Variations of tidal exposures and seasons on growth, morphology, anatomy and physiology of the seagrass Halophila ovalis (R.Br.) Hook. f. in a seagrass bed in Trang Province, Southern Thailand

• Morphological, anatomical and physiological variability of Halophila ovalis studied.
• Best growth and production observed at the lower intertidal zone.
• Limited growth during dry season due to exposure hours, desiccation and high temperature.
• Limited growth during rainy season due to run-off, sedimentation and limited light intensity.
• High spatio-temporal variability of H. ovalis growth, morphology, anatomy and physiology.

Halophila ovalis is a seagrass species which is widely distributed throughout the world and is found in a wide range of habitat along the vertical gradients from upper intertidal to subtidal areas. Differences in tidal exposure control physical factors relating growth and responses of marine plants. To determine variations in growth, morphology, anatomy and physiology of H. ovalis growing in different environments, plants were collected at three levels of tidal exposure (upper intertidal, lower intertidal, subtidal), two seasons (dry and rainy) and at two sites (Laem Yong Lam; YL and Koh Muk; KM, on the Andaman coast of Thailand). The seagrass showed the slowest growth rate in the upper intertidal zone as they spent about 4 days to produce new leaf pairs with a rhizome elongation rate of 3.61 mm day−1. The biggest leaf size of H. ovalis was found in the lower intertidal zone. The lowest maximum relative electron transport rate (rETRmax) was found in plants growing in the subtidal zone. Optimum light for photosynthesis (Eopt) of intertidal seagrass was higher than those of the subtidal population. Higher total chlorophyll and carotenoids content were found in subtidal seagrass while higher anthocyanins content was found in seagrass growing in the upper intertidal area. Responses of H. ovalis in this study are concluded to occur mainly as a consequence of light differentiation and desiccation stress in different places and times.