Half of the oxygen we breathe results from the hard work of marine organisms. As their ecosystem becomes increasingly threatened by climate change, so do our lives.

The ocean covers ~70% of the Earth’s surface and contains ~97% of the Earth’s water. It absorbs and retains heat radiated from the Sun, transports the warm water and precipitation around the globe, and regulates the Earth’s climate to ensure the habitability of our planet.

The vast mother ocean is home to millions of creatures ranging from microbes to macrofauna that are supporting our lives in silence. Did you know that phytoplankton, the floating, drifting, microscopic plant-like organisms, are responsible for half of the oxygen produced on Earth? This microalgae provides an example of the significance of marine species in sustaining terrestrial life.

However, the marine ecosystem is threatened. Due to climate change, the storage of oxygen (O2) in the oceans has changed so much that it alters ecosystems.

A group of climate scientists recently discovered a method to forecast O2 loss and its impacts on marine biogeography, biodiversity, and biogeochemistry (click here to read the publication). They inquired the following question:

How does climate change alter the ocean’s O2 content, exacerbating local coastal eutrophication, with wide-ranging impacts on the distribution and diversity of marine life from microbes to microfauna?

In honor of World Oceans Day, we aim to explore their work and share some of the key results of their research findings on the relationship between climate, oxygen, and the future of marine biodiversity.

Ocean, climate change, and oxygen

Deutsch et al. (2024) identified two driving forces of changes in O2 stock in the ocean due to climate change, and both relate to changes in nutrients that occur in the coastal zone, where open ocean waters interact with highly productive, dynamic, and biodiverse environments.

First, warmer water temperatures.

Oxygen in the atmosphere varies with latitude but is otherwise evenly distributed. In water, however, oxygen levels can vary significantly across habitats and temperatures because warm water holds less oxygen than cold water. For aquatic species populations, warmer water temperatures imply an alert sign to save resources and find new homes.

Second, eutrophication.

An increase in human-induced carbon emissions through the burning of fossil fuels and deforestation has significantly heightened the amount of carbon dioxide (CO2) the ocean must absorb. While the ocean acts as a natural carbon sink that absorbs ~25% of all CO2 emissions and captures ~90% of atmospheric heat, the current CO2 value (427.48 ppm) is fumbling with the dynamics of global ocean O2 supply and demand, dissolving an increased amount of CO2 in seawater, declining O2 stock, and acidifying the ocean as never before.

Not only that, fertilizers used in agriculture (especially nitrogen and phosphorus) that are eventually carried to the coastal ocean stimulate phytoplankton growth and further deplete O2 stock in coastal regions. In other words, the denser the phytoplankton population, the less photosynthesis and O2 concentration produced.

Historical observations reveal that these driving forces are causing a ~2% oxygen decline in the upper ocean and an increase in coastal mass mortality events (p.226).

What does this mean for the marine environment?

Deutsch et al. (2024) categorized this into two scales.

At the ocean scale, a reduction in O2 stock means a reduction in net primary productivity (NPP) in the surface ocean and a depletion of nutrients for the marine life therein (p.226). This destabilizes the ocean’s delicate balance and has a profound impact on the global carbon cycle.

At the organism scale, ocean warming alone has already posed a serious challenge for all marine species that experience O2 limitations in their natural habitat. Top that with an additional reduction in O2 stock, different species populations are fighting over oxygen in an increasingly hypoxic ocean with less and less oxygen due to imbalances at the global scale. It’s Mad Max for the marine organisms.

These challenges are observed particularly in coastal zones, where naturally low O2 stock exists and overlaps with high biological activity. Although covering only ~7% of the ocean’s area, coastal regions contain ~15% of its net productivity and ~90% of its animal life (p.218), making it a prime shelter for a diversity of life associated with shallow-water ecosystems.

One example is in the coral reef ecosystems.

Warmer ocean temperatures and eutrophication cause stress to corals and can result in coral bleaching, where corals lose their vibrant colors due to the death of the microscopic algae living in their tissue and producing their food and pigment (zooxanthellae). Although bleached does not necessarily equal death, bleached corals are more susceptible to diseases and starvation, and if they eventually die, regeneration becomes exceedingly slow and difficult.

As the “rainforests of the sea” with ~25% marine inhabitants, threatened corals indicate a threat to the health of our oceans and the biodiversity of the marine ecosystem.

Ocean, climate change, oxygen, and marine biodiversity

Deutsch et al. (2024) also observed the impact of temperature-dependent hypoxia (insufficient oxygen that varies with temperature) on several common biodiversity indicators, including species richness (the number of species within a defined region) and body size.

While species richness patterns are strongly correlated with sea surface temperature (ie, warmer water temperature = higher species richness) (p.231), an opposite relationship is observed with O2 levels.

Along the equator, for example, warmer water temperatures have less influence on species richness when low O2 stock limits the ability of many marine species to sustain normal metabolic processes important for survival, growth, and immunity, among others. Simply put, O2 matters more than temperatures in ensuring species richness.

O2 loss is also responsible for body size reductions among marine animals (p.235), impacting their metabolic demand for O2 and the surface areas from which they receive oxygen supplies.

In small species (~1g) that form the base of the marine food web, such as phytoplankton and algae, 30% of their body size can experience reductions due to warming and O2 loss. Imagine shrinking to a third of your size when you are already invisible to the naked eye.

For many marine species, this means relocating to places with milder water temperatures but higher O2 concentrations.

So, is migration the all-for-one solution?

That depends on the spatiotemporal context. Migration may reduce the need for adaptive strategies required in shorter periods, but it eventually alters the marine ecosystem as a whole through local extirpation, colonization, and possible extinction.

If climate warming and ocean O2 loss push species beyond their carrying capacity and force them into becoming climate refugees, local habitats that previously existed with abundant oxygen may cause species extirpation–the condition in which an organism or species ceases to exist in a specific geographic area while still existing elsewhere. If enough local habitat disappears, any remaining global habitat may not be sufficient to sustain a viable species population, eventually leading to extinction.

Honoring our mother ocean

Although the relationship between climate and oxygen is clear, predicting the impacts of climate dynamics on biogeography, biodiversity, and biogeochemistry of both ocean and land populations in time and space remains an enormous challenge.

What we know is this: Changes in ocean biodiversity from rising temperature to O2 loss and other environmental changes are attributed to human activities in marine ecosystems. As a species that strongly depends on the ocean for necessities, this should be a good enough reason to act and contribute to its health over the coming decades.

References:

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7. Deutsch, C., Penn, J.L. and Lucey, N. (2024) ‘Climate, oxygen, and the future of Marine Biodiversity’, Annual Review of Marine Science, 16(1), pp. 217–245. doi:10.1146/annurev-marine-040323-095231. 

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