A path towards appropriate degradation experiments for assessing carbon sequestration potential of macroalgae

Abstract

The biodiversity and climate crises have increased the urgency to understand turnover rates and drivers of marine carbon sequestration. Efforts have concentrated on measuring degradation of macroalgae to quantify how much of their carbon might be sequestered. We systematically reviewed literature on macroalgal degradation, compiling 364 measurements of exponential decay constant k. To date, most degradation experiments (1) are carried out in aquaria or shallow environments, (2) span short timescales, (3) use dried or frozen material or (4) rarely incorporate environmental gradients driving degradation. Decay constants were higher in intertidal versus subtidal habitats, for pre‐treated algal tissue compared to fresh, and when refractory material was present rather than absent. These patterns have implications for estimates of export efficiency of macroalgal carbon to the deep ocean. Synthesis. We recommend that future studies of macroalgal degradation (1) adopt standardized approaches, such as using fresh material and litter bag mesh size of 10 mm, to make studies more comparative, (2) measure degradation with sufficient frequency to robustly estimate decay constants and/or refractory material and (3) incorporate environmental gradients, including depth, light and oxygen availability, to improve estimates of macroalgal carbon remineralisation rates during transport to deeper water.