Production and fate of macroalgal carbon in the ocean: How much do macroalgal organics matter?

Global carbon sequestration by macroalgae is hypothesized to rival rates in other blue carbon ecosystems. However, quantifying macroalgal carbon sequestration is challenging as it is hypothesized to occur outside macroalgal ecosystems, with 73% of sequestration occurring when dissolved organic carbon (DOC) is exported to deep ocean waters. In part due to the complexity of tracking carbon from coastal ecosystems to deep waters, large uncertainties remain about the rate of macroalgal carbon sequestration and its fate in the ocean. We present a synthesis of literature on macroalgal carbon cycling and place it in the context of the marine carbon cycle with a focus on DOC. Synthesis and critiquing of current estimates, including through a case study, indicates that uncertainty around all macroalgal carbon cycle terms remains high. To reduce uncertainty, we recommend developing and comparing estimates made via independent methods including by modeling, remote sensing, and using geochemical tracers.