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Ocean Deoxygenation: a Primer

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Ocean Deoxygenation: a Primer View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Epsilon Open Archive One Earth Primer Ocean Deoxygenation: A Primer Karin E. Limburg,1,2,* Denise Breitburg,3 Dennis P. Swaney,4 and Gil Jacinto5 1Department of Environmental and Forest Biology, College of Environmental Science and Forestry, State University of New York, Syracuse, NY, USA 2Department of Aquatic Resources, Swedish University of Agricultural Sciences, Lysekil, Sweden 3Smithsonian Environmental Research Center, Edgewater, MD, USA 4Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA 5Marine Science Institute, University of the Philippines Diliman, 1101 Quezon City, Philippines *Correspondence: [email protected] https://doi.org/10.1016/j.oneear.2020.01.001 Earth’s ocean is losing oxygen; since the mid-20th century, 1%–2% of the global ocean oxygen inventory has been lost, and over 700 coastal sites have reported new or worsening low-oxygen conditions. This ‘‘ocean deoxygenation’’ is increasing and of great concern because of the potential magnitude of adverse changes to both global and local marine ecosystems. Oxygen is fundamental for life and biogeochemical processes in the ocean. In coastal and shelf regions and semi-enclosed seas, over-fertilization of waters largely from agri- culture, sewage, and airborne sources creates algal blooms that die and decay, consuming oxygen. Globally, climate warming both exacerbates the problems from eutrophication and reduces the introduction of oxygen to the interior of the ocean. We discuss mechanisms, scale, assessments, projections, and impacts, including impacts to human well-being, at the individual, community, and ecosystem levels. Deoxygenation together with other stressors presents a major environmental challenge to sustainability and human use of the ocean. The Ocean Is Losing Its Breath in a process called upwelling. OMZs, as well as less severely ox- Human pressure on Earth is increasing as human populations ygen-depleted waters, are expanding as the ocean warms, and and the scale of their impacts continue to expand. We highlight in some regions of the world, upwelling is bringing more severely here the global loss of dissolved oxygen in Earth’s ocean—called oxygen-depleted water toward the shore. ocean deoxygenation, which is occurring globally in coastal waters, semi-enclosed seas, and the open ocean. Since the What Is Deoxygenation, and How Is It Defined? mid-20th century, the oceans are estimated to have lost about ‘‘Hypoxia’’ is defined as a state of low oxygen at which physio- 1%–2% of their oxygen inventory, and over 700 coastal systems logical and ecological processes are impaired. Hypoxia is often have reported low oxygen levels. operationally defined as 2 mg/L O2, equivalent to 1.4 mL/L or In estuaries and coastal seas adjacent to continents, over- 63 mmol/L. In reality, there is wide variation among species and fertilization of water primarily from agriculture, sewage, and the processes in oxygen requirements, and defining hypoxia as a burning of fossil fuels has historically been the most important single oxygen level is problematic. Although there are organisms driver. These low-oxygen areas are sometimes referred to as (including some fish species) that have adapted to life in very low ‘‘dead zones,’’ although even where macrofauna are excluded oxygen, many cannot tolerate even 2 mg/L hypoxia for long pe- they host a rich microbial assemblage and are sites of important riods of time without experiencing some negative effects. In fact, biogeochemical processes. many fishes and invertebrates display hypoxia symptoms at In addition, climate warming is an increasingly important factor much higher levels of dissolved oxygen. driving the decline in dissolved oxygen in the world’s oceans, es- ‘‘Anoxia’’ is defined as the absence of oxygen or, in an opera- tuaries, and seas, although this has received little recognition un- tional sense, oxygen concentrations that are too low to be til recently. It is difficult to understand the lack of appreciation of measured with available technology. In some contexts, such deoxygenation as a climate issue. Given that societies have lived as the Baltic Sea, anoxia is even indicated in negative units with sewage- and fertilizer-driven oxygen loss for over a century, because additional oxygen would be necessary to satisfy the it has most likely been treated as ‘‘just another’’ conventional wa- oxidation demand of the transformation of hydrogen sulfide to ter-quality problem. However, it is increasingly becoming clear sulfate under these chemically reducing conditions. to scientists studying ocean oxygen that nutrient and eutrophi- cation effects are further exacerbated by warming, and vast What Causes Deoxygenation? areas of low oxygen in the open ocean that are not driven by As mentioned above, excess nutrient inputs from land-based excess nutrients are expanding. In the open ocean, the most sources can drive hypoxia. Large watersheds such as the Mis- severely oxygen-depleted waters are typically several hundred sissippi River, which drains 41% of the contiguous United meters below the ocean surface and might not reach the sea States, deliver vast amounts of nutrients to the northern Gulf of bottom. These are called oxygen minimum zones or OMZs. A Mexico and other coastal ecosystems. These nutrients fuel combination of winds and ocean circulation can bring the blooms of algae that eventually die off and decay. Microbes deep, low-oxygen waters toward the surface and closer to shore that utilize aerobic (oxygen-dependent) respiration and break 24 One Earth 2, January 24, 2020 ª 2020 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). One Earth Primer down the algae consume oxygen during that decomposition. for those models. The models track the sources and sinks of ox- Hypoxia occurs when the rate of oxygen consumption exceeds ygen as well as the mixing of water masses, governed by ocean- its replenishment through photosynthesis and mixing of the wa- atmosphere interactions. Sources include oxygen diffusing in ter column. from the atmosphere and, importantly, oxygen produced by Climate change is amplifying the problem of deoxygenation as photosynthesis in surface waters. Oxygen from its origin in sur- a result of the effects on the physical properties of water and on face layers must be transported into deeper layers. Sinks include the respiration rates of microbes and animals. Oxygen is not oxygen consumption (respiration) by organisms, decay of easily dissolved in water, which holds less oxygen as it warms. organic matter, and burial as carbonates and other oxides. A Warmer water is also less dense than colder water and tends large amount of uncertainty is evident in model projections; for to stratify above colder water layers. In areas such as parts of example, Oschlies and colleagues found that models of oxygen the Arctic and Antarctic, glacial meltwater from non-saline change in the tropics predict only half of the oxygen loss snow and ice is less dense than seawater and contributes to sur- observed from data collection. Nevertheless, models forced face water layers as well. The net effect of stratification is to with greenhouse gas inputs from Intergovernmental Panel on reduce the mixing of bottom and surface waters such that bot- Climate Change projections point to substantial increases in ox- tom waters are not re-aerated sufficiently (Figure 1). ygen loss throughout the 21st century, even under optimistic sce- Additionally, certain biogeochemical feedbacks can worsen narios of greenhouse gas reduction. Much of the loss will be at deoxygenation. Examples include so-called legacy phosphorus mid- and high latitudes, and some models project some recovery that is buried in sediments from past algal bloom die-offs. Under in OMZs as a result of lower ocean primary production as climate anoxic conditions, the phosphorus is released and can become change progresses, resulting in lower ecosystem respiration. available in the water column as an input to algal blooms, stimu- lating the cycle again. Similarly, anoxia can promote the forma- Effects on Marine Organisms tion of nitrous oxide (N2O), and acidifying dead organic matter Low dissolved oxygen compromises many, but not all, biological can stimulate the formation of methane, which can ultimately functions and life forms. Insufficient oxygen can reduce growth, be released to the atmosphere. Both are powerful greenhouse reproduction, and survival; make animals more susceptible to gases equivalent to many times the heat-trapping ability of car- disease and to predation by more hypoxia-tolerant species; bon dioxide. The contribution of oceans to global methane emis- and alter distributions. Although some organisms can utilize sions is small, however, and the net flux of N2O from the ocean to anaerobic respiration, in which an element other than oxygen the atmosphere is poorly characterized but could be important. is the terminal electron acceptor, the lack of oxygen has strong effects on energetics. Aerobic metabolism is far more efficient The Scale of the Problem than anaerobic respiration at producing energy from organic Prior to 1960, around 45 sites were known to have episodic or matter. A molecule of glucose yields 39 ATP molecules aerobi- chronic hypoxia. According to Robert Diaz and colleagues, the cally but only about three ATP molecules under anaerobic rate of new sites that have reported hypoxia has roughly doubled metabolism. Thus, as oxygen declines, a cascade of physiolog- each decade since the 1960s (Figure 2). Today, there are over ical responses occurs. The ‘‘scope for metabolic activity’’ (the 700 sites of eutrophication-induced hypoxia, of which some difference in metabolic rate of active and inactive organisms) de- are responding to mitigation and restoration measures. How- clines and can be exacerbated by changes in other stressors, ever, there is clearly under-reporting of hypoxia in many parts such as an increase in temperature or changes in salinity.
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