Why is Oxygen Important to Life in the Ocean?
Oxygen is necessary to support life on Earth. Oxygen makes up about 21% of the air we breathe and half of this oxygen is produced by phytoplankton in the ocean. All aerobic life requires oxygen to produce energy. In water, oxygen is found in a dissolved form and is much more limiting. For this reason, we measure dissolved oxygen in water not in % but in units of umol/kg. The highest concentrations of dissolved oxygen in the ocean are found within the surface mixed layer at concentrations of > 200 umol/kg. Below the surface mixed layer, oxygen concentrations are lower due to respiration by all of the trillions of organisms and bacteria that live in the water column.
What is Ocean Deoxygenation?
Ocean deoxygenation refers to the loss of oxygen from the oceans due to climate change (Keeling et al. 2010). Long-term ocean monitoring shows that oxygen concentrations in the ocean have declined during the 20th century, and the new IPCC 5th Assessment Report (AR5 WG1) predicts that they will decrease by 3-6% during the 21st century in response to surface warming. While 3-6% doesn’t seem like much, this decrease will be felt acutely in hypoxic and suboxic areas, where oxygen is already limiting. “Hypoxic” areas are defined as regions where oxygen limitation is detrimental to most organisms. This threshold differs across the world, but is usually defined as anything below 60 umol/kg. Hypoxic zones have oxygen concentrations 70-90% lower than the mean surface concentrations. “Suboxic” areas are areas where oxygen is so low (less than 5 umol/kg) that most life cannot be sustained and significant biogeochemical changes occur due to altered water chemistry. Suboxic zones have oxygen concentrations 98% lower than the mean surface concentrations. A recent study found that a 1°C warming throughout the upper ocean will result in the increase of hypoxic areas by 10% and a tripling of the volume of suboxic waters (Deutsch et al. 2011). To put this in context, a highly optimistic emissions scenario of atmospheric CO2 levels of 550 ppm by 2100 would lead to a 1.2°C warming of the upper ocean (Mora et al. 2013). Therefore, these declines in oxygen are changes we should be prepared to see.
How Are Humans Lowering the Oxygen Levels in the Ocean?
Oxygen content in the water is dependent on photosynthesis (produces oxygen), animal respiration (uses oxygen), and physical mixing. Ocean warming is reducing global ocean oxygen content through several key mechanisms (Keeling et al. 2010) including:
Stratification impacts: Anthropogenic warming causes surface waters to become warmer and thereby less dense, leading to a more stratified (layered) water column, which reduces mixing. Other impacts of climate change to the water cycle can also lead to a more stratified water column. These include inputs of freshwater to the ocean from rain, river runoff, or melting ice.
Warming effects: As a physical rule, warmer water holds less oxygen. As the surface waters warm due to climate change, the ocean loses its ability to hold oxygen, leading to an oxygen decline.
Biological effects: Changes to the biological use and production of oxygen can lead to changes in oxygen content in the water. Warmer ocean temperatures increase oxygen demand from organisms. Increased nutrient inputs (either through coastal runoff or through upwelling) also lead to more oxygen depletion at mid-depths (100-1000m).
Circulation changes: Changes in ocean circulation are also implicated with some of the observed declines in dissolved oxygen (Grantham et al. 2004). Slowing circulation and increased upwelling of oxygen-poor deep-water can lead to reductions in oxygen.
Oxygen Minimum Zones Are Expanding with Climate Change
Low oxygen areas are already present in several parts of the world and are increasing in number, volume, and intensity. Eastern boundary currents and upwelling regions occur on the eastern side of ocean basins and support some of the richest fisheries in the world. However, underlying these productive surface waters are extremely oxygen-depleted waters at 100-1000m depths. These are known as oxygen minimum zones (OMZs), which are midwater regions of the ocean that are naturally low in oxygen due to the combined processes of high oxygen use and limited oxygen replenishment (Wyrtki 1962). Already, OMZs make up ~8% of the total oceanic area (Paulmier and Ruiz-Pino 2009). OMZs are now expanding both horizontally and vertically due to climate change, resulting in habitat loss for organisms that are sensitive to low-oxygen concentrations (Gilly et al. 2013). Since the 1960s, the hypoxic area has increased by 4.5 million km2 at depths of 200-700 m in tropical and subtropical waters (Stramma et al. 2010). Meanwhile, off of California, waters at 200-300m have lost 20-30% of their oxygen in the last 25 years (Bograd et al. 2008). OMZ expansion is evident in all tropical ocean basins and throughout the subarctic Pacific, making habitat compression an increasingly global issue.
Fisheries Impacts from Oxygen Minimum Zone Expansion
The expansion of oxygen minimum zones and habitat compression is predicted to impact oceanic commercial fisheries. Already, some species range shifts have been observed due to changes in oxygen content. For example, along the Japanese continental slope decreases in mid-depth oxygen content over the last 60 years have resulted in Pacific cod shifting their distributions to shallower depths (Gilly et al. 2013, Ono et al. 2010). In the tropical Atlantic, blue marlin and tuna have experienced a 15% reduction in vertical habitat between 1960-2010 due to the expansion of oxygen minimum zones (Stramma et al. 2011). While organisms that are intolerant of low oxygen are losing habitat, other organisms are gaining habitat. Off the West Coast of the US, the Humboldt squid has greatly expanded its range, and the range expansion coincides strongly with areas of significant oxygen declines (Gilly et al. 2013).
How Will Oxygen Loss Affect Marine Ecosystems and Humans?
Oxygen plays a key role in structuring marine ecosystems and controls the distribution of essentially all marine organisms (Gilly et al. 2013). For this reason, oxygen loss in the oceans will have significant ecosystem-level consequences. Organisms exhibit a great range in tolerances to low levels of dissolved oxygen. Some are highly tolerant, such as jellyfish and squid, while other groups like fish and crustaceans require higher oxygen levels and are highly vulnerable to oxygen declines (Vacquer and Sunyer 2010). This sensitivity has been seen in many systems. For example, off Southern California mid-water fishes, which are crucial components of the ocean food web, declined 63% between periods of high and low oxygen (Koslow et al. 2011). The vulnerability of fish and crustaceans is also evident by the large fish and crustacean die-offs that have occurred during low oxygen events, including off Oregon’s coast (Grantham et al. 2004) and in the Gulf of Mexico. While mortality is a direct impact of oxygen loss, marine organisms can also be indirectly impacted leading to changes in behavior, reproduction, growth and behavior. Together, direct and indirect species-level impacts can lead to significant ecosystem-level consequences such as decreased resilience, stability, and resistance to other anthropogenic stressors such as fishing and pollution. Coastal waters are experiencing significant reductions in oxygen due to nutrient pollution, but these impacts are also exacerbated by climate change. Dead zones, areas where most organisms cannot live due to oxygen limitation, are now reported for more than 479 systems, and since the 1960s, their numbers have doubled approximately every decade (Diaz and Rosenberg 2008). Currently, hypoxia and anoxia are among the most widespread deleterious anthropogenic influences on estuarine and marine environments. As surface waters warm, these low oxygen coastal areas, and the animals that live within them, experience even more oxygen stress. These changes impact the people that depend on these waters for resources.
Need for Attention to Deoxygenation By Policy-Makers
Marine ecosystems provide us with fundamental ecosystems services and changes to these ecosystems leave coastal economies vulnerable. Approximately 470 to 870 million of the poorest people in the world rely heavily on the ocean for food, jobs, and revenues and live in countries that will be most affected by the stacked impacts of warming, acidification, and deoxygenation (Mora et al. 2013). Namibia, the Philippines, India, Chile, Peru, and the US are just several of the coastal nations that are vulnerable to the impacts of deoxygenation (Hofmann et al. 2011). Because deoxygenation may have direct economic impacts on marine resources, it warrants attention by policy-makers.
The potential consequences of ocean oxygen loss are profound, but deoxygenation does not act alone. Together, warming, acidification, and deoxygenation present a triple whammy for marine life (Gruber 2011). Warming and deoxygenation are important synergistic stressors for marine organisms because oxygen requirements vary with temperature, and temperature thresholds are often limited by oxygen availability (Portner and Knust 2007). Low oxygen waters are also characterized by low pH, meaning that marine organisms are simultaneously experiencing oxygen and pH stress (Feely et al. 2008). Meanwhile, decreases in pH may require organisms to use more energy to maintain acid-base homeostasis, however, organismal metabolism may be limited by low oxygen conditions. Due to their interacting effects, warming, acidification, and deoxygenation need to be considered together in order to understand how marine ecosystems will respond to climate change.
What Can be Done?
Since climate change is the driving cause of ocean deoxygenation, reducing carbon dioxide (CO2) emissions is the only real solution. However, certain other actions can help to ameliorate the problem especially at a local level. Lessening other anthropogenic stressors such as nutrient pollution and overfishing may improve the resilience capacity of marine communities. Adopting climate-savvy fisheries management strategies that consider the spatial and temporal extent of hypoxia and are reflective of the changing nature of the marine environment, would aid in protecting oxygen-sensitive marine resources.
- written by Natalya Gallo