The Red Sea is one of the warmest seas with shallow seagrass ecosystems exposed to extreme temperatures, in excess of 35°C, during the summer months. Seagrass meadows are net autotrophic ecosystems, but respiration increases faster than primary production with temperature. This may lead to a shift from an autotrophic to a heterotrophic system at the highest temperatures. Although tropical seagrasses are adapted to high temperatures, the metabolic rates of Red Sea seagrasses have not yet been reported. Here we assessed the community metabolism of 2 seagrass ecosystems, an Enhalus acoroides monospecific meadow and a Cymodocea serrulata and Halodule uninervis mixed meadow, located in the central Red Sea. We measured in situ net community production (NCP), community respiration (R), gross primary production (GPP), activation energy and community production−irradiance curves along their natural temperature gradient over 1 yr by measuring diel fluctuations in dissolved oxygen. The results were species-specific; while the monospecific meadow was autotrophic throughout the year (annual weighted average NCP: 64.63 ± 11.89 mmol O 2 m −2 d −1 , GPP:R ratio: 1.42 ± 0.06), the mixed meadow was heterotrophic during the summer months (annual weighted average NCP: −4.15 ± 9.39 mmol O 2 m −2 d −1 , GPP:R: 1.04 ± 0.05). In both seagrass meadows, R and GPP increased with increasing temperature, but differences in activation energies indicated that the mixed meadow is more sensitive to increasing seawater temperatures. These findings suggest contrasting responses in tropical seagrass species to rising temperature, pointing out the potential vulnerability of seagrasses to ocean warming in the Red Sea.

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.

Kelp forests are experiencing substantial declines due to climate change, particularly ocean warming and marine heatwaves, and active interventions are necessary to halt this decline. A new restoration approach termed “green gravel” has shown promise as a tool to combat kelp forest loss. In this approach, substrata (i.e. small gravel) are seeded with kelp propagules, reared in controlled conditions in the laboratory before out-planting to degraded reefs. Here, we tested the feasibility of cultivating Australia’s dominant kelp, Ecklonia radiata on green gravel with the aim of optimising the seeding conditions for E.radiata. We seeded substrata (i.e. gravel), that had different surface texture and size, with E. radiata gametophytes at two average seeding densities: high density of ~230 fragments mL-1 and low density of ~115 fragments mL-1. The tested substrata were small basalt, large basalt, crushed laterite and limestone. Gametophytes successfully adhered to all four tested substrata, however, gametophytes that adhered to the limestone gravel (the natural reef type off Western Australia) suffered extreme tissue bleaching likely due to dissolution and decrease in seawater pH. Gametophytes that adhered to the three other test substrata were healthy, fertilised following seeding and microscopic sporophytes were observed attaching to the gravel. Substrata and seeding density did not affect sporophyte growth (i.e. length) at the time of transferring into aquarium tanks (after three months of rearing in incubators) but over time substrata showed a significant effect on maximum lengths. After 12 months in aquarium tanks, sporophytes on both small and large basalt gravel were significantly larger than those on the crushed laterite. Gametophytes were also found to not only survive on the gravel itself but also detach from the gravel, settle successfully, fertilise and develop into healthy sporophytes ex situ on the surrounding substratum through lateral transfer. Substrata had a significant effect on density of detached gametophytes with rougher and larger gravel showing higher densities of detachment. Our results show the potential for green gravel to be a vector of dispersal for restoration in Western Australia where natural recovery of kelp forests has failed.

<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>