Coastal waters worldwide are rapidly losing oxygen, causing declines in marine life and affecting communities who rely on the health of coastal waters.
Prominent examples of low-oxygen coastal waters are found in the Baltic Sea, for instance, where oxygen loss in recent decades has led to major ecosystem changes. Potentially toxic cyanobacterial blooms have become frequent and widespread, spawning grounds for cod have been greatly reduced, and fish kills have been observed in coastal waters [Conley et al., 2009]. Similar issues have afflicted the Gulf of Mexico, the Adriatic Sea, the East China Sea, and numerous other areas.
The main cause of declining coastal ocean oxygen is well-known: Since the 1950s, phosphorus and nitrogen from agricultural runoff and wastewater have flowed into coastal seas, where they stimulate phytoplankton blooms that, upon their decay, consume oxygen. This process, called eutrophication, is not the only cause of declining oxygen and so-called dead zones in coastal waters: Increasing global temperatures are contributing by reducing both the solubility of oxygen in seawater and vertical mixing of the ocean water column, thereby limiting the aeration of deeper waters [Breitburg et al., 2018].
Indeed, even coastal systems not experiencing eutrophication, such as the Gulf of St. Lawrence in Canada, may be under threat of low oxygen because of changes in ocean circulation linked to climate change [Wallace et al., 2023].
Various means of artificial reoxygenation have been suggested and studied as possible local to regional solutions to coastal oxygen loss.
Long-term reductions in nutrient inputs from land are widely acknowledged as essential to mitigate coastal eutrophication, but such reductions will take time to have an effect. Nutrients have been accumulating in many coastal systems for decades, and switching off inputs will not immediately lead to lower concentrations [Conley et al., 2009]. Moreover, reducing nutrient releases from agricultural lands in many regions is proving challenging. Attempts to curtail the global use of fossil fuels and cut greenhouse gas emissions substantially have also been less successful to date than what is required to affect ocean oxygen [Breitburg et al., 2018].
Amid the challenges of achieving global-scale solutions, various means of artificial reoxygenation have been suggested and studied as possible local to regional solutions to coastal oxygen loss [Stigebrandt et al., 2015]. Yet these approaches come with risks that must be assessed carefully before implementation [Conley et al., 2009].
Such assessments are becoming urgent with the emergence of potential new artificial reoxygenation technologies linked to green hydrogen production. This process of splitting water by electrolysis to generate hydrogen also generates oxygen [Wallace et al., 2023], which could be put to use in coastal waters, particularly where green hydrogen production facilities are located close to the sea.
Oxygen Supply Versus Demand
Coastal seas gain oxygen naturally through air-sea exchange, vertical and lateral mixing of seawater, and photosynthetic production by phytoplankton (Figure 1). They lose oxygen through respiration of organic matter in the water column and the underlying sediment. Surface waters typically remain oxygenated because of rapid air-sea exchange and primary productivity, but in deeper waters, oxygen removal may dominate, especially in systems with limited vertical mixing [Fennel and Testa, 2019].
The main goal of artificial reoxygenation is to increase the supply of oxygen to deeper waters enough that the water and sediment at the seafloor surface become or remain oxygenated. The oxygen supply needed to achieve this goal depends on the local oxygen demand, which itself depends on the input of organic matter from sinking phytoplankton biomass and the eutrophication history of the system. If organic matter inputs remain high or a lot of organic matter has accumulated on the seafloor, oxygen demand may remain high for a long time. This “legacy” effect can hinder the reoxygenation of a coastal system, as shown in the Baltic Sea [Hermans et al., 2019].
A secondary goal of reoxygenation is to limit recycling of phosphorus from sediments, which, in turn, may reduce the availability of phosphorus as a nutrient for phytoplankton in surface waters. Decreasing how much organic matter is produced and then sinks to the seafloor may lower the oxygen demand for respiration and hence increase oxygen concentrations in deeper waters [Conley et al., 2009].
Inspired by methods used to reoxygenate lakes with some success, two broad approaches have been proposed for artificially reoxygenating coastal systems (Figure 2): bubbling pure oxygen or air into the ocean [Koweek et al., 2020; Wallace et al., 2023] and pumping oxygenated surface water to greater water depths, a process called artificial downwelling [Stigebrandt et al., 2015; Lehtoranta et al., 2022].
Bubble diffusers have been used in lakes to oxygenate deep water directly [Koweek et al., 2020] and in shallow coastal systems to destratify and aerate the water column by inducing mixing [Harris et al., 2015]. Artificial downwelling has been tested for local applications in only a few small coastal systems [Stigebrandt et al., 2015; Lehtoranta et al., 2022].
The Imperfections of Artificial Reoxygenation
Studies to date show that artificial reoxygenation can be applied successfully in small estuaries and bays but that its effect lasts only as long as operations are maintained.
Studies to date show that artificial reoxygenation can be applied successfully in small estuaries and bays but that its effect lasts only as long as operations are maintained. This outcome was observed, for example, in two Swedish bays following their reoxygenation through pump-driven downwelling [Stigebrandt et al., 2015; Lehtoranta et al., 2022]. Similarly, when aerators were switched off in a shallow subestuary of the Chesapeake Bay after several decades of aeration, low-oxygen, or anoxic, levels returned within a day [Harris et al., 2015].
The rapid return of anoxia upon discontinuing artificial reoxygenation operations—also known from applications in lakes—illustrates that these approaches alone do not provide permanent solutions to deoxygenation because they do not address its root causes. Moreover, adding oxygen to the water column does not mitigate wider water quality problems. Nuisance algal blooms in many coastal areas will still occur if the availability of nutrients for phytoplankton remains high.
In the Baltic Sea, for example, natural decadal-scale reoxygenation of deeper waters linked to lateral inflow of oxygenated North Sea water does not lead to a removal of phosphorus in the sediment [Hermans et al., 2019]. This lack of an effect results from the highly reducing conditions in the seafloor sediment, which hinder formation of phosphorus-containing minerals. Consequently, reoxygenation of the water column in the Baltic does not necessarily decrease recycling of phosphorus [Hermans et al., 2019], which may continue to fuel cyanobacterial blooms [Conley et al., 2009].
Reoxygenation via artificial downwelling may also be unsuccessful if it causes warming of deeper waters, which is a risk, especially when surface water pumps are used to reoxygenate temperature- and density-stratified coastal waters. Transferring warm surface water to colder, denser depths near the seafloor may weaken stratification and enhance vertical mixing. Although this process may increase the downward transfer of oxygen, it can also boost upward mixing of nutrients, which may enhance biological productivity. This enhancement can ultimately increase the oxygen demand in deeper waters to such an extent that a net decrease in oxygen results [Conley et al., 2009; Lehtoranta et al., 2022].
Warming at depth can also lead to greater metabolic activity and increased respiration of organic matter, further decreasing oxygen concentrations instead of increasing them as intended.
Side Effects on Climate and Habitats
Artificial reoxygenation may have other undesirable effects as well. It can alter the dynamics of greenhouse gases in coastal waters, for example, because increased aerobic respiration increases carbon dioxide production.
Furthermore, bubbling air through shallow coastal waters can enhance upward transport of methane, a potent greenhouse gas, in the water column and its emission to the atmosphere [Lapham et al., 2022]. In eutrophic coastal systems, reoxygenation does not necessarily suppress the release of methane from sediments [Żygadłowska et al., 2024], implying that upon bubbling, methane emissions from coastal waters may be greater than without reoxygenation.
Reoxygenation operations may also alter ocean habitats and have unintended consequences for marine life. Bubbling generates underwater noise, turbulence, and gradients in oxygen pressure that differ from naturally occurring conditions. Artificial downwelling not only changes water column temperatures but also alters vertical salinity distributions, with unknown consequences for marine organisms [Conley et al., 2009; Wallace et al., 2023]. In addition, the return of bottom-dwelling animals with reoxygenation may cause increased sediment mixing that remobilizes sediment contaminants [Conley et al., 2009].
Assessing Artificial Reoxygenation as a Solution
Artificial reoxygenation, when applied, should always be only one of various measures used to improve water quality.
Taken together, the body of evidence from reoxygenation studies to date indicates that long-term improvements in the oxygen levels and quality of coastal waters require reductions in nutrient inputs and greenhouse gas emissions. Hence, artificial reoxygenation, when applied, should always be only one of various measures used to improve water quality.
In heavily managed coastal systems, reoxygenation may be a temporary solution, as illustrated by its successful application in a subestuary of the Chesapeake Bay [Harris et al., 2015]. Elsewhere, such as in the Gulf of St. Lawrence, reoxygenation might be harnessed to maintain the current oxygen state of the system [Wallace et al., 2023]. However, responses to reoxygenation in eutrophic systems with strong legacy effects, where sediments act as a source of nutrients and a sink for oxygen, are very difficult to predict [Conley et al., 2009; Hermans et al., 2019].
The dependence of reoxygenation effects, either from aeration or from pumping, on site-specific biological, chemical, and physical characteristics, which are often poorly known and differ greatly worldwide, also hinders predictions of responses. Yet accurately predicting the effects of artificial reoxygenation before implementing it is critical and consistent with the precautionary principle that in the absence of scientific certainty, we should act to avoid harm.
This principle can be interpreted to suggest that no measures should be taken in some cases and that in other cases, measures should not be postponed because delay could lead to even more harm. Thus, careful case-by-case assessments of the suitability of artificial reoxygenation at given sites are needed—as is careful monitoring when operations are implemented. Modeling studies are valuable for such assessments [e.g., Koweek et al., 2020] but must be paired and validated with field data.
The potential availability of substantial oxygen supplies to support artificial reoxygenation as a result of increasing green hydrogen production further raises the urgency of suitability assessments [Wallace et al., 2023]. If such supplies can be tapped near coastal areas, they may help make artificial aeration operations logistically more viable and sustainable.
Foundations for Responsible Reoxygenation
For areas found to be potentially well suited for artificial reoxygenation interventions, consensus best practices should be followed when initiating pilot studies or larger implementations. As informed by discussions during a recent meeting organized by the United Nations Educational, Scientific and Cultural Organization Intergovernmental Oceanographic Commission’s Global Ocean Oxygen Network, several elements are foundational to these best practices.
Relevant government bodies, such as national and local water management authorities; stakeholders, including representatives of local communities; and scientists should be involved from the outset to safeguard the interests of all parties. Field trials and implementations should consider perceived environmental benefits and risks of the intended intervention, as well as relevant ethical issues, taking into account the intrinsic value of nature.
Monitoring is important for understanding baseline conditions and assessing the effects of reoxygenation on water quality and ecology, including termination effects after an intervention ceases.
Key biological, chemical, and physical parameters of the system where the intervention will occur (as well as of a reference site) should be monitored before, during, and afterward. This monitoring is important for understanding baseline conditions and assessing the effects of reoxygenation on water quality and ecology, including termination effects after an intervention ceases. Continued measurements over years to decades are also critical to determine longer-term effects.
Finally, the results of all field trials, including failures, should be reported completely, transparently, and publicly.
Artificial reoxygenation is unlikely to be a permanent solution to declining ocean oxygen, and it cannot replace essential measures to reduce greenhouse gas emissions and nutrient inputs to ocean waters. But with science-based suitability assessments and ethical, environmentally safe practices, reoxygenation interventions might prove beneficial in some places, allowing temporary mitigation of the detrimental consequences of coastal deoxygenation.
Acknowledgments
This feature article summarizes the discussion of a workshop on marine reoxygenation organized by the Global Ocean Oxygen Network (GO2NE) on 10–11 September 2024. We thank all participants for their contributions: D. Austin, L. Bach, L. Bopp, D. Breitburg, A. Canning, D. Conley, M. Dai, B. DeWitte, H. Enevoldsen, E. Ferrar, A. Galan, V. Garcon, M. Gregoire, B. Gustafsson,, D. Gutierrez, A. Hylén, K. Isensee, R. Lamond, M. Li, K. Limburg, I. Montes, J. Sterling, A. Tan Shau Hwai, J. Testa, D. Wallace, J. Waniek, and M. Yasuhara.
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Author Information
Caroline P. Slomp ([email protected]), Radboud University, Nijmegen, Netherlands; also at Utrecht University, Netherlands; and Andreas Oschlies, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany
