New research brings us one step closer to making seaweed an ‘actionable’ blue carbon ecosystem

By Cecilie Wathne (NBFN), Karen Filbee-Dexter (IMR), Thomas Wernberg (IMR), Trine Bekkby (NIVA), Hege Gundersen (NIVA) and Kasper Hancke (NIVA)

| May 31st, 2024 | Highlights

A new study finds that seaweed forests are responsible for 3-4% of the global ocean carbon sink. Norway’s vast kelp forests have an important climate mitigation role to play – particularly in the face of rising ocean temperatures.

The world’s seaweed forests are declining at an alarming rate. Urgent action is needed to halt – and ideally reverse – this trend. One argument for why governments should better protect, manage, and restore these ecosystems is their ability to capture and bury large amounts of carbon. However, while mangroves, tidal marshes and seagrass are included in the IPCC greenhouse gas accounting guidelines, macroalgae are not. As explained in a recent High Level Panel report, they are considered to be an “emerging” rather than “actionable” blue carbon ecosystem. To reach this category, more and better data on their contribution to long-term carbon storage is needed.

 

Thomas Wernberg and Karen Filbee-Dexter in the field. Copyright IMR.

 

NBFN has supported a series of academic articles to help fill this knowledge gap. The latest article, titled “Carbon export from seaweed forests to deep ocean sinks”, has now been published in Nature geoscience. It concludes that:

  • Seaweed forests export on average about 15% of their captured carbon into deep ocean waters each year.
  • This equates to 56 million tonnes of carbon (between 10 to 170 million tonnes) reaching deep ocean sinks each year.
  • An estimated 4 to 44 million tonnes of this carbon could remain locked away in these deep sinks for at least a hundred years.

Seaweed-carbon export below 200 meters depth accounts for 3-4% of the total ocean carbon sink.

National estimates of seaweed carbon export. Annual potential export of seaweed-derived POC (TgC yr−1 (or million tonnes yr−1)) below 200 m depth within EEZ boundaries. Source: Filbee-Dexter et al. (2024).

 

These results build upon previous NBFN-supported articles, which found that:

  • The global macroalgal biome is comparable, in terms of both area and net primary production, to the Amazon forest – with a total area of 6.06-7.22 million km2, and a net primary production of 1.32 billion tons of carbon (i.e. 1.32 Pg C) a year (Duarte et al., 2022).
  • Low-latitude kelp forests could experience a 9-42% reduced potential to transport carbon to long-term ocean sinks due to ocean warming (Filbee-Dexter et al., 2022).

The studies also point to Norway as an important blue carbon “hot spot.” While the country is ranked 26th in terms of the amount of sequestered carbon that reaches the deep ocean each year (0.66 million tonnes carbon), it has the second largest seaweed biomass in Europe (Filbee-Dexter et al. 2024). Norwegian seaweed forests are also less vulnerable to climate change. While the forests in southern Norway are susceptible to heat waves, rising temperatures have expanded growth possibilities in the north. At the same time, the percent of biomass that is long-term stored in the seafloor is less likely to be affected by rising temperatures than in many other regions of the world due to relatively lower decomposition rate at higher latitudes (Filbee-Dexter et al., 2022).

Giving these findings, policymakers and practitioners in Norway and abroad should scale up efforts to protect and restore seaweed forests. At the same time, researchers need to continue working to fill the remaining knowledge gap so that macroalgae can join the ranks of other blue forests as an “actionable” blue carbon ecosystem.

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