Blue carbon flux and decomposition from warming Australian macroalgal reefsEcosystem Services

Taylor Simpkins¹, Thomas Wernberg^1,2, Albert Pessarrodona^1,3,4, Morten Pedersen⁵, Karen Filbee^1,2

¹The University of Western Australia, Australia - ²The Institute of Marine Research, Norway - ³Conservation International, USA - ⁴International Blue Carbon Institute, Singapore - ⁵Roskilde University, Denmark

Macroalgal forests have been suggested to export substantial amounts of carbon to deep ocean sinks and could account for substantial blue carbon sequestration in Australia annually. However, a major knowledge gap concerns how much particulate organic carbon is exported from Australian macroalgal reefs and how this transport could change with different species and environmental conditions. Using extensive drop camera surveys across six degrees of latitude and eight seasons, we quantified the composition and abundance of detrital accumulations offshore from coastal reefs in Western Australia, and examined how fluxes of detritus varied across space and time. We then ran an ex-situ experiment on detritus from three dominant macroalgae species (Ecklonia radiata, Scytothalia dorycarpa, and Sargassum spp.) to quantify the effect of ocean temperature on detrital decomposition. Detrital material was commonly observed beyond the reefs during autumn and winter, and we recorded more accumulations in cool southern sites compared to warm northern sites. Decomposition rates of detritus were significantly slower in cool (15°C) compared to warmer treatments (25°C) for all species, with this difference most pronounced for the temperate E. radiata. Our study suggests higher macroalgal export and possible sequestration occurs at cool compared to warm range edges, and highlights that the blue carbon flux to deep ocean sinks may decrease with ocean warming.