Dissolved Oxygen and Climate Change

Due to increasing atmospheric CO2 concentrations and associated climate change, the global ocean is undergoing substantial physical and biogeochemical changes. Among these, changes in ocean oxygen and carbonate chemistry have great implication for marine biota. There is considerable uncertainty in the projections of future climate change, and it is unclear how the uncertainty in climate change would also affect the projection of oxygen and carbonate chemistry. To investigate this issue, we use an Earth system model of intermediate complexity to perform a set of simulations, including that which involves no radiative effect of atmospheric CO2 and those which involve CO2-induced climate change with climate sensitivity varying from 0.5 °C to 4.5 °C. Atmospheric CO2 concentration is prescribed to follow RCP 8.5 pathway and its extensions. Climate change affects carbonate chemistry and oxygen mainly through its impact on ocean temperature, ocean ventilation, and concentration of dissolved inorganic carbon and alkalinity. It is found that climate change mitigates the decrease of carbonate ions at the ocean surface but has negligible effect on surface ocean pH. Averaged over the whole ocean, climate change acts to decrease oxygen concentration but mitigates the CO2-induced reduction of carbonate ion and pH. In our simulations, by year 2500, every degree increase of climate sensitivity warms the ocean by 0.8 °C and reduces ocean-mean dissolved oxygen concentration by 5.0%. Meanwhile, every degree increase of climate sensitivity buffers CO2-induced reduction in ocean-mean carbonate ion concentration and pH by 3.4% and 0.02 units, respectively. Our study demonstrates different sensitivities of ocean temperature, carbonate chemistry, and oxygen, in terms of both the sign and magnitude to the amount of climate change, which have great implications for understanding the response of ocean biota to climate change.

Bioen-OSMOSE: A bioenergetic marine ecosystem model with physiological response to temperature and oxygen

Knowledge of the biology of deep‐sea benthic foraminifera was used to interpret the results of multivariate analyses (factor and cluster) on relative abundance data of benthic foraminifera at Deep Sea Drilling Project Site 219 (southeastern Arabian Sea; 1764 m depth) in combination with carbon and oxygen isotope data. Faunal data document major changes in deep‐sea ventilation and productivity over the past 5.5 Ma, including the end of the Miocene‐Pliocene Indo‐Pacific “biogenic bloom” period at ∼4.0 Ma. Interestingly, there is no simple correlation between high productivity and low oxygenation. Productivity fluctuated but became overall more pulsed, whereas overall oxygenation increased. We interpret the records as a combination of local to regional fluctuations in productivity probably caused by changes in monsoonal development, particularly its seasonality, and changes in oxygenation of intermediate depth waters in the Indian Ocean caused by global changes in climate and ocean circulation.

Dissolved oxygen concentrations in the ocean are declining on a global scale. However, the impact of climate change on oxygen in shelf seas is not well understood. We investigate potential future changes in oxygen on the northwest European continental shelf under a business as usual greenhouse gas emissions scenario (Representative Concentration Pathway RCP8.5). Regions of the European shelf are thermally stratified from spring to autumn, which can cause oxygen depletion in sub-pycnocline waters. A transient climate-forced model simulation is used to study how the temperature, salinity and concentration of near bed dissolved oxygen change over the 21st century. In warming and freshening water, the oxygen concentration declines in all shelf regions. The climate change signal emerges first in salinity, then in temperature and finally in near bed oxygen. Regions that currently experience oxygen depletion (the eastern North Sea, Celtic Sea and Armorican shelf) become larger in the future scenario and oxygen depletion lasts longer. Solubility changes, caused by changes in temperature and salinity, are the dominant cause of reducing near bed oxygen concentrations in many regions. Until about 2040 the impact of solubility dominates over the effects of the evolving ecosystem. However, in the eastern North Sea by 2100, the effect of ecosystem change is generally larger than that of solubility. In the Armorican Shelf and Celtic Sea the ecosystem changes partially mitigate the oxygen decline caused by solubility changes. Over the 21st century the mean near bed oxygen concentration on the European shelf is projected to decrease by 6.3%, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. For monthly minimum oxygen the decline is 7.7% with the solubility component being 50% of the total.

As a result of long-term climate change, regions of the ocean with low oxygen concentrations are predicted to occur more frequently and persist for longer periods of time in the future. When low levels of oxygen are present, this places additional pressure on marine organisms to meet their metabolic requirements, with implications for growth, feeding and reproduction. Extensive research has been carried out on the effects of acute hypoxia, but far less on long-term chronic effects of low oxygen zones, especially with regard to commercially important fishes and shellfishes. To provide further understanding on how commercial species could be affected, the results of relevant experiments must support population and ecosystem models. This is not easy because individual effects are wide-ranging; for example, studies to date have shown that low oxygen zones can affect predator-prey relationships as some species are able to tolerate low oxygen more than others. Some fishes may move away from areas until oxygen levels return to acceptable levels, while others take advantage of a reduced start response in prey fishes and remain in the area to feed. Sessile or less mobile species such as shellfishes are unable to move out of depleted oxygen zones. Some species can tolerate low oxygen levels for only short periods of time, while others are able to acclimatize. To advance the knowledge-base further, a number of promising technological and modelling-based developments and the role of physiological data within these, are proposed. These include advances in remote telemetry (tagging) and sensor technologies, trait-based analyses to provide insight into how whole assemblages might respond in the future, research into long-term adaptability of species, population and ecosystem modelling techniques and quantification of economic effects. In addition, more detailed oxygen monitoring and projections are required to better understand the likely temporal and local-scale changes in oxygen.

Millennial‐scale climate events in the North Pacific are thought to be related to changes in the circulation of North Pacific Intermediate Water, which may have formed in the Bering Sea in the past. To advance our understanding of the mechanisms that underlie millennial‐scale events, Bering Sea sediment cores from the Integrated Ocean Drilling Program site U1340 were used to construct high‐resolution, multiproxy climate records of the last 90,000 years. Sediment density records show millennial‐scale events resembling Dansgaard‐Oeschger events, several of which are laminated. Interstadials were characterized by 3–5 °C warming, increased productivity driven by upwelling, and reduced benthic oxygenation. Bering Sea intermediate water also changed over longer timescales; our records show the presence of intermediate water with lower salinity and higher oxygen content than modern beginning around 60 ka and persisting until the beginning of the deglaciation. The Bølling‐Allerød was characterized by high productivity, laminated sediments, and strong denitrification signature. Our data support the idea that productivity‐derived changes in oxygenation at intermediate water source regions may have contributed to the intensification of the North Pacific–wide oxygen minima during the Bølling‐Allerød.

Rapid climate change alters the environment and biological production of the Indian Ocean

Current studies on abiotic impacts on Artemia, a crustacean which is widely used in aquaculture, and ecotoxicology, often focus on endpoint analysis (e.g., hatching rates, survival). Here, we demonstrate that a mechanistic understanding can be obtained through measurement of oxygen consumption in real-time over an extended time period in a microfluidic platform. The platform enables high level control of the microenvironment and direct observation of morphological changes. As a demonstration, temperature and salinity are chosen to represent critical abiotic parameters that are also threatened by climate change. The hatching process of Artemia consists of four different stages: hydration, differentiation, emergence, and hatching. Different temperatures (20, 35, and 30 °C) and salinities (0, 25, 50, and 75 ppt) are shown to significantly alter the duration of hatching stages, metabolic rates, and hatchability. Specifically, the metabolic resumption of dormant Artemia cysts was significantly enhanced at higher temperatures and moderate salinity, however, the time needed for this resumption was only dependent on higher temperatures. Hatchability was inversely related to the duration of the differentiation stage of hatching, which persisted longer at lower temperatures and salinities. The current approach of investigation of metabolism and corresponding physical changes can be employed to study hatching processes of other aquatic species, even those with low metabolic rate.

The understanding of past changes in climate and ocean circulation is to a large extent based on information from marine sediments. Marine deposits contain a variety of microfossils, which archive (paleo)-environmental information, both in their floral and faunal assemblages and in their stable isotope and trace element compositions. Sampling campaigns in the late 19th and early 20th centuries were dedicated to the inventory of sediment types and microfossil taxa. With the initiation of various national and international drilling programs in the second half of the 20th century, sediment cores were systematically recovered from all ocean basins and since then have shaped our knowledge of the oceans and climate history. The stable oxygen isotope composition of foraminiferal tests from the sediment cores delivered a continuous record of late Cretaceous–Cenozoic glaciation history. This record impressively proved the effects of periodic changes in the orbital configuration of the Earth on climate on timescales of tens to hundreds of thousands of years, described as Milankovitch cycles. Based on the origination and extinction patterns of marine microfossil groups, biostratigraphic schemes have been established, which are readily used for the dating of sediment successions. The species composition of assemblages of planktic microfossils, such as planktic foraminifera, radiolarians, dinoflagellates, coccolithophorids, and diatoms, is mainly related to sea-surface temperature and salinity but also to the distribution of nutrients and sea ice. Benthic microfossil groups, in particular benthic foraminifera but also ostracods, respond to changes in water depth, oxygen, and food availability at the sea floor, and provide information on sea-level changes and benthic-pelagic coupling in the ocean. The establishment and application of transfer functions delivers quantitative environmental data, which can be used in the validation of results from ocean and climate modeling experiments. Progress in analytical facilities and procedures allows for the development of new proxies based on the stable isotope and trace element composition of calcareous, siliceous, and organic microfossils. The combination of faunal and geochemical data delivers information on both environmental and biotic changes from the same sample set. Knowledge of the response of marine microorganisms to past climate changes at various amplitudes and pacing serves as a basis for the assessment of future resilience of marine ecosystems to the anticipated impacts of global warming.

Understanding the response of forests to the increases in atmospheric CO2 (ca) is fundamental to implementing innovative management strategies and for assessing impacts on the global carbon and water cycles. Here, we explored correlations between ecophysiological traits and climate variability that influence changes in stable isotope carbon and oxygen (δ13C and δ18O) of tree‐rings. We present these relationships between species of the contrasting genera Abies and Pinus, along a latitudinal transect encompassing different biogeographical regions in North America. We also tested if the rate of intrinsic water‐use efficiency per unit of ca (dW/dca) during two periods (1890–1965 vs. 1966–2016), for fir and pine were different and indicated acclimation to ca increases. We hypothesize that, spatially and temporally, the divergent responses among species to carbon and oxygen isotopes and dW/dca are influenced by the site conditions and the historical increases in ca. From our results, we show that fir and pine species will behave physiologically different as global warming progresses. Firs are more responsive to atmosphere vapor pressure deficit along different geographical zones. The survival of forests species under climate change will rely on the response to water stress and species' traits that influence the regulation of dW. Finally, we want to highlight the concept of “progressive resource limitation” of soil water and nutrients, previously proposed by other authors, that likely indicate fir species that inhabit moister sites will benefit more from increased ca than pine, but this positive effect is likely transitory as global warming increases.

Oxygen minimum zones (OMZs), large midwater regions of very low oxygen, are expected to expand as a result of climate change. While oxygen is known to be important in structuring midwater ecosystems, a precise and mechanistic understanding of the effects of oxygen on zooplankton is lacking. Zooplankton are important components of midwater food webs and biogeochemical cycles. Here, we show that, in the eastern tropical North Pacific OMZ, previously undescribed submesoscale oxygen variability has a direct effect on the distribution of many major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton live near their physiological limits and respond to slight (≤1%) changes in oxygen. Ocean oxygen loss (deoxygenation) may, thus, elicit major unanticipated changes to midwater ecosystem structure and function.

The California Current Marine Ecosystem is a highly productive system that exhibits strong natural variability and vulnerability to anthropogenic climate trends. Relating projections of ocean change to biological sensitivities requires detailed synthesis of experimental results. Here, we combine measured biological sensitivities with high‐resolution climate projections of key variables (temperature, oxygen, and pCO2) to identify the direction, magnitude, and spatial distribution of organism‐scale vulnerabilities to multiple axes of projected ocean change. Among 12 selected species of cultural and economic importance, we find that all are sensitive to projected changes in ocean conditions through responses that affect individual performance or population processes. Response indices were largest in the northern region and inner shelf. While performance traits generally increased with projected changes, fitness traits generally decreased, indicating that concurrent stresses can lead to fitness loss. For two species, combining sensitivities to temperature and oxygen changes through the Metabolic Index shows how aerobic habitat availability could be compressed under future conditions. Our results suggest substantial and specific ecological susceptibility in the next 80 years, including potential regional loss of canopy‐forming kelp, changes in nearshore food webs caused by declining rates of survival among red urchins, Dungeness crab, and razor clams, and loss of aerobic habitat for anchovy and pink shrimp. We also highlight fillable gaps in knowledge, including specific physiological responses to stressors, variation in responses across life stages, and responses to multistressor combinations. These findings strengthen the case for filling information gaps with experiments focused on fitness‐related responses and those that can be used to parameterize integrative physiological models, and suggest that the CCME is susceptible to substantial changes to ecosystem structure and function within this century.

Abstract. Observations and model runs indicate trends in dissolved oxygen (DO) associated with current and ongoing global warming. However, a large-scale observation-to-model comparison has been missing and is presented here. This study presents a first global compilation of DO measurements covering the last 50 yr. It shows declining upper-ocean DO levels in many regions, especially the tropical oceans, whereas areas with increasing trends are found in the subtropics and in some subpolar regions. For the Atlantic Ocean south of 20° N, the DO history could even be extended back to about 70 yr, showing decreasing DO in the subtropical South Atlantic. The global mean DO trend between 50° S and 50° N at 300 dbar for the period 1960 to 2010 is –0.066 μmol kg−1 yr−1. Results of a numerical biogeochemical Earth system model reveal that the magnitude of the observed change is consistent with CO2-induced climate change. However, the pattern correlation between simulated and observed patterns of past DO change is negative, indicating that the model does not correctly reproduce the processes responsible for observed regional oxygen changes in the past 50 yr. A negative pattern correlation is also obtained for model configurations with particularly low and particularly high diapycnal mixing, for a configuration that assumes a CO2-induced enhancement of the C : N ratios of exported organic matter and irrespective of whether climatological or realistic winds from reanalysis products are used to force the model. Depending on the model configuration the 300 dbar DO trend between 50° S and 50° N is −0.027 to –0.047 μmol kg−1 yr−1 for climatological wind forcing, with a much larger range of –0.083 to +0.027 μmol kg−1 yr−1 for different initializations of sensitivity runs with reanalysis wind forcing. Although numerical models reproduce the overall sign and, to some extent, magnitude of observed ocean deoxygenation, this degree of realism does not necessarily apply to simulated regional patterns and the representation of processes involved in their generation. Further analysis of the processes that can explain the discrepancies between observed and modeled DO trends is required to better understand the climate sensitivity of oceanic oxygen fields and predict potential DO changes in the future.

Sediment geochemical records of productivity and oxygen depletion along the margin of western North America during the past 60,000 years: teleconnections with Greenland Ice and the Cariaco Basin

We explore centennial changes in tropical Pacific oxygen (O2) using numerical models to illustrate the dominant patterns and mechanisms under centennial climate change. Future projections from state‐of‐the‐art Earth System Models exhibit significant model to model differences, but decreased solubility and weakened ventilation together deplete thermocline O2 in middle to high latitudes. In contrast, the tropical thermocline O2 undergoes much smaller changes or even a slight increase. A suite of sensitivity experiments using a coarse resolution ocean circulation and biogeochemistry model show that ocean warming is the leading cause of global deoxygenation in the thermocline across all latitudes with secondary contributions from changes in hydrological cycles and wind stress modulating regional changes in O2. The small O2 changes in the tropical Pacific thermocline reflect near‐complete compensation between the solubility decrease due to warming and reduction in apparent oxygen utilization (AOU). We further quantified the changes in AOU due to contributions from changes in water mass age and biological remineralization from the sensitivity experiments. The two effects almost equally contribute to the reduction of AOU in the tropical Pacific thermocline (43% for physical circulations and 57% for biology). Our results suggest that better understanding of water mass changes in the tropical oceans is key to improving projections and reducing the uncertainties of future O2 changes.