How to make kelp aquaculture a better carbon sink

Low-cost carbon removal technologies play a key role in combating climate change. A team of researchers from the University of Maine, in collaboration with Conscience Bay Research, has developed a kelp aquaculture model for the Gulf of Maine that maximizes carbon sequestration and the cost-effectiveness of this natural carbon sink.

Wild macroalgae are one of the most extensive and productive vegetative biomass stocks, but grow mainly in rocky coastal areas not conducive to localized long-term sequestration, which only occurs when macroalgae become incorporated into ocean sediments at depths greater than 1,000 meters or remineralized at depths below the permanent thermocline in areas of the ocean where carbon is prevented from returning to the atmosphere.

Macroalgal aquaculture, such as kelp farming in the Gulf of Maine, could potentially be used to replicate this carbon sequestration process by growing large quantities of kelp at an offshore location, transporting the kelp to a deep-water “sink” and then deep inside is deposited the ocean where the carbon can be bound.

“Farming kelp for the purpose of large-scale carbon removal is an idea that has recently attracted significant attention from the research community, the private sector and the aquaculture industry, considering the environmental impact of this new technology. Many consider the Gulf of Maine not only a national leader in sustainable fish production, but also a potential carbon sink. More information is urgently needed to guide the development of potential ocean carbon removal,” says Struan Coleman, lead author of the study and research associate at UMaine.

A team of researchers led by UMaine wanted to find out how kelp aquaculture could optimize carbon sequestration and cost-efficiency.

“Kelp cultivation for carbon sequestration is not competitive right now, but there are important ways to reduce the cost of kelp production, which is good for everyone,” says Damian Brady, study co-author and associate professor of oceanography the UMaine. “We have focused on production costs because if costs are not competitive with other approaches to carbon sequestration, such as direct air capture, this may dictate where future research funds go.”

Researchers created a detailed model for kelp aquaculture in the region. By tinkering with 18 different variables — including assumptions for the amount of labor at harvest, where the electricity was sourced, and the size of the PVC spools at the nursery where the seaweed spores, or gametophytes, attach and grow — they were able to estimate the costs of carbon sequestration reduce kelp aquaculture from $17,048 per tonne of carbon dioxide, which equates to $1,257.

The results represent a significant cost reduction for carbon removal in seaweed aquaculture; However, the industry cost target for such technologies is around US$100 per tonne of carbon dioxide equivalent to be commercially viable.

“This means that if macroalgae carbon dioxide removal is to be commercially viable, the industry must innovate about the way farms are operated today,” says Adam St. Gelais, co-author of the study and expert in aquaculture innovation at UMaine Aquaculture Research institutes. “Additionally – and just as important in terms of scaling – this model offers ways to lower production costs and reduce production-related emissions for seaweed farming, regardless of their end use.” Insights from the model can now be applied to help producers increase yield and expand margins through optimization if they want to grow.”

Although it is not possible to optimize all parameters described, scientists have identified six steps that will have the greatest impact on production costs, energy consumption and monitoring in kelp aquaculture. First, farms should be able to relocate to larger, contiguous offshore locations to make more efficient use of ocean space and reduce the risk farmers run when leasing a kelp aquaculture farm. Farmers should also automate the sowing and harvesting process, use selective breeding to increase yields, and evaluate the cost-effectiveness of gametophyte nursery crops versus spores, as they are less expensive and allow for better selective breeding.

“Our results are consistent with many research and development needs that the kelp aquaculture industry has been working on for decades. I think the real value of our approach was to examine how variables such as yield, energy consumption within the nursery and farm design impact the cost structure of seaweed farms to a relatively large extent. If this industry is to continue to expand, whether as a contribution to food supply chains or carbon removal, we need to address these issues,” says Coleman.

Seaweed farms in the Gulf of Maine can also decarbonize by sourcing electricity from renewable sources and using materials with low greenhouse gas emissions and long lifespans. Finally, they need to develop cost-effective and accurate monitoring techniques for oceanic carbon removal to reduce uncertainty in carbon accounting.

“Our team looks forward to continuing this work over the next two years and hopes to accelerate kelp farming along the technology cost curve. We drew heavily on our initial analysis to identify the most powerful levers we could use to address pressing research and development questions. Through a combination of field and model studies, we hope to de-risk promising designs and technologies,” says Coleman.

The study was published in August 2022 frontiers of marine science.