Shore Height and Low-tide Exposure as Regulators of Intertidal Distribution Thresholds of Seagrass Enhalus acoroides in the Gulf of Thailand
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Abstract
Understanding environmental controls on seagrass distribution is essential because inappropriate elevation selection can reduce restoration and transplantation success. This study investigated how shore height and low-tide exposure influence the distribution of Enhalus acoroides in the upper intertidal zone of the Gulf of Thailand. Shore height was measured at 261 stations across seven sites during 2022–2023 using a theodolite and a surveyors’ rod, and expressed as height above lowest low water (cm H_LLW) based on predicted tidal levels. Aboveground biomass was estimated using a modified Rapid Visual Estimation Technique. Across shore heights of 0‒120 cm H_LLW, the frequency of E. acoroides occurrence followed a normal distribution (D = 0.09, p = 0.35), with most observations between 40 and 80 cm H_LLW. Preferred shore-height ranges varied among sites, extending from 0 to 106 H_LLW. Percentage cover did not differ significantly among H_LLW intervals of 0‒40 cm, 40‒80 cm, 80‒120 cm. However, aboveground biomass differed significantly among groups, with the highest biomass at 0–20 cm H_LLW (17.77±6.75 g DW·m⁻²), where mean cover was 48.18±35.73%. The upper shore-height threshold of E. acoroides was lower in the eastern Gulf of Thailand (77 cm H_LLW) than in the southern coastal region (106 cm H_LLW) and was substantially lower than thresholds reported for the Andaman Sea. These regional and local differences were closely associated with tidal regime, low-tide level, desiccation duration, and sediment water-level drawdown. The findings provide practical guidance for restoring and transplantation intertidal E. acoroides meadows under changing coastal conditions associated with global warming.
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