Thermal dependence of seagrass ecosystem metabolism in the Red Sea

Abstract

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.

Drivers of the Low Metabolic Rates of Seagrass Meadows in the Red Sea

“…Our metabolic rates for H. uninervis meadows diverge from those recently reported for a mixed meadow of H. uninervis and C. serrulata in the central Red Sea (Burkholz et al, 2019). Their reported GPP and R rates were two and five times higher than what we recorded for mono-specific meadows of H. uninervis.…”

“…Furthermore, NCP rates and GPP/R ratios were about nine and two times higher than in our study. The differences among studies might result from the difference in standing stock above ground biomass among meadows, which was about 20 times higher in the mixed meadow (average 269 g DW m −2 ; Burkholz et al, 2019) than in our mono-specific meadows (average 12 g DW m −2 ). This large difference in biomass could derive from several factors such as (i) mixed meadows achieve higher above ground biomass than the mono-specific meadows of the same species , (ii) we removed the epiphytes on the leaves which can achieve substantial biomass loads (Nelson, 2017), but Burkholz et al (2019) did not, (iii) differences in depth/light availability (0.5-5 m in our study and 0.7 m in Burkholz et al, 2019), and (iv) potential differences in sediment composition.…”

“…These natural gradients render the Red Sea an optimal ecosystem to explore in situ drivers of seagrass productivity in tropical oligotrophic environments. Here seasonal changes in temperature have been identified as a strong driver of seagrass metabolism (Burkholz et al, 2019), as well as for the calcification rates in the common reef-building coral Pocillopora verrucosa (Sawall et al, 2015). Likewise, nutrient availability plays an important role for primary production in pelagic communities across the Rea Sea (López-Sandoval et al, 2019).…”

“…Here seagrass meadows are commonly found in lagoons, on coastal shallow slopes, and around islands Duarte et al, 2018). Knowledge of metabolic rates of seagrass meadows in this region is limited and has only been studied in two locations, the central Red Sea (Burkholz et al, 2019) and the Gulf of Aqaba in the north (Bednarz et al, 2015;Cardini et al, 2018). One recent study reports the metabolic rates of a meadow dominated by Enhalus acoroides, and a mixed meadow of Cymodocea serrulata and Halodule uninervis in a coastal lagoon in the central Red Sea (Burkholz et al, 2019).…”

“…Knowledge of metabolic rates of seagrass meadows in this region is limited and has only been studied in two locations, the central Red Sea (Burkholz et al, 2019) and the Gulf of Aqaba in the north (Bednarz et al, 2015;Cardini et al, 2018). One recent study reports the metabolic rates of a meadow dominated by Enhalus acoroides, and a mixed meadow of Cymodocea serrulata and Halodule uninervis in a coastal lagoon in the central Red Sea (Burkholz et al, 2019). Two additional studies in the Gulf of Aqaba, a cooler and unique northern region, describe the gross primary-and net production of Halophila stipulacea (Bednarz et al, 2015;Cardini et al, 2018).…”