A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration (2022) / Chapter Skim
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5 Seaweed Cultivation
5 Seaweed Cultivation
Pages 127-146
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From page 127... ...
Large-scale farming of seaweed would incorporate dissolved CO2 from the upper ocean into tissue that then can be sequestered at depth either by pumping biomass to depth or by its sinking through the water column. As many seaweeds grow, they release large amounts of dissolved organic carbon (DOC)
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From page 128... ...
concluded that natural macroalgal ecosystems could make substantive contributions to global ocean carbon sequestration primarily through the export of plant biomass to depth and the seafloor and the production of recalcitrant DOC. Recalcitrant DOC is that fraction of the DOC pool that is resistant to rapid microbial degradation (e.g., Hansell, 2013)
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From page 129... ...
One useful conceptualization of macrophyte cultivation as an ocean CDR strategy would be considering large-scale farming of seaweeds to assimilate CO2 from the surface ocean and then purposefully convey this fixed carbon to deep oceanic reservoirs that will remain out of contact with the atmosphere over some relevant planning time horizon, say 100 years. The air–sea CO2 equilibrium timescales relative to surface water residence times also need to be considered, as is the case with all marine CDR approaches considered here (see Section 1.3)
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From page 130... ...
In the near-surface ocean, these effects include reducing ambient nutrient levels and available light. Subsequently, that will likely reduce phytoplankton primary production rates, decrease carbon export from the surface ocean, and may affect trophic exchanges of energy that support fisheries and marine mammal populations.
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From page 131... ...
The Pacific and Indian basins generally have longer sequestration times than the Atlantic Ocean and Southern Ocean. Assessments made of the injected CO2 retained over a 100-year time horizon illustrate that most of the injected carbon will still be in the ocean at injection depths greater than 1,000 meters, with several geographic exceptions such as the western North Atlantic (see further descriptions of this work below)
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From page 132... ...
, we consider the two primary pathways linking farmed macrophytes and carbon sequestration -- the purposeful injection of particulate macrophyte carbon to depth and the release and eventual sequestration of recalcitrant DOC from the growing macrophyte farms, or Seqgoal = 0.1 Gt CO2/yr = 0.027 Pg C/yr = SeqBio + SeqDOC (5.1) The C budget for natural macrophyte ecosystems created by Krause-Jensen and Duarte (2016)
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From page 133... ...
There are many requirements to farm vast amounts of macrophyte carbon biomass. Optimal growth of macrophyte biomass requires adequate nutrient concentrations and light levels (e.g., Jackson, 1977, 1987; Gerard, 1982; Zimmerman and Kremer, 1986)
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From page 134... ...
Second, deeper discharge locations will sequester CO2 longer than shallower ones, and median sequestration times are typically decades to centuries. Third, large differences in sequestration times occur both within and between the major ocean basins, with the Pacific and Indian basins generally having longer sequestration times than the Atlantic Ocean and Southern Ocean.
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From page 135... ...
The decomposition of the added biomass will lower oxygen levels and increase acidity and nutrient levels, leading to increased deoxygenation, acidification, and eutrophication. Further, the nature of the purposeful inputs will likely be highly heterogeneous in space and likely intermittent in time because it seems difficult to ensure that the inputs of organic matter will be or could be dispersed uniformly at depth, especially given that the infrastructure required to cultivate macrophyte biomass carbon to CDR-relevant scales will be localized to a few port cities.
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From page 136... ...
The farm will divert ambient nutrients that drive the upper ocean ecosystem into cropped biomass, likely reducing rates of phytoplankton NPP and thereby in turn reducing the fluxes of C export from the surface ocean into the ocean interior and decreasing the flow of energy into higher trophic levels that support fisheries and other valued marine resources. These effects may increase the need for more ocean CDR to offset these losses.
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From page 137... ...
change in global inventories due to purposeful injections of cultivated seaweed biomass will require severe constraints on the required accuracy and precision of oxygen (O2) measurements and the timescales required to see these changes.
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From page 138... ...
. Other co-benefits with potential greenhouse gas reductions include the additions of macrophyte biomass to animal feeds that could reduce methane emissions (e.g., Maia et al., 2016)
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From page 139... ...
5.5 SUMMARY OF CDR POTENTIAL The criteria for assessing the potential for seaweed cultivation as a feasible approach to ocean CDR, described in Sections 5.2–5.4, is summarized in Table 5.1. 5.6 RESEARCH AGENDA A research agenda for assessing whether seaweed cultivation and sequestration is CDR worthy will require an assessment of the components compiled in Figure 5.2.
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From page 140... ...
Efficacy Medium Confidence What is the confidence level that this approach will The growth and sequestration of seaweed crops should remove atmospheric CO2 and lead to net increase in lead to net CDR. Uncertainties about how much ocean carbon storage (low, medium, high)
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From page 141... ...
Validate and monitor the CDR performance of the demonstration-scale seaweed farming and sequestration systems on local scales; 6. Evaluate the environmental impacts of large-scale seaweed farming and sequestration systems both in the upper ocean where the farming occurs and at depth where the seaweed is transported for sequestration; 7.
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From page 142... ...
. One would also need to evaluate the environmental impacts of large-scale seaweed farming and sequestration both in the upper ocean where the farming occurs and at depth in the water column or the seafloor where the seaweed is conveyed for sequestration.
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From page 143... ...
However, much has been learned already in the MARINER program, and there should be recognition of the many marine engineering accomplishments made by the global oil and gas industries to date. The costs should be less than $100/metric ton CO2; assuming that the MARINER's cost target for growing macrophyte biomass is met ($75/metric ton CO2)
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From page 144... ...
Farms could also reduce the effects of acidification in the upper ocean. 5.2 Engineering studies focused on How do we convey large amounts Minimal for engineering testing Minimal for engineering 2 10 conveying harvested biomass of seaweed biomass to depth or testing to a durable oceanic reservoir seafloor with minimal losses?
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From page 145... ...
5.8 Document "best practices" How should seaweed cultivation be N/A N/A 1 2 and perform spatial planning conducted and where? exercises NOTE: Bold type identifies priorities for taking the next step to advance understanding of seaweed cultivation as an ocean CDR approach.
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