Study on the variation of seagrass population in Coastal waters of Khanh Hoa Province, Vietnam

Khanh Hoa is among of a few coastal provinces where seagrasses are very abundant and form the large meadows in littoral zones, bays, lagoons. It is possible to consider that the coastal waters of Khanh Hoa has the most diverse of seagrass composition among areas of Vietnam. The investigation of seagrass beds in Khanh Hoa province (1996-1997) recorded 9 species of seagrasses, some of which were very interesting because of their large distribution areas, high density and biomass. Study on the variation of density, above ground biomass, leaf growth rate and leaf production of Enhalus acoroides and Thalassia hemprichii at two stations My Giang (Vanphong bay) and Dong Ba Thin (Camranh bay) were conducted two times per month in two years 1998, 1999. At My Giang station: the mean values of density and leaf production of E. acoroides were highest in September: 144±5shoots/m^2 and 5.96±0.3gDW/m^2/day, above ground biomass and leaf growth rate were highest in April 158±2gDW/m^2 and 1.57±0.2cm/day (March). The density of T. hemprichii was highest in May: 1050±30shoots/m^2, biomass was highest in October: 148±10gDW/m^2, leaf growth rate and leaf production were highest in September: 0.66±0.02cm/day and 5.55±0.3gDW/m^2/day. At Dong Ba Thin station: the mean values of density of E. acoroides was highest in November: 120±10shoots/m^2. The above ground biomass, leaf growth rate and leaf production were highest in September. The mean values were 383±15gDW/m^2, 1.8±0.2cm/day and 5.24±0.4gDW/m^2/day respectively. The density, biomass, leaf growth rate and leaf production of T. hemprichii were also highest in September, the mean values were 420±30 shoots/m^2, 110±10gDW/m^2, 0.66±0.05cm/day and 3.79±0.2gDW/m^2/day respectively. The studied results showed that the variation of density, above ground biomass, leaf growth rate and leaf production of seagrass beds depend on seasons in year and the environmental conditions. These values were often high in dry, sunny season (March-October) and often low in the rainy season (November-January). During the low tide (May-August) seagrass beds were exposed to the air that caused the decrease of density, biomass, leaf growth rate and leaf production of seagrasses