Sea creatures store carbon in the ocean – could protecting them help slow climate change?

As the prospect of catastrophic effects from climate change becomes increasingly likely, a search is on for innovative ways to reduce the risks. One potentially powerful and low-cost strategy is to recognize and protect natural carbon sinks – places and processes that store carbon, keeping it out of Earth’s atmosphere.

Forests and wetlands can capture and store large quantities of carbon. These ecosystems are included in climate change adaptation and mitigation strategies that 28 countries have pledged to adopt to fulfill the Paris Climate Agreement. So far, however, no such policy has been created to protect carbon storage in the ocean, which is Earth’s largest carbon sink and a central element of our planet’s climate cycle.

As a marine biologist, my research focuses on marine mammal behavior, ecology and conservation. Now I also am studying how climate change is affecting marine mammals – and how marine life could become part of the solution.

A sea otter rests in a kelp forest off California. By feeding on sea urchins, which eat kelp, otters help kelp forests spread and store carbon. Nicole LaRoche, CC BY-ND

What is marine vertebrate carbon?

Marine animals can sequester carbon through a range of natural processes that include storing carbon in their bodies, excreting carbon-rich waste products that sink into the deep sea, and fertilizing or protecting marine plants. In particular, scientists are beginning to recognize that vertebrates, such as fish, seabirds and marine mammals, have the potential to help lock away carbon from the atmosphere.

I am currently working with colleagues at UN Environment/GRID-Arendal, a United Nations Environment Programme center in Norway, to identify mechanisms through which marine vertebrates’ natural biological processes may be able to help mitigate climate change. So far we have found at least nine examples.

One of my favorites is Trophic Cascade Carbon. Trophic cascades occur when change at the top of a food chain causes downstream changes to the rest of the chain. As an example, sea otters are top predators in the North Pacific, feeding on sea urchins. In turn, sea urchins eat kelp, a brown seaweed that grows on rocky reefs near shore. Importantly, kelp stores carbon. Increasing the number of sea otters reduces sea urchin populations, which allows kelp forests to grow and trap more carbon.

Scientists have identified nine mechanisms through which marine vertebrates play roles in the oceanic carbon cycle. GRID Arendal, CC BY-ND
Carbon stored in living organisms is called Biomass Carbon, and is found in all marine vertebrates. Large animals such as whales, which may weigh up to 50 tons and live for over 200 years, can store large quantities of carbon for long periods of time.

When they die, their carcasses sink to the seafloor, bringing a lifetime of trapped carbon with them. This is called Deadfall Carbon. On the deep seafloor, it can be eventually buried in sediments and potentially locked away from the atmosphere for millions of years.

Whales can also help to trap carbon by stimulating production of tiny marine plants called phytoplankton, which use sunlight and carbon dioxide to make plant tissue just like plants on land. The whales feed at depth, then release buoyant, nutrient-rich fecal plumes while resting at the surface, which can fertilize phytoplankton in a process that marine scientists call the Whale Pump.

And whales redistribute nutrients geographically, in a sequence we refer to as the Great Whale Conveyor Belt. They take in nutrients while feeding at high latitudes then release these nutrients while fasting on low-latitude breeding grounds, which are typically nutrient-poor. Influxes of nutrients from whale waste products such as urea can help to stimulate phytoplankton growth.

Finally, whales can bring nutrients to phytoplankton simply by swimming throughout the water column and mixing nutrients towards the surface, an effect researchers term Biomixing Carbon.