Elemental 2: Oxygen and Oceans

Climate change conjures up images of rising sea levels. But rising seas are not the only oceanic effect of increasing heat. Many of the cascading changes we will see relate to oxygen.

Understanding climate change means understanding at an elemental level how heat, water, CO2, and oxygen interact, and the cascade of impacts that flow from this, through plants and animals, and into human health.

As water warms, it holds less oxygen. Water molecules gain energy as they heat, causing the bonds between oxygen and water to loosen and oxygen to more easily escape.

Deoxygenation goes hand in hand with acidification. As the ocean absorbs more and more CO2, it becomes more acidic. It’s hard to understate just how much the ocean has looked after us over the centuries. Since the 1700s, the ocean has absorbed a third of all human-produced CO2.

As the world has warmed, the saturation of oxygen in the open ocean has already decreases from between 5 to 3 per cent. Warming is occurring from the ocean floor to the surface, but is particularly pronounced in the 50 meters below the surface.

This last summer, the ocean temperature in the Florida Keyes hit 96°F, almost 36°C (the season average normally sits between 83 to 87°F). At this higher temperature the water is almost the same as the body’s core temperature.

Oceans have a higher heat capacity than land. This means they hold heat for longer. Even at night, they will remain warm. Oceans gain and retain their oxygen levels through autotrophic organisms, those that photosynthesise or metabolically convert sunlight into energy and transforms water molecules into oxygen, such as phytoplankton, algae and kelp.

Half of the oxygen in the world (yes half!) is produced by these quiet ocean metabolisers. The turbulence of waves and wind also oxygenate the surface, and currents draw this down into deeper regions.

As ocean water warms, the metabolic rate of most ocean species increases. They must consume more oxygen. Unlike endothermic humans, who can maintain a core body temperature despite a fluctuating environment, most fish are ectotherms. This means their environment determines their temperature and thus the rate of their bodies’ biochemical processes. As fish warm, their physical systems readjust, running at faster rates.  

A term I learned recently is abiotic ecological master factor. Warming oceans lead to more frequent ‘red tides’ – algae blooms – and increasing occurrences of ‘dead zones’. Rusty coloured algae blooms occur along coastal regions when warm waters meet rich nutrients. The increasing storms from climate change mean heavy rains flush agricultural fertilizer into waterways, leaching nitrogen into estuaries, bays and coastal zones.

While the rapidly multiplying algae produce oxygen, as they die they are decomposed by bacteria that use up the oxygen in the water. Algae blooms also block sunlight from reaching photosynthesising plant species on the ocean floor, and can release a neurotoxin that kills fish. This oxygen depletion, or hypoxia leads to dead zones in which very little marine life can survive.

Global maps of hypoxic zones shows dots of red around land on every continent, but they cluster along the coastlines of Europe, the US, Japan and Australia. The largest dead zone recorded extended out from the mouth of the Mississippi River into the gulf of Mexico 23,000 Sq. kilometers.

In a dead zone, wildlife suffocates, creating ‘kill events’. Fish can be seen gasping at the surface, or lying on the waters edge. Even mammals that hold their breath to dive, such as whales, dolphins and seals, are forced to migrate due to the death of their food, fish and krill.

Zooplankton, such as crustaceans and krill, are particularly vulnerable to warming, and can be effected by even a 1% drop in oxygen. Zooplankton also feeds on phytoplankton, providing the vital role of converting the tiny phytoplankton into an edible food for larger species.

‘Plankton’ means to wade or drift. Such tiny creatures cannot generate enough motion to swim directionally, and are moved by ocean currents. They cannot, like other ocean species, migrate to more hospitable regions as waters warm. The warmer oceans become, the more stratified the water becomes between the warmer surface, layer and the cooler, denser layers below. This prevents nutrients flowing easily to the surface where phytoplankton must live. As the foundation upon which the ocean’s food chain depends, any shifts in plankton health will ripple up to every species.

Climate change acts as a multiplier. Increasing ocean heat adds energy to weather patterns, making hurricanes more frequent and severe. When heavy rainfall increases, more nitrogen runoff enters the ocean. As waters warm, phytoplankton become smaller and thus less productive in converting carbon. By the end of the 21st century, scientific models predict that the ocean temperature will increase by between 1 and 6°C. The oxygen levels in the oven will decrease by 5-10%. Respiratory distress will see many fish species migrate away from the equator into cooler zones. Coastal communities in equatorial zones depended on the ocean for food and livelihoods will be impacted. Shellfish will struggle in an increasingly acidic ocean. Without predators, jellyfish will likely thrive, and perhaps become a new food staple for some communities.

As a researcher of heat stress and human health, I’m mindful to avoid a human-centric approach of health. Understanding how the ocean and ocean life are affected at an elemental level helps me to consider how human health works through interaction, feedback, and cascades. How our health is inseparable from our planetary context.

Image by: Martin Falbisoner