Reply to the comment by Filbee-Dexter et al. (2023) “Seaweed forests are carbon sinks that may help mitigate CO2 emissions”

Abstract

Abstract Filbee-Dexter et al. provided commentary on Gallagher et al.’s assertion regarding the limitations of seaweed ecosystems in mitigating CO2 emissions. However, Filbee-Dexter et al. appear to have different understandings of several key aspects, and claims of heterotrophic bias contradict their cited literature upon which our analysis was based. Filbee-Dexter et al.’s reliance on net primary production fails to consider the consumption and remineralization of said production. Their endorsements of high levels of seaweed ecosystem autotrophy taken from the literature were either conceptually, temporally, or community assemblage-inappropriate. The existing literature does not substantiate their claim of methodological bias between different types of net ecosystem production (NEP) measurements. Additionally, all of these direct measurements account for any photo-re-assimilation of respiratory subsidies. Contrary to Filbee-Dexter et al.’s claim, Gallagher et al. consider the export of sequestered seaweed. The study revealed that respiratory subsidies offset the exported sequestration rates from an average of +173 million tonnes C yr−1 as a carbon sink to a carbon source of around −54 million tonnes C yr−1. Nonetheless, there are also points of consensus. It will be necessary to weight NEP for the types of seaweed ecosystems, and account for differences with a seaweeds’ particular degraded or alternative state as more data becomes available. Finally, more research is required to better understand the fate of export, and the impact of net calcification on the atmospheric exchange of CO2.