Ocean Rewilding and Animating the Marine Carbon Cycle

By Steven Lutz, GRID-Arendal, Arendal, Norway

Our marine fingerprint

Bleached coral on a degraded reef in Sri Lanka (Photo: Steven Lutz).

Ocean rewilding is the large-scale restoration of marine ecosystems to the point where nature can take care of itself. However, as a species we have impacted marine ecosystems across the globe from the beaches to the deepest depths of the ocean. Researchers have found plastic trash at a depth of 10,975 meters (36,000 feet) inside the Mariana Trench in the Pacific Ocean (National Geographic 2019). Global climate change has heat stressed and degraded coral reefs across the world (WWF 2021).

In the face of all the threats we inflict on the marine environment, is ocean rewilding and recovery possible?

Fishing the line

Fishing in the Florida Keys (Photo: Matt McIntosh, NOAA).

Throughout the world a fish harvesting tactic called “fishing the line” is commonly employed. This is where no take marine reserves are targeted by fishers. They fish right up to the very boundaries of the marine protected area (MPA) because they realize that this is where the larger fish are. The MPAs provide a refuge for targeted species. Fish inside its boundaries have the time to grow larger than their counterparts outside of the area. Research has shown that the biomass of schools of fish within an MPA is, on average, six to seven times greater than in nearby unprotected areas (Sala and Giakoumi 2018).

By “fishing the line”, fishermen are evidencing that ocean rewilding is possible and effective.

Ocean rewilding as climate mitigation

Marine life not only serves as a resource to be fished from the ocean. Recent science has illuminated that keeping fish and other marine life in the ocean may benefit us.

Carbon is fixed in the ocean through primary production, which includes the growth of marine plants, such as phytoplankton. Like plants on land, phytoplankton need nutrients to grow. The more nutrients in sunlit surface waters, the more phytoplankton can grow. By absorbing carbon dioxide, phytoplankton play a central role in the global carbon cycle. They also form the base of the marine food web.

All marine animals, from krill to whales, can store carbon. Through their feeding activities and other life processes, marine life can help to remove carbon from the atmosphere (Lutz et al 2018). This includes how whales and their role in the oceanic carbon cycle. The waste products of whales – yes, the whale poo – contains exactly what phytoplankton needs to grow, notably iron and nitrogen. Researchers have estimated that in terms of carbon sequestration and climate mitigation, each great whale sequesters an estimated 33 tons of carbon. Essentially, one whale is worth thousands of trees (Chami et al 2018).

Whales help mitigate climate change through four “whale carbon” mechanisms (Fig 1). When whales feed at depth, they bring nutrients up to the ocean surface through their vertical movement (“whale pump”). Through their annual migrations they also transport nutrients through across the oceans (“whale conveyor belt”). This natural ocean fertilizing activity potentially significantly enhances phytoplankton growth in whales’ habitats. During their lifetimes, whales store carbon in their bodies (“biomass carbon”) and upon natural expiration they bring that carbon to the sea floor (“deadfall carbon”).

“Whale Carbon” Mechanisms (credit: GRID-Arendal).

Ocean rewilding – so what?

Keeping carbon out of the atmosphere is key to addressing climate change, and the length of time over which carbon is stored in the ocean depends on how deep it sinks (Turner 2015). Carbon in shallow waters can be stored over the short-term, for months to decades. Carbon that sinks to deep waters can be stored over the long-term, for hundreds to thousands of years. Carbon that finds its way into marine sediments in the deep seafloor – like a sinking dead whale – can be stored for millions of years. These are timescales far greater than carbon stored in terrestrial forests.

With six out of the 13 great whale species listed as endangered or vulnerable (WWF 2021), and fish species like sharks at the risk of extinction from overfishing (Worm et al 2013), there is an urgent need to restore and rewild our ocean.

Saving whales, ocean conservation and rewilding our ocean, may reanimate ocean carbon function and help save us from climate change!