Ecol. Monogr. (2019).

Coastal marine environments such as mangroves and marshes accumulate carbon through plant growth, which can then be buried in sediments and sequestered over longer timescales. Macroalgae fix more CO2 than other large marine plants, yet grow predominately in rocky areas where burial is limited. Much of this carbon is therefore exported further offshore, and the amount that is ultimately sequestered in deeper sediments is not well known.

Credit: AHS Photography — Alex Schregardus/Moment/

Ana Queirós, from Plymouth Marine Laboratory, UK, and colleagues investigated the role of macroalgal biomass in carbon fluxes to deep coastal sediments. Using genomic and isotopic methods combined with process measurements, they found that the net flux to deep sediments was nearly 60 gC m−2 yr−1, approximately 25% of the carbon sequestration in mangroves. Macroalgae represented about 15% of this flux and played an important role in supporting the benthic food web.

These findings indicate that both coastal sediments and macroalgae may play a greater role in marine carbon storage than previously considered. The results provide important constraints on carbon flow between the land and ocean.