ProblemThe temporal dynamics of different seagrass species in the same locality exhibit similar responses to seasonal forcing, but the magnitude of fluctuations in seasonal growth is species-dependent (Marba et al. 1996). In temperate waters, the strong seasonality of seagrasses responds, in the form of plant growth, to seasonal variation in light and temperature (Phillips et al. 1983; Olesen & SandJensen 1994;Laugier et al. 1999). A comparative analysis of the growth dynamics of different seagrass species forming monospecific meadows showed that differences in shoot growth and species-specific differences in seasonal dynamics are associated with differences in seagrass size. Large species are longer-lived and slower-growing, whereas smaller species are shorter-lived and grow faster (Duarte 1991;Marba et al. 1996). Seagrass size differences may determine the type of environment they occupy; small species usually inhabit frequently disturbed habitats, whereas large seagrasses require more stable environments AbstractThe temporal dynamics of two seagrass species, Zostera marina and Z. japonica, were monitored monthly in Dadae Bay, Geoje Island, on the southern coast of Korea. Plant morphological characteristics, shoot density, biomass, leaf production, reproductive effort, and environmental characteristics were monitored from July 2001 to July 2002. Zostera japonica occurred in the intertidal zone and Z. marina occurred in the subtidal zone from 0.5 to 2.5 m below the mean low water level. Shoots and rhizomes were significantly larger in Z. marina than in Z. japonica, whereas the shoot density was greater in Z. japonica than in Z. marina. Despite differences in morphology and shoot density, biomass did not differ significantly between the species. Reproduction occurred from April to June in Z. marina and from May to July in Z. japonica. The proportion of reproductive shoots was approximately three times higher in Z. marina than in Z. japonica. Seasonal variation in the biomass of Z. japonica was caused by changes in both shoot size and density, whereas that of Z. marina was mainly caused by changes in shoot length. Leaf production in Z. marina and Z. japonica showed clear seasonal variation, and leaf production in Z. marina (2.6 ± 0.2 g DWAEm )2 AEday )1 ) was higher than that in Z. japonica (1.7 ± 0.2 g DWAEm )2 AEday )1 ). The mean plastochrone interval was not significantly different between the two species, whereas the leaf lifetime of Z. marina was longer (69 ± 7.8 days) than that of Z. japonica (59 ± 8.3 days). Our results indicated that seasonal leaf growth patterns in Z. japonica are correlated with irradiance and temperature, whereas those in Z. marina respond most to irradiance. Seasonal changes in irradiance appeared to control the temporal variation in above-ground biomass in both species.

Among common estuarine submerged plants, seagrasses are the most extensively studied due to their ecological importance in estuarine ecosystems. Seagrass meadows are important biological habitats for a wide variety of marine animals and plants. They are a source of organic carbon for commercially important animals. Furthermore, seagrasses act as nutrient filters in estuarine and coastal marine ecosystems. As such, mapping the distribution of seagrass beds is important for management and conservation strategies. In order to survey the seagrass distribution within the Seomjin Estuary, We directly observed seagrass beds in Kwangyang and Hadong using SCUBA. The distribution area, species composition, morphology, density, biomass and productivity of seagrass meadows were examined. Seagrass meadows were distributed in the intertidal and subtidal zones of the Galsa tidal flats, and in the subtidal zone of the neighboring POSCO area. Patches of Zostera japonica was found at patches at the Galsa tidal flats intermediate point. The total estimated seagrass distribution area of the Seomjin Estuary was 1.84 km 2. Of the total, 1.83 km 2 was Zostera marina (eelgrass) and 0.01 km 2 was Zostera japonica (dwarf eelgrass). Zostera japonica was found in intertidal zones. Zostera marina was found from the intertidal to subtidal zones at a 2 m mean sea level (MSL) depth. The leaf productivity of Zostera marina was 4.47g DW m-2 day-1. The annual production of eelgrass was 1,632 g DW m-2 yr-1 , which corresponds to 731g C m-2 yr-1. The total production of eelgrass was 3,002 tons DW yr-1 , which corresponds to 1,343 tons C yr-1 .