Alexandra Bijak

Ph.D. Candidate, University of Florida

2022 Conference Travel Grant Type 2

Seagrass Species Assemblage and Identity Affect Greenhouse Gas Emissions

Seagrass meadows are lauded for their potential to capture and store organic carbon, but meadows also generate greenhouse gases, including nitrous oxide (N20) and methane (CH4), through sediment microbial metabolism. Greenhouse gas emissions are important to quantify in seagrass meadows because they could counteract or diminish the seagrass ecosystem service of sediment carbon storage. Seagrass emissions are highly variable globally and few emissions estimates are available for tropical or subtropical multi-species meadows. Meadow species composition may influence emissions because species-specific plant morphology and physiology affect organic matter trapping, rhizosphere oxygenation and sediment microbial activity. We are investigating the effect of seagrass species composition and identity on sediment-water gas fluxes in subtropical meadows in the northern Gulf of Mexico. This region is host to several tropical seagrass species that are underrepresented in greenhouse gas emissions estimates. Using batch­style incubations, we measured N20 and CH4 fluxes in bare sediments and sediments with Thalassia testudinum, Halodule wrightii, or with both species. We found that sediments with seagrass were sources while bare sediments were sinks of CH4 and N20 fluxes were negligible across all treatments. Seagrass sediment CH4 fluxes were within range of the global average of reported rates, at 46.19 µmol CH4 m-2 day-1. Sediments with T. testudinum, the longer-lived, larger species, produced greater CH4 fluxes than sediments with H. wrightii only. These findings suggest species composition and identity are drivers of variability in seagrass sediment greenhouse gas emissions, and future work will address potential mechanisms contributing to observed differences in emissions between species, such as sediment organic content and root morphology.