First description of seagrass distribution and abundance in São Tomé and Príncipe

•Seagrass meadows in São Tomé and Príncipe, eastern Atlantic Ocean, are described here for the first time.•Halodule wrightii was found at two locations in the island of São Tomé.•A highly morphologically different H. wrightii was found at one location in Príncipe. Further research is needed to clarify if this is a different species.•Results suggest an important ecological role of H. wrightii for carbon fixation.

Seagrass meadows in São Tomé and Príncipe, eastern Atlantic Ocean, are described here for the first time. Specifically, we quantified the biomass and density of seagrasses, characterized the plant morphology and measure their nutrient content as a proxy of the nutrient environmental conditions where the meadows develop. The seagrass Halodule wrightii was found in two locations of the northeastern coast of the island of São Tomé: 1) developing throughout an estimated area of 1500 ha surrounding Cabras islet, at a depth range of 4-10 m, on sandy bottom; and 2) at Santana bay with an area of 1500 m2 at 5-10 m depth, on sandy bottom. A highly morphologically different population of Halodule wrightii was found on the northeastern coast of the island of Príncipe, off Abade beach, covering an area of 135 m2 at 4 m depth. Further research is needed to assess if this is a different species. Shoot biomass and density was 10 and 4-fold higher in São Tomé than in Príncipe, respectively. CN ratios of above and belowground tissues of plants collected in São Tomé were also significantly higher than in Príncipe. The carbon content of Halodule leaves from São Tomé and Príncipe (41%) was much higher than that reported for other Halodule species, suggesting that meadows may have an important ecological role for carbon fixation. The presence of H. wrightii in São Tomé and Principe raises ecological and evolutionary questions that warrant further research.