Emission of volatile halocarbons from the farming of commercially important tropical seaweeds

The emission rates of the atmospherically important halogenated trace gases CHBr 3 , CHBr 2 Cl and CHBrCl 2 were measured from two commercially important tropical red seaweed species Gracilaria manilaensis Yamamoto and Trono, and Kappaphycus alvarezii (Doty) L.M.Liao in a range of environments to investigate the potential impact of seaweed aquaculture on atmospheric halocarbon concentrations. Deep convective transport in the tropics can carry such volatile halocarbons to the upper troposphere and lower stratosphere. In this first study of its kind, emissions from these two seaweed species were measured after growing on an offshore platform, in onshore tanks, and in cages at a river mouth in eastern Peninsular Malaysia. Of the three cultivation systems, the amount of CHBr 3 released by G. manilaensis was highest from the cage culture at the river mouth (267 ± 13 ppbv), followed by the offshore platform (78 ± 47 ppbv) and onshore tank (69 ± 69 ppbv). Daylight emissions by G. manilaensis and K . alvarezii from both offshore and onshore cultivations were greater than in the dark. The global production of seaweed is projected to increase further due to the rising demand for seaweed products, the emphasis on clean energy, and climate action. Harvesting of commercial seaweeds could potentially significantly increase local atmospheric abundances of CHBr 3 ; in our study 500 g of these typical seaweeds increased CHBr 3 in a 40 L flux chamber by 41–267 ppbv in 30 min; far above usual background amounts. As our study showed that higher temperatures increase seaweed halocarbon emissions, given impending climate change, and the possible extension of seaweed cultivation to subtropical seas as seawater temperatures rise, this might further increase halocarbon emissions. We estimate that the harvesting of K. alvarezii in the year 2020 in Malaysia could have released 63–322 mol Br for each hour of harvesting activity.