The ocean is currently rapidly losing oxygen. In the open ocean, this change is mostly attributed to global warming and associated circulation changes, whereas in coastal waters, anthropogenic eutrophication is typically the key driver. Ocean deoxygenation has major impacts on marine life and on the cycling of bio-active elements such as carbon, nitrogen, phosphorus, sulfur and trace metals, that, in turn, can feed back on ocean deoxygenation. Thus, for example, oxygen loss can reduce the habitat of pelagic and benthic animals and can greatly enhance rates of marine nitrogen removal and amplify phosphorus recycling. Strongest effects have so far been reported for Oxygen Minimum Zones and coastal systems that are naturally sensitive to hypoxia linked to water column stratification, which might be affected by sampling biases in the heavily undersampled ocean. Ocean deoxygenation has frequently occurred in Earth’s past, thereby also impacting the evolution of marine organisms. In this session, we invite presentations related to all aspects of deoxygenation in the past, present and future ocean. Hence, we invite modeling, field and laboratory studies on the drivers and consequences of loss of oxygen in marine waters on all spatial and temporal scales. This includes, for example, observational studies in the modern coastal and open ocean, results of Earth System Modeling and reports on the development and application of redox proxies for all relevant periods of Earth’s history (e.g. Oceanic Anoxic Events).
Lead Organizer: Caroline P. Slomp, Utrecht University (c.p.slomp@uu.nl)
Co-organizers:
Marilaure Grégoire, University of Liege (mgregoire@ulg.ac.be)
Andreas Oschlies, GEOMAR Helmholtz Centre for Ocean Research Kiel (aoschlies@geomar.de)
Peter Croot, University of Galway (peter.croot@nuigalway.ie)
Presentations
08:30 AM
What do we need to know about ocean oxygen to manage it better? (6524)
Primary Presenter: Marilaure Gregoire, University of Liege (mgregoire@uliege.be)
The sentence ‘every second breath you take comes from the ocean’ is often used in science communication to highlight the importance of the ocean for humans, yet its correctness is debated. In contrast, the problem of ocean deoxygenation in open and coastal waters is under-communicated, and awareness needs to be raised about this looming threat. To this aim the European Marine Board created a policy document to highlight these topics, including the most recent science on ocean oxygen, its influence on the global oxygen cycling, and on the oxygen we breathe. The document covers: (1) the history of Earth’s oxygenation, including oxygen accumulation mechanisms and feedbacks, and the link between previous mass extinctions and ocean deoxygenation events; (2) The modern oxygen cycle, emphasizing ocean oxygen dynamics, natural variability in atmospheric and oceanic oxygen content, and past and present deoxygenation events; (3) Current ocean deoxygenation, including mechanisms of deoxygenation, effects on life and ecosystems, and impacts of deoxygenation on biogeochemical processes; (4) Methods used to study ocean oxygen; and (5) Management, mitigation and adaptation needs to address ocean deoxygenation. To conclude, the document makes recommendations relevant to policymakers and funders of marine science. This presentation will give an overview of the main messages from this document and focus on the identified science research gaps.
08:45 AM
DECADAL TIME-SERIES DEPLETION OF DISSOLVED OXYGEN AT ABYSSAL DEPTHS IN THE NORTHEAST PACIFIC (5549)
Primary Presenter: Christine Huffard, MBARI (chuffard@mbari.org)
A significant decline in dissolved oxygen concentration has been measured over the period from 1989-2018 at abyssal depths in the Northeast Pacific (Station M, 4,000–4,100 m depth). This site sits beneath the California Current off central California. Three principal hypotheses were tested to examine the relationships between oxygen concentration and biological and physical factors. At annual resolution, Ekman pumping, coastal upwelling, particulate matter flux, and sediment community oxygen consumption significantly correlated with bottom water dissolved oxygen concentration. Coastal upwelling was the most important factor, accounting for 65% of the annual variation in bottom water oxygen concentration. Stepwise regressions were used to examine these relationships.
09:00 AM
Oxygen Isotopes as tracers of redox reactions in natural waters - assessment as a tool for ocean deoxygenation studies (6152)
Primary Presenter: Peter Croot, University of Galway (croot.peter@gmail.com)
The isotopes of oxygen (16,17&18) have been utilized as tracers of marine biogeochemical processes impacting dissolved oxygen (O2) since the 1970's, however it is only recently with the development of membrane inlet mass spectrometry (MIMS) that instrumentation was robust enough for at sea measurements. In this presentation we will give a brief overview of the current application of oxygen isotopes to biogeochemical processes of relevance to ocean deoxygenation: sources (e.g. gross oxygen production (GOP) via photosynthesis) and sinks (e.g. community respiration). Data will be presented on GOP by picoplankton (Prochlorococcus and Synechococcus) cultures. Field work from a low oxygen environment (Lough Furnace, Ireland) also suggests that oxygen isotopes may help constrain oxygen fluxes and reveal interactions with nitrogen and sulfur biogeochemistry. Oxygen isotopes are also applicable to reactions involving other Reactive Oxygen Species (ROS: O2, O2- , H2O2 & OH) in seawater, ROS are important roles in phytoplankton health and in redox reactions with trace metals (e.g. Fe, Cu and Mn) and organic matter. Using this approach we employed H218O2 with picoplankton cultures to quantify catalase and peroxidase activity in cells. Picoplankton work was supported by the TRIATLAS project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 817578. Field work in Ireland was supported by the IMBER endorsed project NUTS&BOLTS, co-funded by EPA Research (2018-W-LS-19) and the Marine Institute.
09:15 AM
STABLE ISOTOPE CONSTRAINTS ON THE OXYGEN BUDGET OF EUTROPHIC LONG ISLAND SOUND (5529)
Primary Presenter: Mark Altabet, University of Massachusetts - Dartmouth (maltabet@umassd.edu)
Long Island Sound (LIS) is an estuary receiving nutrients from a population of >8 million people. Resulting eutrophication has led to severe summertime hypoxia particularly at its western end. Despite a ~60% reduction in nitrogen loading, only the beginning of improvements in summertime O2 has been observed. Because LIS’s O2 budget has not been quantitatively well constrained, reliable predictions of the future course of summertime hypoxic have yet to be made. The major terms in LIS’s oxygen budget are 1) local air-sea gas exchange, 2) ventilation through larger scale circulation, 3) near-surface photosynthetic production, 4) water column respiration, 5) sediment respiration. We have used observation of the 18O/16O ratio of O2 (d18O) and model analysis to parse the contribution of these terms as they produce distinct isotopic signatures. Photosynthesis produces O2 with a d18O 24‰ lower than the atmosphere, respiration in the water column increases d18O with a fractionation factor of about 20‰ (directly measured by us), whereas sedimentary respiration creates little isotope fractionation (directly measured). Samples were collected from stations throughout LIS but focusing on the western end from monthly to bi-weekly (summer). In addition, high temporal frequency sampling to capture diel and tidal influences were conducted at several western stations in summer. Overall, decreasing O2 was associated with large 18O enrichments due to the influence of water column respiration but a contribution of photosynthetic O2 to isotope signals even in bottom waters was also clear.
09:30 AM
Sediment Oxygen Consumption and Seasonal Hypoxia— A Case Study in the Coastal South China Sea (6473)
Primary Presenter: Jing Sun, the Hong Kong University of Science and Technology (jsunbk@connect.ust.hk)
Coastal sediments regulate the water-column oxygen budget due to their high rates of carbon mineralization and may contribute significantly to seasonal hypoxia. Using field observations of dissolved oxygen in the water column and sediments and the sediment oxygen consumption rates (SOC), we explore the effects of SOC on hypoxia formation at the Pearl River Estuary and its adjacent coastal seas. We show that sediment remineralization contributes to more than 50% of organic carbon produced in the euphotic zone, leading to disproportionally shallow oxygen penetration. Our results indicate that the vertical stratification of the water column, particularly the thickness of the bottom boundary layer (BBL), determines the effect of SOC on hypoxia formation. We further develop some relationships between SOC and the water column oxygen distributions and test the model using data from similar settings, including the Changjiang Estuary, the northern Gulf of Mexico, and the Louisiana Continental Shelf. We will discuss the possibility of using commonly measured physicochemical properties in the water column to parameterize SOC, which is expensive to quantify in coastal areas but crucial in modeling regional biogeochemical cycling and hypoxia formation.
09:45 AM
Effects of phosphorus limitation on seasonal hypoxia in large river systems: a comparison of the Mississippi and Changjiang rivers (6550)
Primary Presenter: Arnaud Laurent, Dalhousie University (arnaud.laurent@dal.ca)
Excess nutrient loading is a common feature of coastal environments, associated with urbanization, industrial animal production or fertilizer application to crops. In large river plume or estuarine systems, excess nutrients from the watershed result in widespread eutrophication that may be the source of recurring summer hypoxic conditions in subsurface waters, having detrimental effects on benthic organisms. Despite eutrophic conditions, the unbalanced supply of anthropogenic nitrogen (N) and phosphorus (P) relative to the Redfield ratio can lead to seasonal P limitation in surface coastal waters. Recent investigations in the Mississippi and the Changjiang River plumes, two of the world largest river systems, indicate that P limitation has a significant effect on the magnitude, and spatial and temporal distributions of primary production and respiration and, consequently, on hypoxia. Using simulations from two coupled physical-biogeochemical models of the northern Gulf of Mexico and the East China Sea we explore the similarities and differences between the two river systems with regards to P limitation and hypoxia. We draw general patterns that will help mitigating coastal hypoxia in large river systems.
SS015A Deoxygenation in the Past, Present and Future Ocean
Description
Time: 8:30 AM
Date: 8/6/2023
Room: Sala Ibiza A