Abstract: Seagrass meadows composed of larger species are assumed to store larger sediment organic carbon (Corg) stocks, storing more Corg in their tissues and larger leaves promoting greater burial of seagrass and non-seagrass Corg. However, the influence of species composition on sediment Corg stocks remains poorly understood mainly from challenges in isolating it from confounding factors. We assessed Corg stocks in seagrass biomass and sediment of four species compositions in a tropical Caribbean meadow. We hypothesized that larger species would lead to higher sediment Corg stocks, within a limited geomorphic setting and time frame. Seagrass biomass and surficial and sediment profiles were collected to measure seagrass morphometrics, ?13C and ?15N, dry bulk density, Corg and inorganic carbon (Cinorg) stocks, and grain size. Seagrass biomass Corg stocks ranged from 0.04 to 3.7 Mg?ha?1, with higher biomass Corg stocks in compositions with larger species. Surficial sediment Corg and Cinorg stocks (to 1.5 cm) averaged 2.6?±?0.6 Mg Corg ha?1 and 68.8?±?14.6 Mg Cinorg ha?1, respectively, and did not vary among species compositions. Isotopic analyses revealed a ~?50% contribution of seagrasses to surficial sediment Corg in compositions with larger species, compared to a contribution of ~?35% for those of smaller species. This study provides novel blue carbon data from an understudied region and contributes to understanding the role of seagrass species composition on sediment carbon storage.

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