Beach-cast seagrass wrack contributes substantially to global greenhouse gas emissions

Seagrass ecosystems have received a great deal of attention recently for their ability to capture and store carbon, thereby helping to mitigate climate change. However, their carbon-sink capacity could be offset somewhat if exported plant material - which accounts for ∼90% of total leaf production - undergoes microbial breakdown and is emitted into the atmosphere as a greenhouse gas. Here we measured emissions (CO₂ and CH₄) from the breakdown of exported seagrass plant material, focusing on beach-cast 'wrack'. We tested two seagrass species; Zostera nigricaulis and Amphibolis antarctica, which have contrasting morphologies and chemistries. We found that both species of wrack were substantial sources of CO₂, but not CH₄, during the decomposition process. Biomass loss and the coinciding CO₂ emissions occurred over the 30-day experiment, and the pattern of CO₂ emissions over this time followed a double exponential model (R² > 0.92). The initial flux rate was relatively high, most likely due to rapid leaching of labile compounds, then decreased substantially within the 2-9 days, and stabilizing at < 3 μmol g^-1 d^-1 during the remaining decomposition period. Additionally, seagrass wrack cast high up on beaches that remained dry had 72% lower emissions than wrack that was subjected to repeated wetting in the intertidal zone. This implies that relocation of seagrass wrack by coastal resource managers (e.g. from water's edge to drier dune areas) could help to reduce atmospheric CO₂ emissions. Scaling up, we estimate the annual CO₂-C flux from seagrass wrack globally is between 1.31 and 19.04 Tg C yr^-1, which is equivalent to annual emissions of 0.63-9.19 million Chinese citizens. With climate change and increasing coastal development expected to accelerate the rate of wrack accumulation on beaches, this study provides timely information for developing coastal carbon budgets.