Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass ( Halophila stipulacea ) sediments

Abstract. Seagrass meadows are autotrophic ecosystems storing carbon in their biomass and sediments, but they have also been shown to be sources of carbon dioxide (CO 2 ) and methane (CH 4 ). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO 2 and CH 4 fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air-seawater fluxes of CO 2 and CH 4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO 2 and CH 4 fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the C isotopic signature of CO 2 and CH 4 , to gradual warming from 25 °C (winter seawater temperature) to 37 °C, 2 °C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting CO 2 and CH 4 fluxes. We detected distinct differences between vegetated and unvegetated sediments, with the vegetated sediments supporting 6-fold higher CO 2 fluxes, and 10- to 100-fold higher CH 4 fluxes. Warming led to an increase in net CO 2 and CH 4 fluxes, reaching average fluxes of 10,422.18 ± 2,570.12 µmol CO 2  m −2  d −1 and 88.11 ± 15.19 µmol CH 4  m −2  d −1 , while CO 2 and CH 4 fluxes decreased over time in sediments maintained at 25 °C. Prolonged darkness led to an increase in CO 2 fluxes but a decrease in CH 4 fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH 4 , while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhance global warming.