Growth of seagrass Halophila ovalis at dugong trails compared to existing within-patch variation in a Thailand intertidal flat

Abstract

We studied small-scale variation in morphology and growth rate of the seagrass Halophila ovalis in an intertidal flat of Thailand coast where the seagrass bed is subjected to grazing by dugongs. Our objectives were to examine whether morphology and growth rate differ between rhizomes in different positions within a patch (center, edge and dugong trails), and to test whether H. ovalis shows plastic growth response to disturbance by dugong feeding. Seagrass biomass and leaf density were 3 to 4 times greater at the patch center than the patch edge and the dugong trails. In contrast, branching rate and internode length of rhizomes were significantly greater at the patch edge and dugong trails than at the patch center. The results of marking experiments of H. ovalis revealed that net production, leaf production and rhizome elongation rates were 2 to 3 times greater at the patch edge than at the center. Growth and production rates at the edge of the dugong trails and artificial trails mimicking dugong trails were not significantly higher than at the patch center. Nevertheless, significant differences in some morphological parameters such as branching frequency and internode length between the dugong trails and the patch center suggest that morphological changes in H. ovalis can be induced over short time scales. Recovery of H. ovalis atter disturbance by dugong herbivory was estimated to be quite rapid (<20 d). High growth rates of H. ovalis were probably responsible for its persistence under grazing by dugongs.