Blue Carbon: How The Ocean Stores CO2

The role of marine vertebrates is now being explored to determine their importance in powering the biological carbon pump.

Blacktip shark. Via USFWS/Flickr
Blacktip shark. Via USFWS/Flickr

The term “carbon sequestration” typically conjures up images of lush tropical rainforests, rolling prairies, and dense woodland forests, all of which are terrestrial ecosystems packed with photosynthetic life, capable of removing vast amounts of CO2 from our atmosphere for growth and replacing it with life-giving oxygen. But these land-based environments are far from the only places on Earth that trap and store carbon, and we have only to look to the blue heart of our planet for the answer: the ocean.

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Calcium carbonate produced by bony fish acts as a pH buffer that helps mitigate ocean acidification Photo by Alex Rose

Our world ocean is essentially a giant, global carbon sink responsible for absorbing and accumulating 20 to 35% of atmospheric carbon emissions. “Some 93% of Earth’s carbon dioxide is stored and cycled through the oceans” (Nellemann et al. 2009). About 1.5 billion tons of carbon are captured and sequestered annually by high seas ecosystems, an amount that if calculated in monetary terms of social benefit would equate to around USD $150 billion each year. Terrestrial forests and coastal marine ecosystems such as saltwater marshes, mangroves, kelp forests, and seagrass beds are all recognized for their ability to store and sequester atmospheric carbon, but high seas ecosystems are just now starting to be credited for their “blue carbon” capabilities.

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Migratory whales transport nutrients vertically and horizontally within the water column. Photo by Alex Rose

Until recently, phytoplankton and zooplankton have been the primary focus of scientific studies detailing the oceanic carbon cycle, but the role of marine vertebrates is now being explored to determine their importance in powering the biological carbon pump. It seems as though the impact of large marine vertebrates on carbon uptake has been grossly underestimated and that they may be as critical in the carbon cycle as are the microscopic creatures about which we understand so much more. A recent report released by the non-profit Blue Climate Solutions in collaboration with the Norwegian environmental foundation GRID-Arendal refers to this form of blue carbon as “Fish Carbon” even though fish, mammals, and even turtles are included in the group.

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When shark populations are depleted, too many sea turtles can have a negative impact on seagrass communities. Photo by Alex Rose

“In healthy marine ecosystems, marine vertebrates facilitate uptake of atmospheric carbon into the ocean and transport carbon from the ocean surface to deep waters and sediment, thus providing a vital link in the process of long term carbon sequestration.” As outlined by this report, the sequestration of atmospheric carbon by marine vertebrates can be broken down into eight main biological mechanisms.

  1. Trophic Cascade Carbon: Large marine vertebrates maintain the balance of food chains ensuring that the carbon sequestration capabilities of the plants at the base of the trophic cascade are maximized. For example, sea otters regulate the population of kelp grazers such as sea urchins so that the kelp can grow in ideal conditions.
  2. Biomixing Carbon: One-third of the ocean mixing that occurs can be attributed to the movements of marine vertebrates. The mixing of nutrient rich water throughout the water column boots primary production and consequently enhances uptake of atmospheric carbon.
  3. Bony Fish Carbonate: Bony marine fish produce calcium carbonate as a waste product from a combination of metabolic CO2 and calcium. When excreted, it serves as a pH buffer that helps seawater resist acidification and it is estimated that bony fish carbonate accounts for 3- to 15% of total oceanic carbonate production.
  4. Whale Pump: Migratory whales transport nutrients vertically and horizontally within the water column through sloughed skin, placentas, excrement, and carcasses. These nutrients boost phytoplankton growth and consequently enhance atmospheric carbon uptake through photosynthesis.
  5. Twilight Zone Carbon: Daily vertical migrations of deep water fish to feed in surface waters at night and then return to depths below 200 meters for safety during the day transports carbon in the form of feces. This mechanism provides a “direct two-step route from the ocean surface to the deep sea and sediment, where carbon can be stored for millennia or longer” (Lutz et al. 2007).
  6. Biomass Carbon: Large marine vertebrates such as whales and tuna store carbon within their bodies, and because they can live for such a long time carbon is sequestered in their tissues on a timescale comparable to that of a terrestrial forest. Overfishing of these species may reduce the ocean’s overall potential for carbon storage via this method.
  7. Dead-Fall Carbon: Large carcasses that remain in the ocean sink to the bottom and slowly release their sequestered carbon into deep sea ecosystems “where it can be stored on timescales of thousands to millions of years” (Lutz et al. 2007).
  8. Marine Vertebrate Mediated Carbon: Fecal matter of many marine vertebrate species contains large amounts of carbon and sinks quickly without dissolving much, meaning that it effectively transports carbon to depths where it will be sequestered for long periods of time.

It is apparent that this so called “Fish Carbon” plays an important role in sequestering atmospheric carbon and transporting nutrients throughout the ocean, essentially fertilizing the water so that phytoplankton can flourish and absorb more carbon through photosynthesis.

whale shark
Large marine vertebrates like this whale shark store carbon within their bodies, and because they can live for such a long time carbon is sequestered in their tissues on a timescale comparable to that of a terrestrial forest. Photo by Alex Rose

It also helps to buffer seawater so it can more efficiently resist ocean acidification. Perhaps by recognizing and properly valuing the roles played by marine vertebrates in the oceanic carbon cycle, marine conservation policies and ecosystem management strategies will more accurately address the importance of “Fish Carbon” in mitigating climate change. With this updated understanding of blue carbon, maybe we will start viewing schools of fish as mobile vertebrate forests of the ocean, giving us more reason than ever to protect what is left and work to replenish our world’s waters in a effort to implement solutions to our current climate challenge.

Article Categories:
Fish · Lifestyle · Saltwater Fish

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