Abstract. Seagrass meadows are autotrophic ecosystems acting as carbon
sinks, but they have also been shown to be sources of carbon dioxide
(CO2) and methane (CH4). Seagrasses can be negatively affected by
increasing seawater temperatures, but the effects of warming on CO2 and
CH4 fluxes in seagrass meadows have not yet been reported. Here, we
examine the effect of two disturbances on air–seawater fluxes of CO2
and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated
sediments using cavity ring-down spectroscopy. We first characterized
CO2 and CH4 fluxes in vegetated and adjacent unvegetated
sediments, and then experimentally examined their response, along with that
of the carbon (C) isotopic signature of CO2 and CH4, 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 CO2 and
CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated
compared to bare sediments, as well as 10- to 100-fold-higher CH4
fluxes. Warming led to an increase in net CO2 and CH4 fluxes,
reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m−2 d−1 and 88.11±15.19 µmol CH4 m−2 d−1, while CO2 and CH4 fluxes decreased over time in
sediments maintained at 25 ∘C. Prolonged darkness led to an
increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated
sediments. These results add to previous research identifying Red Sea
seagrass meadows as a significant source of CH4, 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 enhancing global warming.