McCoy D, Bianchi D, Stewart AL. Global observations of submesoscale coherent vortices in the ocean. Progress in Oceanography [Internet]. 2020;189 :102452. Publisher's VersionAbstract
Subsurface-intensified anticyclones are ubiquitous in the ocean, yet their impact on the large-scale transport of heat, salt and chemical tracers is poorly understood. These submesoscale coherent vortices (SCVs) can trap and advect waters thousands of kilometers away from the formation region, providing a transport pathway that is unresolved by low-resolution Earth System Models. However, knowledge of the importance of these eddies for the large scale circulation is hindered by the lack of systematic observations. Here, we take advantage of the global network of Argo floats to identify occurrences of these eddies, which appear as weakly stratified anomalous water masses with Gaussian-shaped vertical structures. We develop a general algorithm to detect subsurface eddies that have propagated away from their source region, and apply it to the database of Argo float profiles, resulting in roughly 4000 detections from more than 20 years of observations. We further group detections into regional populations to identify hot-spots of generation and mechanisms of formation. Analysis of regional SCV statistics reveals important sites of SCV generation in Eastern Boundary Upwelling Systems, marginal sea overflows, and mode water formation regions along major open-ocean fronts. Because of the heat and salt anomaly contained within their cores, SCV could leave a significant imprint on the hydrographic properties of water masses in regions of high SCV density.
Kessouri F, Bianchi D, Renault L, McWilliams JC, Frenzel H, Deutsch CA. Submesoscale Currents Modulate the Seasonal Cycle of Nutrients and Productivity in the California Current System. Global Biogeochemical Cycles [Internet]. 2020;34 (10) :e2020GB006578. Publisher's VersionAbstract
In the California Current, subduction by mesoscale eddies removes nutrients from the coastal surface layer, counteracting upwelling and quenching productivity. Submesoscale eddies are also ubiquitous in the California Current, but their biogeochemical role has not been quantified yet in the region. Here, we present results from a physical-biogeochemical model of the California Current run at a resolution of 1 km, sufficient to represent submesoscale dynamics. By comparing it with a coarser simulation run at 4 km resolution, we demonstrate the importance of submesoscale currents for the seasonal cycles of nutrients and organic matter and highlight the existence of different regimes along a cross-shore gradient. In the productive coastal region, submesoscale currents intensify quenching and reduce productivity, further counteracting wind-driven upwelling. In the offshore oligotrophic region, submesoscale currents enhance the upward transport of nutrients, fueling a dramatic increase in new production. These effects are modulated by seasonality, strengthening near the coast during upwelling and offshore in wintertime. The intensification of the transport by submesoscale eddies drives an adjustment of the planktonic ecosystem, with a reduction of plankton biomass, productivity, and size near the coast and an increase offshore. In contrast, organic matter export by sinking particles and subduction of detritus and living cells are enhanced nearly everywhere. Similar processes are likely important in other regions characterized by seasonal upwelling, for example, other eastern boundary upwelling systems.
Wilson ST, Al-Haj AN, Bourbonnais A, Frey C, Fulweiler RW, Kessler JD, Marchant HK, Milucka J, Ray NE, Suntharalingham P, et al. Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment. Biogeosciences [Internet]. 2020;17 (22) :5809–5828. Publisher's Version
Yang S, Chang BX, Warner MJ, Weber TS, Bourbonnais AM, Santoro AE, Kock A, Sonnerup RE, Bullister JL, Wilson ST, et al. Global reconstruction reduces the uncertainty of oceanic nitrous oxide emissions and reveals a vigorous seasonal cycle. Proceedings of the National Academy of Sciences. 2020;117 (22) :11954–11960.
Bryndum-Buchholz A, Boyce DG, Tittensor DP, Christensen V, Bianchi D, Lotze HK. Climate-change impacts and fisheries management challenges in the North Atlantic Ocean. Marine Ecology Progress Series. 2020;648 :1–17.
Guiet J, Galbraith ED, Bianchi D, Cheung WWL. Bioenergetic influence on the historical development and decline of industrial fisheries. ICES Journal of Marine Science. 2020.
Howard EM, Penn JL, Frenzel H, Seibel BA, Bianchi D, Renault L, Kessouri F, Sutula MA, McWilliams JC, Deutsch C. Climate-driven aerobic habitat loss in the California Current System. Science Advances. 2020;6 (20) :eaay3188.
Palter J, Cook L, Neto AG, Nickford S, Bianchi D. SIDEBAR. Acoustic Backscatter Patterns. Oceanography. 2019;32 (3) :140–141.
Galbraith ED, Le Mézo P, Solanes Hernandez G, Bianchi D, Kroodsma D. Growth Limitation of Marine Fish by Low Iron Availability in the Open Ocean. Frontiers in Marine Science [Internet]. 2019;6 :509. Publisher's VersionAbstract
It is well-established that phytoplankton growth can be limited by the vanishingly low concentrations of dissolved iron found in large areas of the open ocean. However, the availability of iron is not typically considered an important factor in the ecology of marine animals, including fish. Here we compile observations to show that the iron contents of lower trophic level organisms in iron-limited regions can be an order of magnitude less than the iron contents of most fish. Although this shortfall could theoretically be overcome if iron assimilation rates were very high in fish, observations suggest this is not the case, consistent with the high recommended iron contents for mariculture feed. In addition, we highlight two occurrences among fish living in iron-poor regions that would conceivably be beneficial given iron scarcity: the absence of hemoglobin in Antarctic icefish, and the anadromous life history of salmon. Based on these multiple lines of evidence, we suggest that the iron content of lower trophic level organisms can be insufficient to support many fish species throughout their life cycles in iron-poor oceanic regions. We then use a global satellite-based estimate of fishing effort to show that relatively little fishing activity occurs in High Nitrate Low Chlorophyll (HNLC) regions, the most readily-identified iron-poor domains of the ocean, particularly when compared to satellite-based estimates of primary production and the observed mesozooplankton biomass in those waters. The low fishing effort is consistent with a low abundance of epipelagic fish in iron-limited regions, though other factors are likely to contribute as well. Our results imply that the importance of iron nutrition extends well beyond plankton and plays a role in the ecology of large marine animals.
Lotze HK, Tittensor DP, Bryndum-Buchholz A, Cheung WWL, Galbraith ED, Barange M, Barrier N, Bianchi D, Blanchard JL, Bopp L, et al. Global ensemble projections reveal trophicamplification of ocean biomass declineswith climate change. Proceedings of the National Academy of Sciences [Internet]. 2019. Publisher's VersionAbstract

While the physical dimensions of climate change are now routinely assessed through multimodel intercomparisons, projected impacts on the global ocean ecosystem generally rely on individual models with a specific set of assumptions. To address these single-model limitations, we present standardized ensemble projections from six global marine ecosystem models forced with two Earth system models and four emis- sion scenarios with and without fishing. We derive average biomass trends and associated uncertainties across the marine food web. Without fishing, mean global animal biomass decreased by 5% (±4% SD) under low emissions and 17% (±11% SD) under high emissions by 2100, with an average 5% decline for every 1 °C of warming. Projected biomass declines were primarily driven by increasing temperature and decreasing primary production, and were more pronounced at higher trophic levels, a process known as trophic amplification. Fishing did not substantially alter the effects of climate change. Considerable regional variation featured strong biomass increases at high latitudes and de- creases at middle to low latitudes, with good model agreement on the direction of change but variable magnitude. Uncertainties due to vari- ations in marine ecosystem and Earth system models were similar. Ensemble projections performed well compared with empirical data, emphasizing the benefits of multimodel inference to project future outcomes. Our results indicate that global ocean animal biomass con- sistently declines with climate change, and that these impacts are am- plified at higher trophic levels. Next steps for model development include dynamic scenarios of fishing, cumulative human impacts, and the effects of management measures on future ocean biomass trends.

Carozza DA, Bianchi D, Galbraith ED. Metabolic impacts of climate change on marine ecosystems: Implications for fish communities and fisheries. Global Ecology and Biogeography [Internet]. 2018.

Aim: Climate change will reshape marine ecosystems over the 21st century through diverse and complex mechanisms that are difficult to assess quantitatively. Here, we characterize expectations for how marine community biomass will respond to the energetic consequences of changes in primary production and temperature‐depend‐ ent metabolic rates, under a range of fishing/conservation scenarios.


Location: Global ocean.


Time period: 1950–2100.


Major taxa studied: Commercially harvested marine ectotherms (‘fish’).


Methods: We use a size‐structured macroecological model of the marine ecosystem, coupled with a catch model that allows for calibration with global historical data and simulation of fishing. We examine the four energetic mechanisms that, within the model framework, determine the community response to climate change: net pri‐ mary production, phytoplankton cell size, and the temperature dependencies of growth and natural mortality.


Results: Climate change decreases the modelled global fish community biomass by as much as 30% by 2100. This results from a diminished energy supply to upper trophic levels as photosynthesis becomes more nutrient limited and phytoplankton cells shrink, and from a temperature‐driven increase of natural mortality that, together, overwhelm the effect of accelerated somatic growth rates. Ocean circulation changes drive regional variations of primary production, producing patterns of winners and los‐ ers that largely compensate each other when averaged globally, whereas decreasing phytoplankton size drives weaker but more uniformly negative changes. The climate impacts are similar across the range of conservation scenarios but are slightly amplified in the strong conservation scenarios owing to the greater role of natural mortality. Main conclusions: The spatial pattern of climate impacts is mostly determined by changes in primary production. The overall decline of community biomass is attributed to a temperature‐driven increase of natural mortality, alongside an overall decrease in phytoplankton size, despite faster somatic growth. Our results highlight the importance of the competition between accelerated growth and mortality in a warming ocean.

Yamamoto A, Palter JB, Dufour CO, Griffies SM, Bianchi D, Claret M, Dunne JP, Frenger I, Galbraith ED. Roles of the ocean mesoscale in the horizontal supply of mass, heat, carbon and nutrients to the Northern Hemisphere subtropical gyres. Journal of Geophysical Research: Oceans [Internet]. 2018. Publisher's VersionAbstract

Horizontal transport at the boundaries of the subtropical gyres plays a crucial role in providing the nutrients that fuel gyre primary productivity, the heat that helps restratify the surface mixed layer, and the dissolved inorganic carbon that influences air‐sea carbon exchange. Mesoscale eddies may be an important component of these horizontal transports; however, previous studies have not quantified the horizontal tracer transport due to eddies across the subtropical gyre boundaries. Here we assess the physical mechanisms that control the horizontal transport of mass, heat, nutrients, and carbon across the North Pacific and North Atlantic subtropical gyre boundaries using the eddy‐rich ocean component of a climate model (Geophysical Fluid Dynamics Laboratory Climate Model version 2.6) coupled to a simple biogeochemical model (mini‐BLING). Our results suggest that horizontal transport across the gyre boundaries supplies a substantial amount of mass and tracers to the ventilated layer of both Northern Hemisphere subtropical gyres, with the Kuroshio and Gulf Stream acting as main exchange gateways. Mass, heat, and dissolved inorganic carbon supply is principally driven by the time‐mean circulation, whereas nutrient transport differs markedly from the other tracers, as nutrients are mainly supplied to both subtropical gyres by downgradient eddy mixing across gyre boundaries. A budget analysis further reveals that the horizontal nutrient transport, combining the roles of both mean and eddy components, is responsible for more than three quarters of the total nutrient supply into the subtropical gyres, surpassing a recent estimate based on a coarse‐resolution model and thus further highlighting the importance of horizontal nutrient transport.

Claret M, Galbraith ED, Palter JB, Bianchi D, Fennel K, Gilbert D, Dunne JP. Rapid coastal deoxygenation due to ocean circulation shift in the northwest Atlantic. Nature Climate Change [Internet]. 2018 :1. Publisher's VersionAbstract
Global observations show that the ocean lost approximately 2% of its oxygen inventory over the past five decades, with important implications for marine ecosystems. The rate of change varies regionally, with northwest Atlantic coastal waters showing a long-term drop that vastly outpaces the global and North Atlantic basin mean deoxygenation rates. However, past work has been unable to differentiate the role of large-scale climate forcing from that of local processes. Here, we use hydrographic evidence to show that a Labrador Current retreat is playing a key role in the deoxygenation on the northwest Atlantic shelf. A high-resolution global coupled climate–biogeochemistry model reproduces the observed decline of saturation oxygen concentrations in the region, driven by a retreat of the equatorward-flowing Labrador Current and an associated shift towards more oxygen-poor subtropical waters on the shelf. The dynamical changes underlying the shift in shelf water properties are correlated with a slowdown in the simulated Atlantic Meridional Overturning Circulation (AMOC). Our results provide strong evidence that a major, centennial-scale change of the Labrador Current is underway, and highlight the potential for ocean dynamics to impact coastal deoxygenation over the coming century.
Bianchi D, Weber TS, Kiko R, Deutsch C. Global niche of marine anaerobic metabolisms expanded by particle microenvironments. Nature Geoscience [Internet]. 2018. Publisher's VersionAbstract

In ocean waters, anaerobic microbial respiration should be confined to the anoxic waters found in coastal regions and tropical oxygen minimum zones, where it is energetically favourable. However, recent molecular and geochemical evidence has pointed to a much broader distribution of denitrifying and sulfate-reducing microbes. Anaerobic metabolisms are thought to thrive in microenvironments that develop inside sinking organic aggregates, but the global distribution and geochemical significance of these microenvironments is poorly understood. Here, we develop a new size-resolved particle model to predict anaerobic respiration from aggregate properties and seawater chemistry. Constrained by observations of the size spectrum of sinking particles, the model predicts that denitrification and sulfate reduction can be sustained throughout vast, hypoxic expanses of the ocean, and could explain the trace metal enrichment observed in particles due to sulfide precipitation. Globally, the expansion of the anaerobic niche due to particle microenvironments doubles the rate of water column denitrification compared with estimates based on anoxic zones alone, and changes the sensitivity of the marine nitrogen cycle to deoxygenation in a warming climate.

Frenger I, Bianchi D, Stührenberg C, Oschlies A, Dunne J, Deutsch C, Galbraith E, Schütte F. Biogeochemical role of subsurface coherent eddies in the ocean: Tracer cannonballs, hypoxic storms, and microbial stewpots?. Global Biogeochemical Cycles [Internet]. 2018;32 (2) :226-249. Publisher's VersionAbstract

Subsurface coherent eddies are well-known features of ocean circulation, but the sparsity of observations prevents an assessment of their importance for biogeochemistry. Here, we use a global eddying (0.1° ) ocean-biogeochemical model to carry out a census of subsurface coherent eddies originating from eastern boundary upwelling systems (EBUS), and quantify their biogeochemical effects as they propagate westward into the subtropical gyres. While most eddies exist for a few months, moving over distances of 100s of km, a small fraction (< 5%) of long-lived eddies propagates over distances greater than 1000km, carrying the oxygen-poor and nutrient-rich signature of EBUS into the gyre interiors. In the Pacific, transport by subsurface coherent eddies accounts for roughly 10% of the offshore transport of oxygen and nutrients in pycnocline waters. This "leakage" of subsurface waters can be a significant fraction of the transport by nutrient-rich poleward undercurrents, and may contribute to the well-known reduction of productivity by eddies in EBUS. Furthermore, at the density layer of their cores, eddies decrease climatological oxygen locally by close to 10%, thereby expanding oxygen minimum zones. Finally, eddies represent low-oxygen extreme events in otherwise oxygenated waters, increasing the area of hypoxic waters by several percent and producing dramatic short-term changes that may play an important ecological role. Capturing these non-local effects in global climate models, which typically include non-eddying oceans, would require dedicated parameterizations.

Galbraith ED, Carozza DA, Bianchi D. A coupled human-Earth model perspective on long-term trends in the global marine fishery. Nature Communications [Internet]. 2017;8 :14884. Publisher's VersionAbstract

The global wild marine fish harvest increased fourfold between 1950 and a peak value near the end of the 20th century, reflecting interactions between anthropogenic and ecological forces. Here, we examine these interactions in a bio-energetically constrained, spatially and temporally resolved model of global fisheries. We conduct historical hindcasts with the model, which suggest that technological progress can explain most of the 20th century increase of fish harvest. In contrast, projections extending this rate of technological progress into the future under open access suggest a long-term decrease in harvest due to over-fishing. Climate change is predicted to gradually decrease the global fish production capacity, though our model suggests that this is of secondary importance to social and economic factors. Our study represents a novel way to integrate human-ecological interactions within a single model framework for long-term simulations.

Carozza DA, Bianchi D, Galbraith ED. Formulation, General Features and Global Calibration of a Bioenergetically-Constrained Fishery Model. PloS one [Internet]. 2017;12 (1) :e0169763. Publisher's VersionAbstract

Human exploitation of marine resources is profoundly altering marine ecosystems, while climate change is expected to further impact commercially-harvested fish and other species. Although the global fishery is a highly complex system with many unpredictable aspects, the bioenergetic limits on fish production and the response of fishing effort to profit are both relatively tractable, and are sure to play important roles. Here we describe a generalized, coupled biological-economic model of the global marine fishery that represents both of these aspects in a unified framework, the BiOeconomic mArine Trophic Size-spectrum (BOATS) model. BOATS predicts fish production according to size spectra as a function of net primary production and temperature, and dynamically determines harvest spectra from the biomass density and interactive, prognostic fishing effort. Within this framework, the equilibrium fish biomass is determined by the economic forcings of catchability, ex-vessel price and cost per unit effort, while the peak harvest depends on the ecosystem parameters. Comparison of a large ensemble of idealized simulations with observational databases, focusing on historical biomass and peak harvests, allows us to narrow the range of several uncertain ecosystem parameters, rule out most parameter combinations, and select an optimal ensemble of model variants. Compared to the prior distributions, model variants with lower values of the mortality rate, trophic efficiency, and allometric constant agree better with observations. For most acceptable parameter combinations, natural mortality rates are more strongly affected by temperature than growth rates, suggesting different sensitivities of these processes to climate change. These results highlight the utility of adopting large-scale, aggregated data constraints to reduce model parameter uncertainties and to better predict the response of fisheries to human behaviour and climate change.

Holzer M, DeVries T, Bianchi D, Newton R, Schlosser P, Winckler G. Objective estimates of mantle 3He in the ocean and implications for constraining the deep ocean circulation. Earth and Planetary Science Letters [Internet]. 2017;458 :305 - 314. Publisher's VersionAbstract

Abstract Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in 3He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the 3He/4He isotope ratio to constrain the ocean's mantle 3He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal 3He source distribution that minimizes the mismatch between modeled and observed δ3He. In this way, we are able to fit the observed δ3He distribution to within a relative error of ∼15%, with a global 3He source that ranges from 640 to 850 mol yr−1, depending on circulation. The fit captures the vertical and interbasin gradients of the δ3He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ3He distributions still display some systematic biases, especially in the deep North Pacific where δ3He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ3He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of \CFCs\ and radiocarbon for either eddy parameterization.

Carozza DA, Bianchi D, Galbraith ED. The ecological module of BOATS-1.0: a bioenergetically constrained model of marine upper trophic levels suitable for studies of fisheries and ocean biogeochemistry. Geoscientific Model Development. 2016;9 (4) :1545–1565.Abstract

Environmental change and the exploitation of marine resources have had profound impacts on marine communities, with potential implications for ocean biogeochemistry and food security. In order to study such global-scale problems, it is helpful to have computationally efficient numerical models that predict the first-order features of fish biomass production as a function of the environment, based on empirical and mechanistic understandings of marine ecosystems. Here we describe the ecological module of the BiOeconomic mArine Trophic Size-spectrum (BOATS) model, which takes an Earth-system approach to modelling fish biomass at the global scale. The ecological model is designed to be used on an Earth-system model grid, and determines size spectra of fish biomass by explicitly resolving life history as a function of local temperature and net primary production. Biomass production is limited by the availability of photosynthetic energy to upper trophic levels, following empirical trophic efficiency scalings, and by well-established empirical temperature-dependent growth rates. Natural mortality is calculated using an empirical size-based relationship, while reproduction and recruitment depend on both the food availability to larvae from net primary production and the production of eggs by mature adult fish. We describe predicted biomass spectra and compare them to observations, and conduct a sensitivity study to determine how they change as a function of net primary production and temperature. The model relies on a limited number of parameters compared to similar modelling efforts, while retaining reasonably realistic representations of biological and ecological processes, and is computationally efficient, allowing extensive parameter-space analyses even when implemented globally. As such, it enables the exploration of the linkages between ocean biogeochemistry, climate, and upper trophic levels at the global scale, as well as a representation of fish biomass for idealized studies of fisheries.

Bianchi D, Mislan KAS. Global patterns of diel vertical migration times and velocities from acoustic data. Limnology and Oceanography [Internet]. 2016;61 (1) :353–364. Publisher's VersionAbstract

Diel vertical migrations (DVM) of zooplankton and micronekton are observed throughout the world ocean, where they influence ecological interactions and biogeochemical cycles. Despite their common occurrence, descriptions of the characteristics of these migrations are currently limited at the large scale. We analyze trajectories of migrations from a global dataset of acoustic backscatter to identify the large-scale patterns of the timing and speed of DVM. Sound scattering layers (SSL) leave the surface 21 ± 20 min before sunrise, and return to it 17 ± 23 min after sunset, while changes in bulk surface backscatter appear to be nearly synchronous to sunrise and sunset. Mean downward migrations (7.6 ± 3.6 cm s−1) are significantly faster than mean upward migrations (6.5 ± 3.5 cm s−1). Furthermore, coherent and predictable variations of migration properties at the scale of ocean basins are evident. These variations appear to be related to the depths of migration, such that deeper migrations, observed for example in the subtropical gyres, the western tropical Pacific and the Southern Ocean, show earlier departures and later arrivals than shallower migrations. Vertical velocities peak in the tropical and subtropical regions, and decline towards the poles, with the strongest declines observed in the North Pacific. Migration velocities are also correlated to migration depths, with deeper migrations being faster than shallow migrations. These new constraints on the characteristics of migrating SSL could help shed light on the physiological, ecological, and environmental controls that regulate DVM behavior.