Seagrass community-level controls over organic carbon storage are constrained by geophysical attributes within meadows of Zanzibar, Tanzania

Abstract. The aim of this work was to explore the feasibility of using seagrass functional traits to predict differences in sediment carbon storage. At 19 sites within highly diverse seagrass meadows of Zanzibar, Tanzania, species cover was estimated along with three community traits hypothesized to influence sediment carbon storage (amount of above and belowground biomass, seagrass tissue nitrogen content, and shoot density). We identified five distinct seagrass communities that had notable variations in key plant traits but these differences did not translate into differences is sediment organic carbon (OC) storage. Across all communities, sediment OC was very low (ranging from 0.15 % to 0.75 %) and there were no differences in OC storage among communities, which was considerably lower (33.97.7 Mg C ha−1) than the global average (194.220.2 Mg C ha−1) reported for other seagrass ecosystems. In spite of high seagrass diversity and clear zonation among plant communities, sediments in all communities were shallow (ranging from 19 to 78 cm) and composed of medium-coarse grained carbonate sand on top of carbonate rock. We propose that geophysical conditions of the sediment were not conducive to OC stabilization, and outweighed any variation in the quantity or quality of plant litter inputs, ultimately leading to low OC storage within all seagrass communities. This highlights the complexity of OC cycling in seagrass ecosystems and cautions against the use of plant traits as a proxy for OC storage across all seagrass ecosystems.