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

• Main Authors: C. Burkholz, N. Garcias-Bonet, C. M. Duarte
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/1717/2020/bg-17-1717-2020.pdf

<p>Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (<span class="inline-formula">CO₂</span>) and methane (<span class="inline-formula">CH₄</span>). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> in Red Sea <i>Halophila stipulacea</i> communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span>, to gradual warming from 25&thinsp;<span class="inline-formula">^∘</span>C (winter seawater temperature) to 37&thinsp;<span class="inline-formula">^∘</span>C, 2&thinsp;<span class="inline-formula">^∘</span>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 <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> fluxes. We detected 6-fold-higher <span class="inline-formula">CO₂</span> fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher <span class="inline-formula">CH₄</span> fluxes. Warming led to an increase in net <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> fluxes, reaching average fluxes of 10&thinsp;422.18&thinsp;<span class="inline-formula">±</span>&thinsp;2570.12&thinsp;<span class="inline-formula">µ</span>mol&thinsp;<span class="inline-formula">CO₂</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> and <span class="inline-formula">88.11±15.19</span>&thinsp;<span class="inline-formula">µ</span>mol&thinsp;<span class="inline-formula">CH₄</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span>, while <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> fluxes decreased over time in sediments maintained at 25&thinsp;<span class="inline-formula">^∘</span>C. Prolonged darkness led to an increase in <span class="inline-formula">CO₂</span> fluxes but a decrease in <span class="inline-formula">CH₄</span> fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of <span class="inline-formula">CH₄</span>, 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.</p>