CO₂ Removal by Macroalgae

CO₂ Removal by Macroalgae can be quantified as:

$CO_2eMacroalgae=Macroalgae_{Accumulated}+Biomass_{Accumulated}-Shed-BAF-BFR-PMF-Shal$

Where:

$Macroalgae_{Accumulated}$= Mass of carbon absorbed by deployed macroalgae attached to carbon buoys at the point of sinking.

• Ocean observation platforms capture photos of deployed carbon buoys to provide in-situ visual validation of macroalgae biomass accumulation and growth rates up to the point the buoys begin to sink. Image analysis (through machine vision) is used to extract growth characteristics (blade length, width, area) to correlate to biomass and carbon content. This imaging informs macroalgae loss or shedding that occurs during the growth process and float period.

• Models are trained using machine vision to map macroalgae mass using the camera systems deployed on the open ocean observation platforms. Preliminary data has shown high correlations in direct measurements of macroalgae biomass and carbon elemental analysis.

• Yields will depend on macroalgae species and genotype, environmental conditions such as macro and micronutrient availability, and episodic mortality events such as disease and storms.

• Future quantification will include refined analysis of imaging through water, which will help account for distortion from refraction and turbidity. Currently, image quality is more altered by physical constraints on the system (power, lighting, electronics, etc.).

$Biomass_{Accumulated}$= Mass of carbon absorbed by non-seeded macroalgae that accumulates on carbon buoys via natural recruitment at the point of sinking.

• Biomass accumulation in addition to the specific species of macroalgae seeded on the buoys. Carbon buoys and other materials placed in the ocean are subject to natural photosynthetic ‘biofouling’, i.e. the growth of microorganisms, algae, and other species on submerged surfaces. While this process tends to be undesired for maritime vessels or oceanographic sensors, much like seeded macroalgae recruitment, they represent natural recruitment and growth of carbon-fixing organisms that if sunk are additional to natural biological pump activity.

• Quantification of additional biomass accumulation is contingent on the ability to recognize macroalgae species and effectively image and model growth rates in the same fashion as macroalgae seeded on deployed carbon buoys described above.

• Similarly, this additional biomass is subject to further benthic and pelagic ecological investigations in relation to the species identified.

$Shed$= Portion of macroalgae carbon shed during the growth and sinking process.

• Open ocean observation platforms are collecting data on macroalgae biomass accumulation up to the point of sinking, and buoy design promotes an accelerated sinking rate compared to phytoplankton and particulate organic carbon, which will minimize potential shedding rates. Any shed material that has already started sinking is expected to behave similarly to the macroalgae material attached to the buoy.

• It is well established that many fish species have a natural affinity to aggregate near floating structures, and as such may view new macroalgae biomass as a potential food source as it moves through the water column. This impact is expected to be immaterial for initial deployments due to intervention design, but further research is being conducted into the potential impact of pelagic organisms on macroalgae consumption and the fraction of that consumed biomass that is transported to the seabed as fecal pellets (i.e., transported to slow carbon cycle) versus being remineralized to CO₂ in the surface water and released back to the fast carbon cycle.

$BAF$ = Biogeochemical Additionality Factor, or the secondary effects on phytoplankton carbon uptake associated with competition with cultivated macroalgae for nutrients or light in the open ocean.

• The addition of macroalgae into the open ocean where nutrients are already limiting for photosynthesis has the potential to decrease phytoplankton net primary productivity indirectly through nutrient consumption by macroalgae. Since macroalgae have much higher C:N ratios than phytoplankton, they can form one unit of carbon biomass using a smaller nutrient load than phytoplankton. Although this makes macroalgae more efficient with respect to nutrient utilization, and prevents macroalgae from displacing phytoplankton on a 1:1 basis, macroalgae photosynthesis may still displace phytoplankton photosynthesis to some extent in waters where nutrients are limiting.

• For initial deployments, given that algal photosynthesis in the subpolar North Atlantic is primarily iron-limited [Moore et al. 2013], it is expected that phytoplankton-macroalgae competition will be driven by the micronutrient iron.

• Net primary productivity tradeoffs are expected to be immaterial for initial deployments. Additional research will be conducted to determine the expected nutrient tradeoff and the associated reduction in total carbon removed that must be accounted for in larger scale interventions. From a carbon perspective, only the expected carbon that would have been naturally removed via the biological pump should be discounted; however, the broader nutrient and ecosystem impacts must also be considered.

$BFR$= Biogeochemical Feedback Responses, or the range of potential environmental responses that develop in response to the introduction of macroalgae into the open ocean environment. As a specific example, it is possible that the growth of calcifying epibionts (i.e. an organism that lives on the surface of another living organism) may be observed as macroalgae grows, though their presence is not expected.

• This variable reflects a range of potential biogeochemical responses that may be observed when macroalgae grows in a new environment, and which may impact total carbon removed.

• Specific to calcifying organisms, it is unclear whether calcifying organisms will actually recruit onto the macroalgae over a short multi-month timeframe in the open ocean, given their preferred benthic environment on the coastal shelf. In the event calcifying organisms are observed, image analysis through machine vision will be used to identify the species and appropriately discount the modeled carbon content at the point of sinking.

$PMF$= Physical Mixing Factor. The effect of ocean mixing processes on the efficiency of moving CO₂ from the fast-to-slow cycles – i.e., surface ocean subduction to deeper waters before carbon removed from seawater by macroalgae growth is completely re-equilibrated.

• The removal of carbon from the fast to slow cycle in the macroalgae pathway occurs during organic carbon production (photosynthesis) and sinking. Like OAE, this leaves a deficiency of dissolved CO₂ compared to the baseline, particularly due to the high C:N ratio of some macroalgal species, some of which will instantaneously be replaced by a small amount of bicarbonate. If subduction of surface water due to physical mixing processes occurs prior to complete re-equilibration, this amount should be accounted for with a discount factor.

• It should be noted that the timescales of surface water subduction versus CO₂ re-equilibration, and the methods to quantify this, are an active area of research within the scientific community.

$Shal$ = Portion of macroalgae carbon that ends up beached or sunk in areas too shallow or at risk of upwelling.

• Similar to terrestrial biomass sinking, spatial evolution of the buoy population can be characterized through a combination of computational modeling, in-situ empirical observation, and laboratory testing, alongside a Monte Carlo analysis to determine the terminal carbon buoy distribution on the seafloor.

• Given the deployment areas and float times, this impact is expected to be immaterial for initial deployments.