Macroalgae detritus decomposition and cross-shelf carbon export from shallow and deep reefs

Macroalgal forests have been suggested to export substantial amounts of carbon to deep ocean sinks and could account for 27–34% of annual blue carbon sequestered in Australia. However, a major knowledge gap concerns how carbon in the detrital tissue of the dominant seaweed species is remineralized as it is exported offshore. We quantified decomposition and carbon content in detrital tissue of dominant canopy-forming seaweeds Ecklonia radiata and Scytothalia dorycarpa at three depths (10, 20, and 50 m) in a 50 d in situ litterbag experiment in Western Australia. We then combined these rates with a particle tracking model to estimate the potential export of macroalgae detritus from our experiment sites into deeper waters. Decomposition of particulate organic carbon was fast relative to other cooler regions globally, and there were no significant differences between species and most depths. One-half of the detritus was remineralized within 12 (± 2) days for E. radiata and 8 (± 2) days for S. dorycarpa, with ~ 8% remaining for both species after 50 d in situ. Based on simulated transport times and decomposition, 10% and 11% of the E. radiata and S. dorycarpa detritus from shallow reefs (10–20 m) were exported beyond the shelf break (≥ 200 m) whereas 47% and 37% were exported from deep reefs (50 m). These estimates highlight the variable but substantial carbon sequestration potential across the coastal zone.