Maine is one of the first states to consider the carbon storage potential of seaweed in its carbon budget, a biennial accounting of the balance between locally produced greenhouse gas emissions and the amount of carbon being safely stored in the environment.

The carbon budget already considers the storage capacity of Maine forests, which cover 89% of the state. Forests absorb atmospheric carbon through photosynthesis and store 412 million metric tons of carbon in the trees themselves and a whopping 1.6 billion metric tons of carbon in their soils.

The state carbon budget, which was first introduced in Maine’s last greenhouse gas report, also includes traditional blue carbon sources like salt marshes and eelgrass beds, which have high carbon storage rates but not as much impact as forests because they aren’t as plentiful.

The impact of blue carbon could grow, however, if the board that oversees rule changes at the Maine Department of Environmental Protection grants the agency’s request to add seaweed to the method it uses to calculate its annual greenhouse gas emissions.

Some of the members of the Board of Environmental Protection who heard the proposal Thursday were clearly surprised to learn that seaweed had any carbon storage potential, and were more familiar with the traditional role of green carbon sequestration.

“I’ve witnessed kelp get battered by storms and going into little bits,” said board member Barbara Vickery of Richmond, a conservation biologist who spent 33 years with the Nature Conservancy. “The idea of it being buried in the sense of sequestering that we think of soils, does that happen?”

According to Stacy Knapp, who produces the DEP’s biennial emissions inventory, the answer is yes, but it’s more complicated than forest carbon storage, and much of the science on how this will be calculated is evolving.

“Some of this organic material will be eaten by zooplankton or other animals and then excreted out as snow,” Knapp said, “that can be eaten by another organism or sometimes it falls all the way to the ocean floor and it is in fact buried.”

Lush seaweed beds, or macroalgae, can be found in the cold, nutrient-rich waters along Maine’s rocky shore. These underwater meadows use sunlight to absorb carbon dioxide and make sugars through photosynthesis, and provide food and habitat for marine animals.

Seaweed stores carbon in its blades. Over time, the sea absorbs atmospheric carbon dioxide to replace what the seaweed took in. But without roots, seaweed can’t sink carbon into the seabed like a forest can with soil. It releases carbon when it’s buffeted by waves, is eaten by marine life or is harvested.

That explains why scientists didn’t initially consider seaweed to be a viable blue carbon candidate. In fact, some scientists still believe that wild mature seaweed beds can, under normal conditions, release more carbon than they sequester.

But researchers began to reassess seaweed’s storage potential when they found traces of macroalgae carbon in deep-sea sediments. They now believe seaweed fragments drift and sink to such depths that they are sequestered, far beyond the reach of any current.

Nichole Price, a senior research scientist at Bigelow Laboratory for Ocean Sciences, is leading the U.S. component of an Oceans 2050 study that has found seaweed carbon deposition under two dozen active agricultural seaweed farms spanning five continents, from Maine to China.

“There is still so much we don’t know,” Price said. “Seaweed has no stock assessment. We don’t have a list of species by state or country. But the data is clear: it can store carbon, both in the wild and when it’s farmed. Now we need more data.”

To estimate its seaweed carbon storage potential, Maine must first map its wild beds and determine carbon sequestration rates for each species. After deposition rates are set, storage values can then be assigned to Maine’s state-permitted seaweed farms based on state landings data.

That won’t be possible until the state’s next greenhouse gas emissions report is published, which is expected to come out in January, but writing seaweed into the rule that lays out how greenhouse gas emissions are calculated will drive the research needed to quantify its storage potential.

In 2019, Maine enacted legislation that set a goal of reducing gross greenhouse gas emissions to 45% below 1990 levels by 2030 and to 80% below 1990 levels by 2050. Later that year, Gov. Janet Mills signed an executive order to achieve carbon neutrality in Maine by 2045.

The last carbon budget indicates Maine is approximately 75% of the way to being carbon neutral, which means 75% of the greenhouse gases emitted in 2016 are balanced by environmental sequestration in forests, soils and blue carbon sinks such as salt marshes.

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