Topic 1: Prediction and Monitoring
Laure Resplandy • Stephanie Henson • Nicolas Gruber
Topic 2: Ecosystem Impacts
Lisa Levin • Alf Norkko • Arnaud Bertrand
Topic 3: Ventilation and Oxygen Supply
Peter Brandt • Ivonne Montes • Laurent Bopp
Topic 4: Microbial Communities and their Impact on Biogeochemical Cycles in Oxygen Minimum Zones
Osvaldo Ulloa • Bess Ward • Bo Thamdrup
Topic 5: Major Upwelling Systems
Marina Lévy • Damian Arévalo-Martínez • Carolin Löscher
Topic 6: Physiological Effects of Oxygen and Interactions with Multiple Stressors
Denise Breitburg • Brad Seibel • Tom Fenchel
Topic 7: Impacts on Fisheries/Socioeconomics
Kenny Rose • Michele Casini • Rashid Sumaila
Topic 8: Coastal Systems: From Understanding to Management
Daniel Conley • Nancy Rabalais • Jianping Gan
Topic 9: Ocean Deoxygenation - How the Past can Inform the Future
Eric Galbraith • Zunli Lu • Dimitri Gutierrez
Topic 10: Biogeochemical Cycles: Feedbacks and Interactions
Wei-Jun Cai • Donald Canfield • Tom Jilbert
Gendered Research and Innovations
Londa Schiebinger
Topic 1: Prediction and Monitoring
Constraints on ocean de-oxygenation and warming from atmospheric oxygen observations
Laure Resplandy
O2 inventories in the ocean and the atmosphere are linked. As the ocean warms, it loses O2 to the atmosphere. The amount of O2 lost by the ocean could be quantified with the complementary change observed in the atmosphere, using precise atmospheric O2 measurements spanning nearly three decades. This method is not limited by data sparseness, as fast mixing in the atmosphere efficiently integrates the global ocean signal, and could provide an independent constraint on the global ocean O2 loss obtained from hydrographic data. Here, we use atmospheric O2 and C O2 measurements combined into the tracer “atmospheric potential oxygen” (APO) to remove land biosphere influences in atmospheric O2. The signature of ocean changes (solubility, ocean circulation and biological photosynthesis and respiration) in APO is further isolated from the impacts of industrial processes, anthropogenic aerosol deposition etc. We show that a clear positive signal in APO emerges, i.e. the global ocean response to climate change has unambiguously led to a net ocean outgassing of O2 and CO2. We use this observed ocean-driven change in APO to evaluate ocean de-oxygenation and discuss the uncertainties related to this method that implicitly assumes a carbon-to-heat ratio in the ocean. Hydrographic observations and Earth system models indicate, however, that this ocean-driven APO signal is largely dominated by solubility changes, and is therefore a good proxy of the global change in ocean heat content. Using this tight link between APO and heat, we provide a much-needed independent constraint on the ocean heat uptake, at the high end of previous in-situ temperature based estimates (1.41 ± 0.36 x 1022 J /y since 1991).
Emergence and detection of climate change-driven trends in oceanic oxygen concentration
Stephanie Henson
Climate change is expected to modify ecological responses in the ocean, with the potential for important effects on the ecosystem services provided to humankind. As part of the effort towards detection of long‐term trends, a network of ocean observatories and time series stations provide high quality data for a number of key parameters, such as oxygen concentration. The temporal and spatial scales over which observations of thermocline oxygen concentration must be made to robustly detect a long‐term trend are assessed. As a global average, continuous time series are required for ~ 26 years to distinguish a climate change trend in oxygen concentration from natural variability. Regional differences are extensive, with temperate latitudes generally requiring shorter time series (<~30 years) to detect trends than other areas. In addition, the ‘footprint’ of existing and planned time series stations, that is the area over which a station is representative of a broader region, is quantified. The existing network of observatories is representative of oxygen concentrations over only 9% of the global ocean.
Climate-driven changes in oxygen concentration are unlikely to occur in isolation and multiple factors may act additively or synergistically to increase the impact of deoxygenation. How rapidly multiple drivers of marine ecosystem change, including oxygen concentration, develop in the future ocean is assessed. By analysing an ensemble of models we find that, within the next 15 years, the climate change-driven trends in multiple ecosystem drivers emerge from the background of natural variability in 55% of the ocean and propagate rapidly to encompass 86% of the ocean by 2050 under a ‘business-as-usual’ scenario. However, we the exposure of marine ecosystems to climate change-induced stress can be drastically reduced via climate mitigation measures; with mitigation, the proportion of ocean susceptible to multiple drivers within the next 15 years is reduced to 34%. Mitigation slows the pace at which multiple drivers emerge, allowing an additional 20 years for adaptation in marine ecological and socio-economic systems alike.
Variability and extremes in ocean (de)oxygenation and water column denitrification in the eastern tropical Pacific
Nicolas Gruber, Simon Yang, Ana Franco, Matthias Münnich, and Meike Vogt
The Eastern Tropical Pacific (ETP) hosts two of the world’s three Oxygen Deficient Zones (ODZs), large bodies of suboxic water that are subject to high rates of water column denitrification (WCD). In the mean, these two ODZs are responsible for 15 to 40% of all fixed N loss in the ocean, but knowledge is limited on how this loss varies in time. Here, we use hindcast simulations with both a global and a regional model to assess the variability and the extremes in the (de)oxygenation of the ETP and its impact on WCD. Using the global model, i.e., the ocean component of the NCAR Community Earth System Model, we showed already that the El Niño–Southern Oscillation (ENSO) is a major driver for extreme conditions in the ODZ. Namely, we found that ENSO causes large variations in WCD, with mature La Niña (El Niño) conditions having peak denitrification rates that are up to 70% higher (lower) than the mean rates (Yang et al., 2017). This large variability is the result of wind-driven changes in circulation and isopycnal structure concurrently modifying the thermocline distribution of O2and organic matter export in such a way that the response of WCD is strongly amplified. Of particular importance is the shoaling (deepening) of the upper boundary of the ODZs, as this results in a much larger fraction of the exported organic matter being subject to anaerobic remineralization, i.e., WCD. While the global model is well positioned to diagnose and analyze such large-scale events. its usefulness is limited to assess the role of smaller, shorter, but often more intense extreme events. To this end, we will be using hindcast simulations with a high-resolution regional model of the ETP region based on the Regional Oceanic Modeling System (ROMS). Initial analyses suggested that a some of these events are associated with mesoscale eddies that create very strong low oxygen environments, while other events are the result of regional-scale oceanic or atmospheric "weather" conditions. Of particular relevance are those events when ENSO and local processes push the system toward very extreme conditions. Given the highly non-linear nature of the marine oxygen and nitrogen cycle, such extreme events can leave a disproportional impact on the overall (de)oxygenation of the ETP and its WCD, with strong implications on the global-scale balance of the marine N cycle and the emission of the greenhouse gas N2O.
Topic 2: Ecosystem Impacts
Benthic Ecosystem Responses to Open Ocean Deoxygenation
Lisa Levin
Knowledge of benthic ecosystem response to ocean deoxygenation can be derived from the paleo-record, natural gradients, laboratory and mesocosm experiments and by using environmental proxies. Here I will focus on how natural oxygen gradients on upwelling margins associated with oxygen minimum zones offer tremendous insight into the consequences of ocean deoxygenation for benthic ecosystems. Changes in ecosystem structure include declines in biodiversity manifested as altered taxonomic composition, number and distribution of species, declines in body size, biomass and architectural complexity of taxa, as well as changes in vertical and geographic distribution. These structural changes are translated into altered ecosystem functions involving production, habitat provision, bioturbation, colonization potential and resilience, and trophic functions reflected in feeding modes, symbioses and carbon fixation pathways as well as visual behaviors, species interactions and bentho-pelagic coupling. Functional shifts are then manifested as altered ecosystem services. Fisheries may diminish under dysoxic conditions but flourish where individuals aggregate at hypoxic zone margins. Enhanced local C sequestration may emerge from limited remineralization and high deposition within OMZs or from carbonate precipitation under anaerobic methane oxidation, whereas stratification-induced nutrient limitation may ultimately limit atmospheric C drawdown. Genetic novelty in OMZs could fuel new industrial applications. Changes in greenhouse gases, habitat availability and species distributions create climate and hydrographic feedbacks that could further modify ecosystems. Exceedingly small increases or declines in oxygen (of 5 mMol kg-1 or less) can lead to state changes in benthic ecosystems when they are initiated at low oxygen concentrations. Thus, even small changes in oxygenation projected to occur in the next century may have large consequences on margins with oxygen minima. Our challenge is to identify where and when these will matter most and manage accordingly. In the majority of open ocean settings, oxygen rarely changes without concurrent shifts in temperature, CO2 and food supply. Understanding the interplay of these factors in shaping ecosystem structure, function and services is of increasing importance in a rapidly changing ocean.
Coastal hypoxia and benthic ecosystem functioning
Alf Norkko
Soft-sediment habitats are important in the global cycles of elements and energy. The benthic fauna play pivotal roles in affecting benthic-pelagic coupling and sediment biogeochemistry, but rapidly changing oxygen conditions are profoundly changing benthic–pelagic processes. In coastal seas, excessive organic enrichment and rising temperatures are resulting in spreading hypoxia, comprising not only the integrity of faunal communities, but also their contribution to key processes and ecosystem functions in coastal ecosystems, including nutrient transformation and retention. Over the past 40 years, since the publication of the Pearson & Rosenberg model on benthic succession, our understanding of structural and functional change associated with increasing organic enrichment has grown tremendously. Nevertheless, our knowledge of how feedbacks between the biodiversity of macrofaunal communities and key ecosystem processes such as organic matter mineralization, burial, and nutrient transformation pathways change across gradients of hypoxic stress are still limited. Hence our ability to generalize how changing biodiversity modifies ecosystem functions in real world settings is limited due to the complexity of natural ecosystems. In this talk I will provide examples from a combination of in situ field experiments, large-scale field surveys and modelling efforts across the Baltic Sea, which aim to resolve how macrofaunal communities process organic matter and mediate nutrient fluxes across the sediment-water interface, while exploring how gradients in eutrophication and hypoxia modify these relationships. These studies suggest that the relationship between benthic community structure and nutrient fluxes is highly context dependent and dictated by local communities and environmental conditions. Changes in the abundance and distribution of functionally important species due to progressing eutrophication and climate change thus has important feedbacks on the functioning of seafloor ecosystems. Embedding experimental work along environmental gradients may thus be particularly powerful for resolving such context dependency in benthic-pelagic coupling and ecosystem functioning, to meet the demands of management and conservation.
How the OMZ can drive ecosystem functioning from plankton to seabirds and fishers
Arnaud Bertrand
Eastern boundary current systems (EBUS) present contrasted levels of anoxia. A shallow oxycline reduces the habitat for most species, concentrating pelagic life within a thin surface layer. This is particularly significant at night when diel migrant organisms occupy the surface oxygenated layer to ‘pay their oxygen debt’. Such organisms’ concentration can enhance trophic interactions among species. Some winners indeed benefit from the lower energetic cost to forage. On the other hand, certain species cannot survive within a too narrow oxygenated habitat and are expulsed from the system. In addition, habitat compression does not occur in the vertical plane only. Indeed the oxycline/pycnocline is not a flat boundary but is shaped by internal wave, submeso- and meso-scale processes. This lead to local downward deformation of the clines, generating small-scale oases for life. In this context, the objective of this work is to review the impact of the oxygen minimum zone, in particular the spatiotemporal dynamics of its upper boundary, on planktonic and nektonic (fish and squids) species in EBUS. In addition, we will discuss on the consequences for population dynamics and foraging efficiency of air-breathing predators, in particular seabirds and fishers.
Topic 3: Ventilation and Oxygen Supply
Drivers and mechanisms of thermocline oxygen changes in the eastern tropical North Atlantic
Peter Brandt
The tropical thermocline that covers the strongly stratified region below the oceanic mixed layer accommodates extended oxygen minimum zones (OMZs). The subtropical-tropical circulation of the Pacific and Atlantic oceans characterized by the wind-driven equatorward and westward flow of newly subducted water masses sets the poleward boundaries of the OMZs in both oceans. Energetic circulation along the equator results in enhanced oxygen supply to the eastern basins from the well-ventilated western boundary thereby separating the OMZs of both hemispheres by an equatorial oxygen maximum.
Observations indicate a significant and broad-scale decrease of oxygen levels and associated increase in the volume of oxygen deficient waters of the eastern tropical North Atlantic (ETNA) during the past 50 years, which is part of the widespread deoxygenation of the global ocean’s thermocline. During the recent decade covered by enhanced observations in the framework of the German collaborative research centre SFB754 a more distinct pattern emerges consisting of regions with decreasing and increasing oxygen along the 23°W repeat section cutting through the ETNA OMZ. The observed spatial variations in the obtained ten-year trend may help to understand the underlying drivers and mechanisms of long-term oxygen changes. Current models do not reproduce observed patterns of oxygen changes in the ocean’s thermocline. Nevertheless, they reveal a close correlation between declining oxygen levels and increases in water age on isopycnal surfaces in the thermocline suggesting decreasing ventilation as the dominant driver of thermocline deoxygenation whereas changes in respiration seem to play only a minor role. Here we investigate and review possible drivers and mechanisms of the observed oxygen changes in the tropical thermocline including changes in the wind-driven and eddy-driven circulation, alterations of ventilation pathways, and changes in stratification by taking into account a large data base of hydrographic, oxygen and current data as well as idealized process and realistic models.
Physical Forcings of Eastern Pacific OMZ
Ivonne Montes, Boris Dewitte, Joel Sudre, Martin Hernandez-Ayon, Aurélien Paulmier, and Véronique Garçon
The Eastern Pacific (EP) hosts one of the most productive eastern boundary upwelling systems (EBUS) of the world oceans, encompassing an extended Oxygen Minimum Zone (OMZ). EBUS is characterized by complex dynamical processes spanning a wide range of spatio-temporal variability due to the strong coupling between the ocean and atmosphere. In particular, the EP OMZ is strongly modulated by the remote variability exerted by the equatorial dynamic (i.e., circulation, El Niño), local variability exerted by coastal upwelling driven by the divergence in Ekman transport and by high-frequency variability exerted by mesoscale activity. We provide here a brief synthesis of current research related to dynamical processes, both physical and biogeochemical, involved in the EP OMZ.
Ocean deoxygenation in the 21st century: the role of ventilation
Laurent Bopp
All Earth System models project a consistent decrease in the oxygen content of the ocean for the coming decades. The processes at play in each of the models are ocean warming, reduced ventilation and increased stratification. At the regional scale and for the subsurface ocean however, these model projections are much less robust, which reflect a complex interplay of processes responsible for the the changes in ocean oxygen levels. In this presentation, I make use of simulations performed in the context of the Coupled Model Intercomparison project 5 (CMIP5). I will show that the sub-surface deoxygenation trend is partly balanced by a reduction in sub-surface respiration. I will also show that in many of these models, the tropical ocean undergoes partial re-ventilation, and hence a rejuvenation of the sub-surface water masses, even in case of increased vertical stratification. This may lead to a partial tropical sub-surface re-oxygenation with global warming. To get more insights into the role of ventilation in oxygen trends, changes in oxygen fluxes are analysed at the based of the mixed layer in one of the CMIP5 models. I also turn to past climates, such as the Last Glacial Maximum, or even deep time anoxic events, to decipher the role of ventilation in driving past changes in oxygen.
Topic 4: Microbial Communities and their Impact on Biogeochemical Cycles in Oxygen Minimum Zones
New microbial players in oxygen-minimum-zone biogeochemistry
Osvaldo Ulloa
Key microbial players from oxygen minimum zones (OMZs) have been identified, but the majority remains unrecognized or uncharacterized. The application of single-cell genomics -in combination with microbial-community gene content, transcription, and process rate measurements- offers the possibility to gain some insight into the distribution, metabolic potential, and activity of previously uncharacterized bacterial and archaeal groups. Such approach is revealing new potentially import players for OMZ biogeochemistry, as well as the presence of some apparently overlooked pathways. Quantifying the importance of such microbial processes at the local and global scale and putting them into a general ecological framework remain a challenge.
Nitrogen biogeochemistry of oxygen minimum zones: what controls the distribution of microbes and N transformation reactions?
Bess Ward, Amal Jayakumar, Xin Sun, and Claudia Frey
Oxygen minimum zones are distinguished by characteristic distributions of the inorganic nitrogen compounds nitrate, nitrite, and nitrous oxide. Although many of the reactions that produce and consume these compounds are known, the mechanisms by which characteristic features, such as the secondary nitrite maximum and the subsurface nitrous oxide maximum, are maintained are not clear. Oxygen is one of the most important constraints on the distributions of chemistry and microbial reactions. Nevertheless, there is evidence for “incompatible reactions”, such as oxidation of nitrite in the absence of oxygen and consumption of nitrous oxide in the presence of oxygen. Data from stable isotope tracer incubation experiments, distributions of natural abundance of stable isotopes, and molecular data on the composition, diversity and distribution of microbes and their key functional genes related to these processes will be considered. These data will be used to characterize the functional response of microbial N transformations involved in nitrite and nitrous oxide production and consumption in relation to oxygen concentration and other variables. Relationships between rate measurements and molecular data will be used to investigate the depth distribution of microbes, processes, and chemistry, and to develop hypotheses about control of the “incompatible reactions”.
Anaerobic methane oxidation is an important sink for methane in the ocean’s largest oxygen minimum zone
Bo Thamdrup, Herdís G. R. Steinsdottir, Laura A. Bristow, Cory C. Padilla, Anthony D. Bertagnolli, Nastassia V. Patin, and Frank J. Stewart
The eastern tropical North Pacific (ETNP) harbours the largest of the oceans’ three major oxygen minimum zones and is further characterized by what appears to be the largest open-ocean accumulation of methane, with peak concentrations located in the anoxic core. Benthic methanogenesis is a major methane source in the ETNP OMZ but it remains unknown to which extent methane is cycled within the OMZ or released to the surrounding oxic waters, thus representing a source of methane to the atmosphere. However, the recent discovery of transcriptionally active bacteria closely related to the anaerobic methanotroph “CandidatusMethylomirabilis oxyfera” of the NC10 clade in OMZs, suggests that the OMZ methane pool might be more dynamic than previously recognized.
We measured methane oxidation rates in anoxic incubations of ETNP OMZ waters using tritium-labelled methane. The highest rates were found at depths intermediate between the secondary nitrite maximum and the deeper peak in methane concentrations within the OMZ core. By contrast, activity was at or below detection in the lower oxycline and at the oxic/anoxic interface. In oxygen addition experiments the process was inhibited by low micromolar oxygen concentrations, which further indicated an anaerobic metabolism, possibly performed by nitrite-dependent NC10 bacteria. The rate constant of anaerobic methane oxidation was approx. 0.3 y-1. Given available estimates of water residence time, this implies that anaerobic methane oxidation is a substantial methane sink in the ETNP OMZ and hence attenuates the emission of methane from this and possibly other oxygen minimum zones.
Topic 5: Major Upwelling Systems
The combination of high biological production and weak oceanic ventilation over coastal upwelling systems cause large-scale oxygen minimum zones (OMZs) that profoundly affect marine habitats and alter key biogeochemical cycles. These low subsurface oxygen levels are determined by a balance between remineralization and ventilation, which both vary over multiple time-scales and have local as well as remote sources.
In this presentation, I will highlight key factors affecting the variability of the Arabian Sea OMZ over seasonal, interannual and climate-change time scales, using high-resolution model simulations of the Northern Indian Ocean performed using a range of physical (NEMO, ROMS) and biogeochemical (NPZD, PISCES) models.
The Arabian Sea OMZ has unusual characteristics compared to the two other main OMZ, which are found in Eastern Boundary Upwelling Systems, along the eastern Pacific and Atlantic, respectively. In the Arabian Sea, the strongest coastal upwelling is driven in summer by southwest monsoon winds and is located along the western coast, while the OMZ is shifted eastward from this region of highest productivity and reaches the Indian coastal waters. Furthermore, the Arabian Sea OMZ is the thickest of the three oceanic OMZ, with near-total depletion of oxygen (suboxia) at depths 200-1000m associated with intense denitrification. Suboxic events are particularly dramatic when they occur along coastal areas, as they induce episodes of fish mortality and shorter fishing season, inducing a sharp decline in fish catches. The economy based on fisheries of these dense population regions is thus highly vulnerable to the variability in the strength of this OMZ and to the frequency of suboxic events.
I will examine several factors that were found to prevent and/or limit anoxia, over intra-seasonal to interannual time-scales. First, eddy-driven ventilation strongly limits their extent, limiting the associated denitrification. This has a positive feedback on primary production with the counter-intuitive consequence of increasing the extent of hypoxia. Second, anoxia occurs when the OMZ is raised toward the surface. This is reinforced by remineralisation fluxes that occur essentially in the upper twilight zone. Over inter-annual time-scales, the strongest coastal anoxic events occur essentially during negative Indian Ocean Dipole events which are associated with an upwelling of the thermocline and of the oxycline. Finally, over climate change time scale, variations in monsoon wind intensity and large-scale ventilation both affect the volume occupied by the OMZ.
The “bad” breath of the ocean: Greenhouse gas emissions from Eastern Boundary Upwelling Ecosystems
Damian Arévalo-Martínez
Eastern Boundary Upwelling Ecosystems (EBUS) are well-known “hotspots” for production of greenhouse gases (GHG). Although each year tons of GHG are emitted out of the global ocean, EBUS are focal points with a disproportionally high share of the total efflux of these gases. Given that EBUS are a fundamental component of the socio-economic development of the bordering countries, the associated anthropogenic activities bear the potential to exacerbate the already increasing GHG emissions. Since GHG-driven warming is thought to be the major cause for ocean deoxygenation, it is of utmost importance to understand the distribution and variability of these gases over wide temporal and spatial scales. Throughout the last decade great progress has been achieved in the development of methods for continuous in situmonitoring of GHG, which not only contributes to fill the existing gaps in data coverage but also helps improving modelling approaches for predicting the emission trends of GHG with future climate change. In this talk I will present observational trends of GHG in all four EBUS and discuss how to find reasonable temporospatial scales for linking observational and modelling programs. Likewise I will address some of the major environmental problems leading to increased deoxygenation and how they influence the emissions of the major GHG in EBUS.
Does the Ocean lose its breath?
Carolin Löscher
Oxygen (O2) plays a critical role for life on Earth, but over the last 50 years, O2concentrations in the Ocean have decreased massively as a consequence of human activity. These activities include greenhouse gas emissions and nutrient discharge to coastal waters, with major impacts on Ocean biogeochemistry and ecology. This effect is particularly intense in O2depleted tropical Ocean areas, also referred to as oxygen minimum zones (OMZ). These are sensitive areas as OMZs are connected to nutrient-rich coastal upwelling systems and support some of the world’s most prolific fisheries. While continuous OMZ expansion would lead to a decreasing habitat for fish and other organisms, feedback mechanisms which counteract O2depletion and stabilize the system’s ecological functionality may however explain and sustain high productivity.
Microbes are essential to regulating ocean biogeochemistry, oxygen and nutrient turnover and may those hold the key to stabilizing the oxygen budget to a certain extent. In this talk, I will discuss the potential of microbes involved in the nitrogen cycle to mitigate against Ocean oxygen depletion via feedback cycling from an integrated molecular, biogeochemical, and modeling perspective.
Topic 6: Physiological Effects of Oxygen and Interactions with Multiple Stressors
It’s not just oxygen: Understanding and managing a multiple stressor world
Denise Breitburg
Deoxygenation does not occur in isolation. Anthropogenic nutrient enrichment, acidification, fisheries, rising temperatures, and other consequences of human activities can all influence the process of deoxygenation and alter physiological and ecological consequences of oxygen decline. The effects of multiple stressors can depend on the timing and order of exposures as well as severity; interactive effects can occur at levels of organization ranging from physiology to landscapes. The difficulty of clearly predicting combined effects of multiple stressors complicates our ability to project futures under a variety of scenarios, and to develop and implement effective management strategies.
This talk will focus on recent and new ideas on the general issue of multiple stressors, and especially the combined effects of oxygen and other stressors in coastal systems and semi-enclosed seas. Proximity to large human populations results in these systems being altered by activities within their local watershed, airshed and waters. In addition, they are strongly affected by global consequences of increased greenhouse gas emissions, and are warming and becoming more acidic as well as more oxygen-depleted as a result. Although the large temporal and spatial variation in physical characteristics of these systems may have already selected for species with wide physiological tolerances, both experiments and models show that the combined effects of multiple stressors can strongly affect both individual organisms and food webs. Rising temperatures, deoxygenation and acidification are especially tightly linked; warming and nutrient enrichment increase respiration, which depletes oxygen and releases CO2. Some stressors, such as increased temperatures, can make animals more sensitive to low oxygen, while others such as high mortality rates due to fisheries, may mask oxygen effects. Development of a framework for understanding effects of multiple stressors will require us to consider processes across life stages, generations and landscapes, as well as how behavioral responses to hypoxia can determine exposure to other stressors.
Critical oxygen levels of marine animals and the consequences of ocean deoxygenation and warming
Brad Seibel, Curtis Deutsch, Karen Wishner, Chris Roman, Allison Mislan, C. Tracy Shaw, Matthew Birk
Oxygen supply to the sites of cellular respiration requires a partial pressure (PO2) gradient from the environment to the mitochondria to drive diffusion. In surface waters of the ocean, as in air, the PO2 drops from 21 kPa in ambient water to less than 1 kPa at the mitochondria. In some marine environments, such as mesopelagic oxygen minimum zones (OMZ), the PO2 in the ambient water is less than 0.5 kPa. Thus, the PO2 gradient driving oxygen utilization is a mere fraction of that required for aerobic metabolism in terrestrial, and most marine, environments. While animal diversity and abundance are reduced in the OMZ core, life does thrive there. Here we review hypoxia tolerance (critical oxygen partial pressures, Pcrit) in marine animals and show that hypoxia tolerance varies widely across species. This variation is unrelated to metabolic rate, phylogeny, body mass or temperature due to the fact that O2 demand and environmental hypoxia are compensated for by evolved physiological adjustments that enable effective O2 extraction, transport, and utilization. Temperature is the primary driver of intraspecific variation in Pcrit due to effects on oxygen demand. However, because temperature increases O2 supply, the average temperature sensitivity of Pcrit is less than that for metabolic rate. Despite substantial physiological adaptation and diversity, distributions of most species appear to be constrained by oxygen supply relative to demand. We demonstrate that changes in oxygen of only a few micromolar, which may occur over very short vertical and horizontal distances, are important in structuring some mesopelagic communities, even though these ecosystems contain species with the greatest hypoxia tolerance (lowest Pcrits) of any animals measured to date. Ocean deoxygenation will reduce environmental oxygen supply while global warming will increase metabolic demand, reducing the metabolically available habitat and dramatically restructuring mesopelagic communities.
Microbial Biota in Anaerobic and Microaerobic Habitats
Tom Fenchel
Anaerobic zones occur in stratified water columns and almost universally in aquatic sediments at various depths. Anaerobic zones are covered by zones characterized by more or less steep oxygen gradients. These habitats harbor characteristic biota of unicellular organisms with zonation patterns determined by oxygen tension. The presentation will especially emphasize eukaryotic microbes in microaerobic and anaerobic habitats with respect to physiological adaptations and constraints and to zonation patterns exemplifying both sediments and the stratified water column.
Topic 7: Impacts on Fisheries/Socioeconomics
Deoxygenation effects on fisheries: a mosaic of effects and responses
Kenny Rose, Dimitri Gutierrez Aguilar, Denise Breitburg, Daniel Conley, J. Kevin Craig, Halley E. Froehlich, R. Jeyabaskaran, V. Kripa, Baye Cheikh Mbaye, K.S. Mohamed, Shelton Padua, and D. Prema
Fisheries (harvest) are an ecosystem service that provide employment and nutrition in the global food system. Worldwide production of wild fisheries has leveled off, while demand continues to increase. Over harvesting switches fisheries from an ecosystem service to a stressor. Deoxygenation is anticipated to expand over the next decades, and can affect fisheries through negative effects on growth, survival, and reproduction, and effects on movement that, in turn, affects the availability and location of the harvestable biomass. Quantifying the effects of deoxygenation on fisheries is challenging because of the effects of other stressors (e.g., warming, acidification) that partially covary with oxygen and because the dynamics of oxygen and fisheries are highly site-dependent. We review existing studies on the effects of deoxygenation on fisheries. In statistical (correlation-based) analyses spanning multiple coastal hypoxia ecosystems, stock biomass and landings were positively related to nitrogen loadings but it was difficult to isolate a direct hypoxia effect. However, the analysis suggested that trophic efficiency (landings per unit nitrogen loadings) was low in systems with extensive hypoxia. Eight case studies show the various ways deoxygenation can affect fisheries: Atlantic croaker and shrimp in the Gulf of Mexico, Dungeness crab in Hood Canal (Puget Sound), cod in the Baltic Sea, anchovy and other species in the Peruvian EEZ, Indian Oil Sardine in the southwest coast of India, white grouper in coastal west Africa, and billfishes in the eastern tropical Pacific. Case studies include low oxygen effects on the population itself through reduced recruitment and population abundance, and examples of spatial distribution effects resulting in changes in the dynamics of the fishing vessels. Modeling analyses demonstrate that, in those situations when hypoxia alone may have small to moderate population-level effects, the effects become amplified when hypoxia is combined with other stressors. A prevalent effect of deoxygenation documented in the case studies was changes in fishing locations in response to fine-scale distributional changes of the fish that then affect the catchability and bioeconomics of fishing. Further refining the role of deoxygenation on fisheries will increase the effectiveness of management by enabling proper interpretation of population fluctuations and spatial dynamics, more accurate view of the vulnerability of fish to harvest (e.g., catchabilities), and the derivation of robust population indices used in stock assessments. There is little room for management mis-calculations; too many people depend on effective management of fisheries to ensure sustainable harvests and healthy ecosystems.
Effect of deoxygenation on fish biology and fisheries in an enclosed brackish marine ecosystem
Michele Casini
Increased deoxygenation has affected worldwide the biology and behavior of fish and invertebrates, and the structure and functioning of marine ecosystems. Hypoxia can affect organisms through several direct and indirect mechanisms, which are not mutually exclusive and whose effects may sum up to lead to the observed changes. The species that are directly affected by deoxygenation are often highly important commercial species with large consequences on the fisheries relying on these resources. The effects of low-oxygen conditions on the fishery can span from lower quantity and quality of the catches as well as increased fishing efforts and changes in exploitation areas and gears. In some areas, such as the brackish Baltic Sea, deoxygenation has also coincided with an increased difficulty to determine fish age using otoliths, hampering the reliability of stock assessment estimates that are used as base for management advice. This has made the Baltic cod fishery losing its quality certification and eco-labelling with potential repercussions on the industrial revenues. In this talk, an overview of the effects of increased hypoxia on exploited fish species and the industry will be provided with focus on the Baltic Sea.
Fish, ocean oxygen depletion and the food security of current and future generations
Rashid Sumaila
It turns out that the ocean carries a heavy burden of climate change by absorbing more than 30% of the carbon produced on land. While this is a great service in the first instance it results in physical and chemical changes that are already changing the biophysics of the ocean, triggering acidification and de-oxygenation. Here, I will explore the potential food security and economic consequences of increasing depletion of ocean oxygen via its impacts on fish populations. This analysis is needed because it has been predicted that oxygen levels in the world’s oceans have already declined by 2% on average in the last 50 years, with the fall in the Pacific Ocean much higher than the average. This decline is predicted to accelerate into the future if we fail to take action to reduce C02emissions. In conducting our analysis, we will develop a number of scenarios and use economic indicators such as catch, revenues and profits to explore how and to what extent de-oxygenation would likely threaten the food security (defined broadly) of both current and future generations of people through its impact on fish stocks.
Topic 8: Coastal Systems: From Understanding to Management
The Baltic Sea: From Understanding to Management
Daniel Conley
Excessive nutrient inputs over the last century have altered the subtle balance between oxygen supply and oxygen consumption and changed the Baltic Sea from a state with hypoxia confined to the deepest bottom waters to widespread hypoxia in most bottom waters. The Baltic Sea is naturally susceptible to hypoxia because the pronounced vertical stratification in the water column prevents the resupply of oxygen below the permanent halocline, and the salt water inputs from the adjacent North Sea through shallow sills influences both horizontal and vertical water exchange. Analysis of the extensive data available from different countries, their monitoring program and research cruises, has allowed for the computation of basin-wide trends of oxygen conditions over more than a century. The low oxygen zone has increased by a factor of 10 over the last 115 years and has grown from about 5,000 km2around 1900 to more than 60,000 km2in recent years. Anthropogenic nutrient inputs are the primary cause of the hypoxia, however, global warming has exacerbated low oxygen conditions. In the estuarine and coastal systems of the Baltic hypoxia is much more variable and strongly dependent on processes controlling vertical mixing. The low oxygen conditions have altered many biogeochemical cycles (P, N, Fe, Mn, S, etc.) and influenced many processes including the nutrients limiting phytoplankton production, altering microbial communities and changing the burial of elements in sediments. In addition, the enhanced accumulation of organic-rich sediments with hypoxia, e.g. the legacy of eutrophication, has increased benthic oxygen demand. Although reductions in nutrient loads have reduced overall eutrophication, especially local conditions, a response is not yet evident in the dynamics of hypoxia. The time lag in responses to current efforts to reduce nutrients is slow, which also challenges management efforts to reduce eutrophication. Additional efforts to achieve nutrient reductions from catchments will be necessary to improve oxygen conditions in the Baltic Sea.
Greening of the Land and the Coastal Ocean
Nancy Rabalais
Increasing human population. Increasing need for food, fiber and fuel. Increasing use of fossil fuel. Increasing generation of reactive-Nitrogen into airscapes, landscapes, and seascapes. Increasing incidences of coastal ocean oxygen-depleted waters (or hypoxia), especially since the 1950s.
Increases in coastal areas of oxygen depletion parallel large-scale landscape changes, including deforestation, expansion of agricultural lands, more intense fertilizer use, cropping choices including corn for corn-based ethanol, loss of wetlands and riverine buffer zones, changes in cropland drainage, especially subsurface tile drains, and hydrologic controls restricting the floodplain removal of nutrients (i.e., levees or impervious surfaces). This trend for well-developed countries and developing countries is especially evident in agriculture landscapes and highly populated coastal urban centers.
Physical/biological dynamics generate varying levels of dissolved oxygen concentrations that affect living resources in multiple ways. The assemblages of organisms in estuaries and coastal waters are exposed to deoxygenated waters in ecosystems ranging from mostly permanent hypoxia, seasonally but annually persistent hypoxia, intermittent deoxygenation where physical forces disrupt longer periods of seasonal deoxygenation, and during diel cycles in areas with subaquatic vegetation. Oxygen deficiency creates unsuitable feeding habitat for demersal organisms, including the commercially important penaeid shrimps, crabs, lobsters, cod, red snapper, and other prized fish. Consider that cod eggs sink to their preferred density/depth within macroalgal beds affected by low oxygen, and where they now die and do not contribute to future recruitment, and that a 20,000 km2 swath of severely low oxygen waters in the northern Gulf of Mexico occurs at the same time brown shrimp need to migrate from estuaries to deeper waters and greater secondary production.
Hypoxia restoration requires the reduction of the high nutrient loads to coastal waters, which are primarily the result of expanded agribusiness (artificial fertilizers), intensified animal husbandry, insufficiently treated wastewater, and unnecessary consumption of fossil fuels. The societal shifts to a less consumptive life style are not always politically palatable, but some governmental units have come together and implemented multi-faceted plans that partially reduced nutrient loading. Ecosystem recovery may take years to decades following long-term exposure to long-lasting hypoxia, and a serious commitment by individuals, societies and governments will be needed to improve coastal water quality.
Coupled Physical-Biogeochemical Study of Eutrophication/Hypoxia in the Pearl River Estuary off Hong Kong
Jianping Gan, and Minhan Dai
The coastal waters around Hong Kong are affected by persistent and increasing eutrophication. This deteriorating situation may increase the frequency of HABs, expand the area of hypoxic zones and lead to other ecosystem disruptions and worse of all, offset the environmental improvements achieved through the costly Harbour Area Treatment Scheme over the last decade. Eutrophication/hypoxia in Hong Kong waters is primarily caused by the ecosystem’s responses to the increasing nutrient discharge from the Pearl River and local sewage effluent. Highly variable oceanic currents transport the nutrients in the interactive river-estuary-shelf (RES) waters around Hong Kong, which undergo complex coupled physical-biogeochemical processes and modulate eutrophication/hypoxia. To date, these key processes have not been investigated in a comprehensive manner in the RES waters, and they remain largely unresolved in similar ecosystems elsewhere in the world. Understanding the full spectrum of intrinsic coupled physical and biogeochemical processes in eutrophication is crucial to predicting and mitigating the impacts of eutrophication, and it remains a huge scientific challenge regionally and globally. By conducting an interdisciplinary study, we investigate the coupled physical-biological-chemical processes in this interactive RES system, and diagnose the eutrophication/hypoxia in the study region. We conducted interdisciplinary mapping and time-series measurements, and based on them, developed a novel coupled physical-biogeochemical modelling system under a grand OCEAN_HK project to determine: sources and sinks of nutrients, their biogeochemical controls, ecosystem dynamics, and physical controls on the eutrophication/hypoxia in the RES waters.
Topic 9: Ocean Deoxygenation - How the Past can Inform the Future
Large changes in ocean oxygenation during the last ice age: observations, mechanisms and ecosystem responses to change
Eric Galbraith
The controls on dissolved oxygen are complex, involving oxygen saturation concentrations, ocean circulation, air-sea gas exchange and the rate of organic matter respiration. Marine sediment records of oxygenation during the last ice age provide a means to gauge the relative importances of these mechanisms, as well as the response of the marine ecosystem, under climate forcings that are comparable in magnitude to the expected future changes. During the peak of the last ice age - the last glacial maximum - observations show that, despite greater oxygen solubility caused by lower ocean temperatures, the oxygen concentrations throughout the deep ocean were generally lower. This implies that greater oxygen utilization overwhelmed the solubility changes, which could have been due to more sluggish ocean circulation, more restricted air-sea exchange, and/or a more rapid sinking flux of organic matter to depth. Radiocarbon measurements from the last glacial maximum have previously been interpreted as showing more sluggish circulation, but model simulations show that the measured radiocarbon values can instead be explained by a change in ocean circulation patterns, which would not necessarily change oxygen concentrations. On the other hand, model simulations show that a significant decrease in the preformed oxygen content of Antarctic bottom waters could have occurred under glacial conditions, due to blocking of air-sea exchange by very extensive sea ice cover. In addition, a greater flux of organic matter to the deep sea may have contributed to the low deep ocean oxygen concentrations, due to an increased supply of iron and nitrogen to the surface, and/or ecosystem changes that increased the export fraction, transfer efficiency, or both. Relatively abrupt changes in the Atlantic Meridional Overturning Circulation that punctuated the glacial period and deglaciation were accompanied by dramatic changes in oxygen concentrations, particularly at intermediate-depths of the northern Indian and Pacific oceans. Qualitatively, these abrupt changes are well captured by models, but it is not clear if the models correctly simulate the magnitude of changes, owing to a paucity of quantitative observational constraints. Microscopic fossil assemblages, including fish debris, show large responses to changes in dissolved oxygen concentrations, pointing to an unrealized potential for sediment records to inform the sensitivity of marine ecosystems to changes in oxygenation.
Looking back into the future with a geochemical oxygenation proxy (I/Ca)
Zunli Lu
Forecasting trends in deoxygenation is evolving to become a major task for research communities focusing on the ocean-climate system. Geological records provide great opportunities to investigate environmental perturbations occurred in the past, in order to improve constraints on the rate and magnitude of modern climate change impacts. It is beneficial to examine events across different time scales, particularly for gauging the sensitivity of various controls on ocean oxygenation. Proxy approaches are key to unveiling past ocean conditions, however, oxygenation reconstructions have their unique challenges. This talk will review results of a carbonate-based proxy iodine to calcium ratios (I/Ca). I advocate that well-established approaches in paleoceanographic studies (e.g. temperature and pH) may require adaptations to best serve the deoxygenation community.
Past variability and recent trends of subsurface ocean oxygenation in the Eastern Tropical South Pacific: insights from proxy records
Dimitri Gutierrez
The Eastern Tropical South Pacific (ETSP) oxygen minimum zone (OMZ) is one of the largest regional OMZs in the global ocean. At multidecadal to millennial scales, the intensity and extension of this OMZ is modulated by changes of the Walker circulation, which control the thermocline depth –and thus productivity and respiration– in the ETSP, as well as the subsurface ventilation associated to the equatorial currents. Further south, the OMZ is also influenced by the meridional shifts of the subtropical front and by the intensity of the South Pacific Subtropical High (SPSH), which control the ventilation at higher latitudes and the alongshore wind forcing, respectively. Here we review a number of sedimentary proxy records that have been studied to reconstruct past variations of this OMZ under climate variability, in order to gain insights of the OMZ response to past global warm periods and this its potential sensitivity to current and future global warming. After the Last Glacial Maximum, a rapid deoxygenation process took place following the Southern Hemisphere signature of the deglaciation warming, leaving a distinct imprint in d15N sediment records along the Peru-Chile margin, up to maximal values during Heinrich Stadial 1. Thereafter, an average positive trend of oxygenation characterized the late deglaciation, overimposed with millennial fluctuations. A weaker OMZ during the warm mid-Holocene, as inferred by multiple proxies in the Peruvian slope, has been attributed to the intensification of the eastward subsurface equatorial currents and the poleward undercurrent, ultimately as a consequence of a much stronger Walker circulation. For the late Holocene, proxy records of oxygenation and productivity have evidenced significant shifts at multi-centennial time-scales, associated to global ‘cold’ and ‘warm’ climatic periods. Thus, for the Little Ice Age (LIA, 1400-1850 AD), proxy records off Peru and Northern Chile indicate an increased oxygenation in the water column and lower export production, under both weak Walker circulation and SPSH conditions. By contrast, for the late Medieval Climatic Anomaly (MCA, 1100 – 1350 AD) and after the LIA, the records report an increase in subsurface deoxygenation and a higher export production, while the Walker circulation and SPSH intensified. For the twentieth century, in the subtropical region, several shelf/slope records suggest an increased subsurface and bottom oxygenation with lower export production. At lower latitudes, a slight oxygenation is noticeable for the last half century that seems decoupled from increased export production. It needs to be elucidated in what extent the recent oxygenation trend could be explained by enhanced advection of the more oxygenated subsurface equatorial waters, lower productivity and respiration, increased northward advection of oxygen-rich subsurface and intermediate waters from the south and/or other mechanisms.
Topic 10: Biogeochemical Cycles: Feedbacks and Interactions
A comparative study of coastal ocean hypoxia and acidification in two large river dominated systems (northern Gulf of Mexico and East China Sea)
Wei-Jun Cai, Jianfang Chen, Bin Wang, Kui Wang, Zong-Pei Jiang, Katja Fennel, and Qian Li
The northern Gulf of Mexico (nGOM) and the East China Sea (ECS) face similar physical drivers and anthropogenic stressors. Most importantly, both systems are strongly influenced by large rivers (the Mississippi and Changjiang) and intense eutrophication due to agriculture and population growth. Bottom water hypoxia and acidification appear to grow more severe in recent years in both systems despite the fact that riverine nutrient supply has been stabilized since the 1980’s in the nGOM, however, it is increasing in the ECS.
In the surface water of the nGOM and ECS, the spatial distributions of O2and pH are associated with the trajectory of the river plumes and in situ biological activity driven by riverine nutrients. In both plume regions the highest O2 and pH values and lowest pCO2 values were observed at intermediate salinities where light and nutrient were both favorable for phytoplankton production. In the bottom layer, low O2 and pH values were observed in hypoxic waters. The subsurface pH shows correlations with DIC and apparent oxygen utilization (AOU), suggesting that decomposition of organic matter was the dominant factor regulating pH variability. In addition to the low O2and pH in the hypoxic bottom water, there was a layer of low O2 and pH at mid-water depth in the nGOM. T-S diagrams and numerical modeling suggest that this mid-water acidification and hypoxia was not caused by respiration of organic matter from local surface production, but was a result of intrusion of low O2 and pH water from a nearshore bottom layer. This extension of hypoxia and acidification from the nearshore bottom to the offshore mid-depth can form rapidly after a storm disruption and can then extend further to the bottom. This process might be a threat to marine organisms in offshore mid-water depths once thought to be unaffected by bottom hypoxia. Lateral transport also plays an important role in the formation of hypoxia and acidification in the ECS. We further reveal that the intensity and extensiveness of hypoxia and acidification events closely correlated to a climate change index in the ECS. We will discuss the common drivers and the differences between these two large-river dominated, eutrophic coastal systems with examples from recent and historical cruises.
The Regulation of Oxygen to Low Concentrations in the Bay of Bengal
Donald Canfield, Carolin R. Löscher, Beate Kraft, and Richard A. Boyle
The Bay of Bengal (BoB) is a peculiar oxygen minimum zone (OMZ). With traditional oxygen sensing techniques, mid-level water oxygen concentrations are below detection, yet, geochemical analysis have provided little evidence for N2 production as might be expected in the absence of oxygen. New, ultra-high sensitivity STOX oxygen measurements show, in contrast, that low, sub-micron, levels of oxygen are a persistent feature of BoB OMZ waters. Molecular genetic analyses show that these waters contain a suite of anaerobic bacteria capable of producing N2 (both anammox bacteria and the denitrifiers), as well as nitrifying bacteria and archaea, as typical for OMZs. Thus, microbes capable of a completely coupled anaerobic/aerobic nitrogen cycle are present. Despite these microbial populations, our geochemical analyses confirm that rates of N2 production within the BoB OMZ are very low. Thus, extremely low-oxygen conditions, but with occasional anoxia allowing limited N2 production, seem to have been a long-term feature of the BoB OMZ. This persistence of low oxygen conditions is remarkable. Simple modeling shows that such a situation is extremely unstable without an active stabilizing mechanism. In this talk, we review our current understanding of BoB biogeochemistry and microbial ecology. We also propose biological feedback mechanisms, in the context of a simple box model, that may act to stabilize oxygen in the BoB to low sub-micromolar concentrations. Such feedback mechanisms could well operate elsewhere, but they cannot keep an OMZ from becoming anoxic if the carbon flux to OMZ waters is high enough.
Impacts of a changing global phosphorus cycle on coastal ocean deoxygenation
Tom Jilbert, Caroline P. Slomp, Matthias Egger, Thilo Behrends, Mariette Wolthers, Niels A.G.M. van Helmond, Bo G. Gustafsson, Nikki Dijkstra and Peter Kraal
Phosphorus (P) is an essential element for all life on Earth due to its presence in critical intracellular compounds such as adenosine triphosphate (ATP) and nucleic acids. Hence the global distribution of bioavailable P strongly controls the distribution of life. In the oceans, P is regarded as the ultimate limiting nutrient for phytoplankton production, thus changes in the supply of P from the continents on geological timescales influence marine productivity, with impacts on the biogeochemical cycles of carbon, nitrogen and oxygen.
The global P cycle today is rapidly changing due to human activities, including the exploitation of mineral P reserves for fertilizer production and the subsequent dissipation of this P into the environment, the widespread damming of rivers, and the effects of climate change. All these factors may impact on the net flux of P from the continents to the oceans, as well as the forms in which P enters the oceans and the spatial distribution of P inputs.
Coastal regions in particular have experienced direct eutrophication as a consequence of enhanced recent inputs of P, as well as nitrogen, leading to the expansion of hypoxic and anoxic zones in near-shore areas. This talk will focus on the cycling of P in coastal regions impacted by human activities, highlighting the current state of knowledge and the key focus areas for future research. Special attention will be given to feedbacks in the cycling of P between sediments and the water column, which introduce strong non-linearities in the relationship between external P loading and coastal hypoxia. Understanding these feedbacks is critical to predicting ‒ and managing ‒ the future development of low-oxygen conditions in the coastal oceans.
Gendered Research and Innovations
Gendered Innovations in Science, Climate Change, Machine Learning, and Robotics
Londa Schiebinger
How can we harness the creative power of gender analysis for discovery and innovation? In this talk I identify three strategic approaches to gender in research, policy, and practice: 1) "Fix the Numbers of Women" focuses on increasing women's participation; 2) "Fix the Institutions" promotes gender equality in careers through structural change in research organizations; and 3) "Fix the Knowledge" or "Gendered Innovations" stimulates excellence in science and technology by integrating sex and gender analysis into research. This talk focuses on the third approach. I will discuss several case studies, including sex and gender analysis in basic science, climate change, machine learning, and robotics. To match the global reach of science and technology, Gendered Innovations was developed through a collaboration of over a hundred experts from across the United States, Europe, Canada, and Asia. Major funders for Gendered Innovations include the European Commission, the U.S. National Science Foundation, and Stanford University.