in the pelagic community. Some demersal species have become more robust and less sensitive. Pelagic structural keystone species, predominately the tunas and billfish, have been replaced by demersal fish species. The changes in species overlap were strongly correlated with regional oceanographic changes, in particular increasing ocean warming and the southward transport of warmer and saltier water with the East Australian Current, but less correlated with fisheries catch. Our study illustrates how large‐scale multispecies distribution changes can help identify structural changes in marine ecosystems associated with climate change.     

Estuary–ocean connectivity: fast physics,slow biology

Estuaries are connected to both land and ocean so their physical, chemical, and biological dynamics are influenced by climate patterns over watersheds and ocean basins. We explored climate‐driven oceanic variability as a source of estuarine variability by comparing monthly time series of temperature and chlorophyll‐a inside San Francisco Bay with those in adjacent shelf waters of the California Current System (CCS) that are strongly responsive to wind‐driven upwelling. Monthly temperature fluctuations inside and outside the Bay were synchronous, but their correlations weakened with distance from the ocean. These results illustrate how variability of coastal water temperature (and associated properties such as nitrate and oxygen) propagates into estuaries through fast water exchanges that dissipate along the estuary. Unexpectedly, there was no correlation between monthly chlorophyll‐a variability inside and outside the Bay. However, at the annual scale Bay chlorophyll‐a was significantly correlated with the Spring Transition Index (STI) that sets biological production supporting fish recruitment in the CCS. Wind forcing of the CCS shifted in the late 1990s when the STI advanced 40 days. This shift was followed, with lags of 1–3 years, by 3‐ to 19‐fold increased abundances of five ocean‐produced demersal fish and crustaceans and 2.5‐fold increase of summer chlorophyll‐a in the Bay. These changes reflect a slow biological process of estuary–ocean connectivity operating through the immigration of fish and crustaceans that prey on bivalves, reduce their grazing pressure, and allow phytoplankton biomass to build. We identified clear signals of climate‐mediated oceanic variability in this estuary and discovered that the response patterns vary with the process of connectivity and the timescale of ocean variability. This result has important implications for managing nutrient inputs to estuaries connected to upwelling systems, and for assessing their responses to changing patterns of upwelling timing and intensity as the planet continues to warm.     

Interannual Changes in Biomass Affect the Spatial Aggregations of Anchovy and Sardine as Evidenced by Geostatistical and Spatial Indicators

Geostatistical techniques were applied and a series of spatial indicators were calculated (occupation, aggregation, location, dispersion, spatial autocorrelation and overlap) to characterize the spatial distributions of European anchovy and sardine during summer. Two ecosystems were compared for this purpose, both located in the Mediterranean Sea: the Strait of Sicily (upwelling area) and the North Aegean Sea (continental shelf area, influenced by freshwater). Although the biomass of anchovy and sardine presented high interannual variability in both areas, the location of the centres of gravity and the main spatial patches of their populations were very similar between years. The size of the patches representing the dominant part of the abundance (80%) was mostly ecosystem- and species-specific. Occupation (area of presence) appears to be shaped by the extent of suitable habitats in each ecosystem whereas aggregation patterns (how the populations are distributed within the area of presence) were species-specific and related to levels of population biomass. In the upwelling area, both species showed consistently higher occupation values compared to the continental shelf area. Certain characteristics of the spatial distribution of sardine (e.g. spreading area, overlapping with anchovy) differed substantially between the two ecosystems. Principal component analysis of geostatistical and spatial indicators revealed that biomass was significantly related to a suite of, rather than single, spatial indicators. At the spatial scale of our study, strong correlations emerged between biomass and the first principal component axis with highly positive loadings for occupation, aggregation and patchiness, independently of species and ecosystem. Overlapping between anchovy and sardine increased with the increase of sardine biomass but decreased with the increase of anchovy. This contrasting pattern was attributed to the location of the respective major patches combined with the specific occupation patterns of the two species. The potential use of spatial indices as auxiliary stock monitoring indicators is discussed.     

Temperature and zooplankton size structure: climate control and basin‐scale comparison in the North Pacific

The global distribution of zooplankton community structure is known to follow latitudinal temperature gradients: larger species in cooler, higher latitudinal regions. However, interspecific relationships between temperature and size in zooplankton communities have not been fully examined in terms of temporal variation. To re‐examine the relationship on a temporal scale and the effects of climate control thereon, we investigated the variation in copepod size structure in the eastern and western subarctic North Pacific in 2000–2011. This report presents the first basin‐scale comparison of zooplankton community changes in the North Pacific based on a fully standardized data set obtained from the Continuous Plankton Recorder (CPR) survey. We found an increase in copepod community size (CCS) after 2006–2007 in the both regions because of the increased dominance of large cold‐water species. Sea surface temperature varied in an east–west dipole manner, showing the typical Pacific Decadal Oscillation pattern: cooling in the east and warming in the west after 2006–2007. The observed positive correlation between CCS and sea surface temperature in the western North Pacific was inconsistent with the conventional interspecific temperature–size relationship. We explained this discrepancy by the geographical shift of the upper boundary of the thermal niche, the 9°C isotherm, of large cold‐water species. In the eastern North Pacific, the boundary stretched northeast, to cover a large part of the sampling area after 2006–2007. In contrast, in the western North Pacific, the isotherm location hardly changed and the sampling area remained within its thermal niche throughout the study period, despite the warming that occurred. Our study suggests that while a climate‐induced basin‐scale cool–warm cycle can alter copepod community size and might subsequently impact the functions of the marine ecosystem in the North Pacific, the interspecific temperature–size relationship is not invariant and that understanding region‐specific processes linking climate and ecosystem is indispensable.     

Global impacts of the 1980s regime shift

Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change‐point analysis and a sequential t‐test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.