Global and regional variation in seaweed export to potential carbon sinks

Jorge Assis³, Trine Bekkby⁴, Dr Karen Filbee-Dexter, Carlos M. Duarte⁶, Albert Pessarrodona², Michael T. Burrows⁵

¹Institute of Marine Research; Norway, ²University of Western Australia, Australia, ³CCMAR, CIMAR, Universidade do Algarve, Portugal, ⁴Norwegian Institute for Water Research, Norway, ⁵Scottish Association for Marine Science, United Kingdom, ⁶King Abdullah University of Science and Technology, Saudi Arabia, ⁷Sorbonne Université, , France, ⁸Aarhus University, Denmark, ⁹Fisheries and Oceans Canada, Canada, ¹⁰Department of Earth Sciences, Utrecht University, The Netherlands, ¹¹Newcastle University, United Kingdom, ¹²Roskilde University, Denmark, ¹³University of Porto, Portugal, ¹⁴Plymouth Marine Laboratory, United Kingdom, ¹⁵Marine Biological Association of the United Kingdom, United Kingdom

Correspondence: Karen Filbee-Dexter, kfilbeedexter@gmail.com

The world’s seaweed forests are the largest vegetated ecosystem in the ocean with productivity rates comparable to phytoplankton. Yet, the fate of the large flux of carbon exported from seaweed habitats is a key unknown in the oceanic carbon budget and prevents accurate estimates of seaweed blue carbon potential. Here we create the first spatially explicit global estimate of seaweed carbon export to the open ocean by combining global models of seaweed forest distribution and net primary production (NPP), with seaweed decomposition rates and oceanographic models of coast-ocean exchange. Export potential of seaweeds across the shelf break (>200 m depth) was on average 10.6% of annual NPP or 27.9 g C m^-2 y^-1, which equates to 47.5 Tg C per year globally. National estimates show that some regions have high fluxes of seaweed carbon to the open ocean, likely due to a combination of narrow continental shelves, submarine canyons and unique oceanographic features, as well as large total seaweed NPP. Significant detrital seaweed carbon crosses the continental shelf near the meso-bathypelagic boundary, suggesting this carbon can reach deep ocean zones of long-term storage and slow rates of surface exchange. These findings demonstrate high spatial variability in seaweed fluxes to deep ocean areas adjacent to the coastal zone, which has consequences for energy inputs to mesopelagic ecosystems and estimates of overall carbon sink potential of seaweed forests.