Seasonality and depth zonation of two common intertidal northern Vietnamese seagrass species, Halophila ovalis and Zostera japonica, were investigated in a 50 m long intertidal transect from 1999 till 2001. Z. japonica occurred in the middle intertidal (1.2 m above mean chart datum to mean sea level at 2.1 m above chart datum). H. ovalis occurred deeper in the intertidal (between 1.0 and 1.4 m above chart datum). Based on seasonally different sensitivity to reciprocal transplants, it is concluded that this zonation is most likely due to different tolerances to low light availability (less in Z. japonica) and desiccation (less in H. ovalis). Both species reached maximum shoot densities in September-October with total biomasses around 70 g DW m-2. Flowering in Z. japonica occurred in April only, whilst H. ovalis flowered in November after the rainy season and again in April. Seasonality in density and biomass was apparent and similar between the two species. It was largely coupled to the rainy season, which brings spates of turbid water during May-August, thereby possibly limiting light availability and hence growth.
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