A model of macroalgal decomposition

Macroalgae are currently the focus of blue carbon, biological carbon dioxide (CO2) removal in the ocean. Carbon exported as seaweed detritus is equivalent to ∼7.5% of anthropogenic CO2 emissions. Macroalgal blue carbon hinges on the fate of this detritus, so a reliable model of its decomposition is key. However, macroalgal decomposition has been neither consistently nor reliably modelled. Here I present a model of macroalgal decomposition. Informed by the theory of detrital photosynthesis, it treats detritus as an entity in limbo that can initially either grow or decompose. This is achieved by replacing the rate constant of the conventional exponential decay model with a logistic function of time, mirroring the sigmoidal decay of detrital photosynthesis. The resulting mathematical model has three parameters that describe the initial exponential growth rate, the photosynthetic half-life, and the final exponential decay rate once physiology is factored out. The third parameter is equivalent to the rate constant of the conventional model for dead detritus. Using Bayesian data analysis in R and Stan, I show that the statistical implementation of the mathematical model provides meaningful inference given empirical data from a variety of ecological contexts and routinely outperforms the conventional model. The presented model can describe any macroalgal decomposition trajectory, which enables formal comparison across studies for the first time. I anticipate that this will prove instrumental in resolving the debate around macroalgal CO2 removal.