Responses of the seagrass Halophila stipulacea to depth and spatial gradients in its native region (Red Sea): Morphology, in situ growth and biomass production

The tropical seagrass Halophila stipulacea is the most dominant and widespread seagrass species in the Gulf of Aqaba (GoA, northern Red Sea). In light of the ecological services provided by H. stipulacea in its native regions, and its reported colonization and dispersion in non-native areas, there is a growing global interest in understanding its vegetative development and growth strategies under varying environmental gradients. Aiming to compare growth and biomass production rates in native vs. invasive regions in the near future, it is rather surprising that such baseline data hardly exists for H. stipulacea. Focusing on the northern GoA, we investigated morphological characteristics, in situ growth rates, and biomass production at three different sites (NB, TY and SB), and at each site, at two depths (5 and 14 m) during the summer months (June-July 2019). Significantly larger (11 %) rhizome internodes and longer (19 %) and wider (15 %) leaves were observed in deeper plants compared to those inhabiting shallow meadows. On the contrary, shoot and internode formation rates in shallow plants were markedly higher than in deep-adapted plants, with production values of one shoot and one internode every 5.9 ± 0.8 and 6.9 ± 0.6 days, respectively. Rhizome elongation rates did not vary significantly across sites and depth with an average value of 0.2 cm d⁻¹. In addition, larger leaf area, number of leaves, and higher above and below-ground biomass were observed in the NB site, compared with TY and SB, suggesting that more favourable growth conditions for H. stipulacea existed at NB which is exposed to high anthropogenic pressures. Results highlight how the species is highly responsive to in situ conditions, and thus contribute to an improved understanding of its vegetative dynamics and growth strategy as a basis for future comparisons and temporal assessments.