Carbon Dioxide Removal in the Cabo Verde Islands

Abstract

Pelagic sargassum (S. fluitans and S. natans), originally from the Sargasso Sea, has been blooming every year since 2011 across the tropical Atlantic and in the Cabo Verde region. sargassum is an essential habitat for many crustaceans, fish, and turtle species. It also plays a crucial role in the marine ecosystem due to its capacity to take up carbon from the ocean surface and export it to the deep ocean when it sinks naturally. This natural process, if enhanced, could be applied as an ocean Carbon Dioxide Removal (CDR) method. To understand the potential of Cabo Verde’s sargassum for CO2 uptake and export, different investigations were made, such as particulate organic matter composition analysis of pelagic sargassum collected around Cabo Verde, as well as sargassum seasonal abundance obtained from a satellite data. In addition, to test the hypothesis that iron coming from the Saharan dust plumes might enhance sargassum’s growth, a growth experiment was performed with benthic sargassum available close to the coast of Mindelo. The mean particulate organic carbon, nitrogen, and phosphorus in pelagic sargassum were found to be 338 ± 40 μg.mg-1, 14 ± 2 μg.mg-1, and 2 ± 0 μg.mg-1 of dry weight, respectively. The average Inorganic Carbon: Organic Carbon ratio, which provides information on the degree of calcifiers attached to the sargassum, was 0.10 ± 0.09. The molar C:N, C:P, and N:P are respectively 24 ± 4, 149 ± 29, and 6 ± 1. The peak biomass period of sargassum around Cabo Verde in the last four years is mainly in June and July (note that some benthic species were found floating together with pelagic species in Cabo verde). The relative growth rate of benthic sargassum (Sargassum cymosom) was 2.42 g/day in control and 3.01 g/day for the addition of dust, showing the positive impact of the Saharan dust on the growth rate of the sargassum. Furthermore, it was found that during the 14-day experiment, the CO2 uptake by the sargassum averaged 3.58 kg of CO2 for every 0.9 kg of wet biomass. Estimations based on the results obtained show that in the Cabo Verde region over the last four years, sargassum has taken up between 2.56 Mt and 4.28 Mt of CO2 annually.