Abstract
Tropical seagrass meadows are highly productive ecosystems that thrive in oligotrophic environments. The Red Sea is characterized by strong N-S latitudinal nutrient and temperature gradients, which constrain pelagic productivity. To date, the influence of these natural gradients have not been assessed in metabolic rates for local seagrass communities. Here we report metabolic rates (gross primary production, respiration, and net community production) in four common species of seagrass (Halodule uninervis, Halophila ovalis, Halophila stipulacea, and Thalassia hemprichii) along latitudinal and thermal gradients in the Red Sea. In addition, we quantified leaf nutrient concentration (nitrogen, phosphorous, and iron), and correlate this with latitude. Our results show that average metabolic rates and aboveground biomass of seagrass meadows in the Red Sea were generally in the lower range when compared to global values reported for the same species forming meadows. The optimum temperature of Red Sea seagrass meadows varied among species with declines along the sequence: H. stipulacea > T. hemprichii > H. uninervis ~ H. ovalis. Gross primary production for H. uninervis – a seagrass thermophile – was lowest in higher latitudes and increased toward lower latitudes during the summer months. While temperature was identified as a strong driver of metabolic rates across seagrass meadows, leaf concentration of phosphorous and iron (but not nitrogen), were below nutrient sufficiency thresholds, indicating these two elements might be limiting for seagrass meadows in the Red Sea.
Highlights
Seagrass meadows are found in shallow marine waters and are among the most productive ecosystems on Earth (Duarte and Chiscano, 1999)
The mean net community production (NCP) of Red Sea seagrass meadows ranged from 47 ± 28 (T. hemprichii) to 58 ± 31 (H. uninervis) mmol O2 m−2 d−1, with all meadows being autotrophic (GPP/R > 1) on average (Table 1)
Thermal performance curves for meadows dominated by two species of seagrasses (H. stipulacea and T. hemprichii) revealed an optimum temperature (Topt) of 30.59 and 30.89◦C for gross primary production (GPP), 29.71 and 29.78◦C for R, and 29.88 and 30.78◦C for NCP, respectively (Figure 3 and Supplementary Table S3)
Summary
Seagrass meadows are found in shallow marine waters and are among the most productive ecosystems on Earth (Duarte and Chiscano, 1999). As such, they have the capacity to act as intense carbon sinks and are a fundamental blue carbon habitat (Duarte et al, 2010; Fourqurean et al, 2012). Like many other marine organisms (Sibly et al, 2012; García et al, 2018; South et al, 2018), seagrass generally increase its metabolic rate [e.g., photosynthesis and respiration (R)] until reaching an optimum temperature beyond which they decline, sometimes abruptly (Marbà and Duarte, 2010).
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