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

Abstract

<p><strong>Abstract.</strong> 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<sub>2</sub>) and methane (CH<sub>4</sub>). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO<sub>2</sub> and CH<sub>4</sub> fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air-seawater fluxes of CO<sub>2</sub> and CH<sub>4</sub> in Red Sea <i>Halophila stipulacea</i> communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO<sub>2</sub> and CH<sub>4</sub> fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the C isotopic signature of CO<sub>2</sub> and CH<sub>4</sub>, 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<sub>2</sub> and CH<sub>4</sub> fluxes. We detected distinct differences between vegetated and unvegetated sediments, with the vegetated sediments supporting 6-fold higher CO<sub>2</sub> fluxes, and 10- to 100-fold higher CH<sub>4</sub> fluxes. Warming led to an increase in net CO<sub>2</sub> and CH<sub>4</sub> fluxes, reaching average fluxes of 10,422.18 ± 2,570.12 µmol CO<sub>2</sub> m<sup>−2</sup> d<sup>−1</sup> and 88.11 ± 15.19 µmol CH<sub>4</sub> m<sup>−2</sup> d<sup>−1</sup>, while CO<sub>2</sub> and CH<sub>4</sub> fluxes decreased over time in sediments maintained at 25 °C. Prolonged darkness led to an increase in CO<sub>2</sub> fluxes but a decrease in CH<sub>4</sub> fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH<sub>4</sub>, 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.</p>

Photo-physiology and morphology reveal divergent warming responses in northern and southern hemisphere seagrasses

“…The replacement of climax seagrass species, generally characterized by high biomass and productivity, by pioneer species will also decrease the capacity of seagrass meadows to mitigate the effects of carbon emissions (Gattuso et al 2018). Under some warming scenarios, seagrasses ecosystem may even switch metabolism from autotrophic to heterotrophic (Burkholz et al 2019), and enhance CO2 and methane fluxes from the meadows into the atmosphere (Burkholz et al 2020).…”

Methane emission and sulfide levels increase in tropical seagrass sediments during temperature stress: A mesocosm experiment

“…How seagrass systems might contribute to these emissions has received comparably little attention, although valuable studies have been published (Bahlmann et al, ; Barber & Carlson, ; Deborde et al, ; Garcias‐Bonet & Duarte, ; Oremland, ). Temperature increases have been shown to enhance methane emissions from freshwater systems (Yvon‐Durocher, Hulatt, Woodward, & Trimmer, ; Yvon‐Durocher, Montoya, Woodward, Jones, & Trimmer, ), and recently, it has been shown that methane emission from seagrass meadows rises substantially when seagrasses are disturbed (Burkholz, Garcias‐Bonet, & Duarte, ; Lyimo et al, ), and based on calculations of methane emission in seagrass sediments from the Red Sea, it has been suggested that the present estimations of methane emissions from natural systems might have to be increased by about 30% to account for hitherto unrecognized contributions from seagrass systems (Garcias‐Bonet & Duarte, ). In general, the methane production of biological systems is closely correlated with the productivity of the plants within the system (Borges, Speeckaert, Champenois, Scranton, & Gypens, ; Bridgham et al, ), and for wetlands in particular, there is a clear positive correlation between emission of methane and net ecosystem production (Whiting & Chanton, ).…”