11. What can we do? Adaptation and solutions to declining ocean oxygen Denise Breitburg, Daniel J. Conley, Kirsten Isensee, Lisa A. Levin, Karin E. Limburg and Phillip Williamson What can we do? Adaptation and 11 solutions to declining ocean oxygen Denise Breitburg1, Daniel J. Conley2, Kirsten Isensee3, Lisa A. Levin4, Karin E. Limburg5 and Phillip Williamson6 1 Smithsonian Environmental Research Center, Edgewater, MD, USA. 2 Department of Geology, Lund University, Lund, Sweden. 3 Intergovernmental Oceanographic Commission of UNESCO, Paris, France. 4 Scripps Institution of Oceanography, University of California, La Jolla, CA, USA. 5 State University of New York College of Environmental Science and Forestry, Syracuse, NY, USA. 6 University of East Anglia, Norwich, UK. Summary • The oxygen content of the open ocean and coastal waters has declined since the middle of the 20th century, and is expected to decline further during the 21st century as a result of climate change and increased nutrient discharges. Consequences of this ocean oxygen decline include decreases in biodiversity, shifts in species distributions, displacement or reduction in fisheries resources, and changes in biogeochemical cycling. • Fossil fuel combustion and agriculture contribute to both global warming and over-enrichment of waters with nutrients. Sewage – biomass in untreated sewage and nitrogen and phosphorus in both treated and untreated sewage effluent – is also a major contributor to oxygen depletion in coastal waters. • Nutrient reduction strategies that have been most effective have utilized legal requirements, set specific targets, and have employed monitoring to detect problems and responses to management strategies. A range of potential solutions to nutrient reduction exists and can be tailored to local needs and economies. • Reducing the rate of oxygen decline in the global ocean, and minimizing the contribution of climate change to deoxygenation of coastal waters, requires a dramatic climate mitigation effort, primarily through global reduction in greenhouse gas emissions due to human activities. Restoring oxygen lost over the past century on less than millennial time scales will likely also require reducing atmospheric greenhouse gas concentrations to levels lower than the present, through active greenhouse gas removal. • Continued and enhanced efforts to quantify trends in deoxygenation and project future oxygen conditions, to understand deoxygenation effects on biological, biogeochemical and ecological processes, and to incorporate deoxygenation in development of fisheries and other management strategies are needed. • Governance at scales ranging from local jurisdictions to international bodies such as the United Nations plays important roles in identifying the problem of deoxygenation, and in mitigation and adaptation efforts to reduce deoxygenation and its negative consequences. CHAPTER 11 Ocean deoxygenation: Everyone’s problem 545 11. What can we do? Adaptation and solutions to declining ocean oxygen • Solutions to ocean deoxygenation, and development of adaptation strategies in its presence, depend on sound and sufficient science. The international scope of scientific collaboration on this issue is notable; scientific working and expert groups can help facilitate communication among different stakeholders, and support decision makers to take measures required to stem increasing deoxygenation at local, regional and global scales. Further progress is needed, however, in the science of ocean deoxygenation, especially to improve predictions of future conditions and impacts on human welfare. 11.1 Introduction less severely oxygen-depleted waters alter distributions, growth rates and disease in marine organisms. The Declining oxygen in the world’s ocean, including its combined effects of oxygen decline with those of other coastal waters, is one of the starkest examples of anthropogenic stressors — warming, acidification and degradation of ocean ecosystems caused by human population-level and food web-effects of fisheries, for activities (Breitburg et al., 2018) (Figure 11.1). Since the example — can be more severe than effects of oxygen middle of the 20th century, the open ocean has lost an decline alone. Oxygen is fundamental to multicellular life estimated billion metric tons of oxygen, and the volume and to many microbes, and when in short supply, it can of water in which oxygen is completely lacking1 has greatly alter the functioning of marine ecosystems, with increased 4 fold (Schmidtko et al., 2017). Much of this the potential to negatively affect human wellbeing. oxygen loss is attributable to global warming through its effects on oxygen solubility, stratification, ocean How do we move forward, given the magnitude of the circulation and respiration rates (IPCC, 2019; Oschlies problem of ocean oxygen decline? The first step is to et al., 2018). Over a similar period of time, more than raise awareness of the issue among policy-makers 500 estuaries, semi-enclosed seas and other coastal and within civil society. Increasing awareness and water bodies have reported first instances or expanded understanding of ocean2 oxygen decline is a major areas of dissolved oxygen concentrations at or less than goal of this report, as well as of several other recent -1 -1 2 mg L or 63 μmol L (hypoxia) as a result of an over- publications (e.g. Breitburg et al., 2018; GO2NE, 2018; supply of nutrients from agriculture, aquaculture, human Isensee et al., 2015; Levin & Breitburg, 2015; Limburg et sewage and the combustion of fossil fuels (Breitburg et al., 2017). Although the problem of low oxygen caused al., 2018; Diaz & Rosenberg, 2008; Isensee et al., 2015). by nutrients and sewage in coastal waters has been Inaccessibility of information, and lack of monitoring, has recognized for over a century, progress in correcting the likely led to an undercount of such systems, particularly problem has generally been slow except where solutions in developing nations. Numerical models project that also address issues that create a direct threat to human continued warming and increasing human populations health (e.g. discharge of raw sewage into waterways). will increase the severity of the problem of oxygen The magnitude of the effect of global warming on ocean decline in both the open ocean and coastal waters (e.g. oxygen content is only recently becoming understood Cocco et al., 2013). (Levin, 2018). It is important to raise the profile of oxygen decline within the larger conversation on the effects Severely oxygen-depleted waters, and those in which that increasing greenhouse gas (GHG) emissions are oxygen is completely lacking, represent habitat that is having on the global ocean. Improved understanding unusable by most multicellular marine organisms that of the economic consequences of oxygen decline, lost are important to marine food webs and fisheries. Instead, ecosystem services and effects on human well-being these areas host rich microbial communities that alter may contribute to this effort. biogeochemical cycles, increasing production of toxic compounds and greenhouse gases, and reducing the In the remainder of this chapter, we discuss the way supply of biologically available nitrogen in regions of forward – the steps and strategies needed, as well the ocean where nutrients can be in short supply. Even as opportunities to stem the loss of ocean oxygen, and especially in nutrient-enriched coastal waters, to 1 The designation of an area as ‘anoxic’ or ‘completely lacking in oxygen’ is based on detection limits of instrumentation used to measure oxygen and the presence of biogeochemical processes that are inhibited by oxygen. As 2 We use the term ‘ocean’ or ‘global ocean’ to include the open ocean, instrumentation improves, we are finding that some areas classified as anoxic and coastal water bodies such as semi-enclosed seas, estuaries, and similar contain nanomolar concentrations of oxygen, but these are levels that are too systems. ‘Open ocean’ refers to areas where the influence of local watershed low to support most multicellular organisms and many microbes. processes is secondary or undetectable. 546 Ocean deoxygenation: Everyone’s problem 11. What can we do? Adaptation and solutions to declining ocean oxygen A B Figure 11.1 Oxygen has declined in both (A) coastal waters and (B) the open ocean since the middle of the 20th Century (reproduced from Breitburg et al., 2018). (A) Coastal waters that have reported oxygen concentrations <2 mg L-1 (63 μmol L-1 or 61 μmol kg-1 ) (red dots) (Diaz & Rosenberg, 2008; Isensee et al., 2016). Most systems shown in this figure reported their first incidence of low oxygen after 1960. (B) Estimated change in oxygen content of the global ocean in percentage decade-1 since 1960 (Schmidtko et al., 2017). Red tones denote oxygen declines, and blue tones increases. restore this vital resource to healthier levels. Addressing fuel and agriculture are major contributors to both deoxygenation will require action on multiple fronts, with problems. Fossil fuel combustion, the major source of mitigation and adaptation actions at the global, regional, GHG emissions, also produces nitrogen oxides that are national and local levels, and thus engagement of a deposited onto land and directly into coastal waters, broad range of stakeholders. where they stimulate primary production and, ultimately, oxygen decline (Seitzinger & Phillips, 2017). Atmospheric 11.2 Solutions to slow and reverse oxygen deposition of iron and fixed nitrogen