Magnitude of within‐patch variation in seagrass Halophila ovalis growth affected by adjacent Thalassia hemprichii vegetation

Seagrass beds in South‐east Asia sometimes consist of a mosaic of different species in monospecific patches. We examined whether the magnitude of within‐patch variation in the seagrass Halophila ovalis is affected by the presence or absence of surrounding vegetation consisting of another seagrass species Thalassia hemprichii in an intertidal flat in Thailand waters. We measured biomass and growth rates of H. ovalis at the edges and centers of two different types of patches: (i) H. ovalis patches adjoining T. hemprichii vegetation (HT patches), and (ii) H. ovalis patches adjoining unvegetated sand flats (HS patches). Furthermore, we examined the possible effects of interspecific interactions on the growth of H. ovalis by experimentally removing adjoining T. hemprichii at the edges of HT patches. The biomass of H. ovalis was greater at the patch centre than the patch edge in both types of patches. For the growth rate of H. ovalis , significant interactions were detected between patch types and positions in patches. The difference in growth was significant and more than 4‐fold between edges and centers of the HS patches, whereas the growth was not significantly different between edges and centers of the HT patches. The removal of T. hemprichii did not significantly affect the growth rate of H. ovalis at the edge of the HT patches. These findings demonstrate that the magnitude of within‐patch variation in H. ovalis growth is affected by the conditions of adjoining habitats. However, any effects of local competition with T. hemprichii on H. ovalis growth were not evident in this short‐term manipulative experiment.