Sediment organic carbon stocks were similar among four species compositions in a tropical seagrass meadow

Seagrass meadows composed of larger species are assumed to store larger sediment organic carbon (C-ore) stocks, storing more C org in their tissues and larger leaves promoting greater burial of seagrass and non-seagrass C-ore. However, the influence of species composition on sediment C-ore stocks remains poorly understood mainly from challenges in isolating it from confounding factors. We assessed C org 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 C-ore stocks, within a limited geomorphic setting and time frame. Seagrass biomass and surficial and sediment profiles were collected to measure seagrass morphometrics, delta C-13 and delta N-15, dry bulk density, C-ore and inorganic carbon (C-inorg) stocks, and grain size. Seagrass biomass C org stocks ranged from 0.04 to 3.7 Mg ha(-1), with higher biomass C-ore stocks in compositions with larger species. Surficial sediment C-ore and C-inorg stocks (to 1.5 cm) averaged 2.6 +/- 0.6 Mg C(ore )ha(-1) and 68.8 +/- 14.6 Mg C-inorg ha(-1), respectively, and did not vary among species compositions. Isotopic analyses revealed a similar to 50% contribution of seagrasses to surficial sediment C-ore in compositions with larger species, compared to a contribution of similar to 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.