Community structure, growth, and productivity of seagrasses in the coastal area of Tando, Nueva Valencia, Guimaras Island, Central Philippines

The community structure of the seagrass meadow in the coastal area of Tando, Guimaras Island, Central Philippines was described in terms of a) the environmental conditions of the seagrass habitat; b) composition, distribution and seasonality in biomass, growth, and productivity of seagrasses; c) seasonality in biomass of periphyton attached to selected seagrasses; and d) vegetative and reproductive strategies of some seagrasses in the area. Six seagrass species formed the mixed meadow, namely: Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, Syringodium isoetifolium, Halodule uninervis (narrow-leaf variety) and Halophila ovalis. Detrended Canonical Correspondence Analysis (DCCA) showed that seagrass distribution is influenced by depth and substrate with larger seagrass species such as Enhalus acoroides and Thalassia hemprichit more capable of occupying deeper areas dominated by coarse sand. Smaller species such as Cymodocea rotundata and Halodule uninervis were restricted in shallow areas with fine sand as the dominant substrate. Leaf-marking method revealed seasonality in shoot growth, shoot size, and productivity of the larger seagrass species such as Enhalus acoroides and Thalassia hemprichii which increased during summer (March-April) and decreased during the rainy months (July-September). Likewise, shoot growth, shoot size and productivity were positively correlated with solar irradiance for these larger species. Periphyton biomass associated with selected seagrasses amounted to 20-50% of seagrass aboveground dry weight and from 8-13% of seagrass aboveground ash-free dry weight in the meadow. Periphyton biomass was also high in summer, and low during the rainy months. The age reconstruction technique based on leaf plastochrone interval (PI) estimate was used to describe the demography as well as the vegetative and reproductive strategies of Thalassia hemprichii and Cymodocea rotundata. Results showed that these two species have median ages of 1.15 and 0.85 yr, and maximal observed longevities of 6.11 and 5.17 I, respectively. Mortality and recruitment rates revealed that Thalassia hemprichi had a slightly declining population, while Cumodocea rotundata had a fairly stable population within the last five years. Vertical internode growth for these two species showed distinct annual patterns, with growth minima occurring in January and August, and higher growths during the summer months. The same patterns were observed in shoot growth and productivity of these two species using the leaf-marking technique. Horizontal elongation rate was higher in Cymodocea rotundata (24.58 cm apex\u27 yr\u27) compared with Thalassia hemprichii (18.10 cm apex\u27 yr\u27). These values were comparable with those in low sedimentation areas in the country and the Carribean. The age reconstruction technique also showed that 18% of Thalassia hemprichit in the meadow are reproductive shoots. Each reproductive shoot produced on the average two flowers in its lifetime. Flowering for this species is a whole vear event, showing a unimodal trend which peaks in November. This study demonstrates that temporal variations in seagrass density, growth, biomass, and production, as well as in periphyton biomass correlate well with light availability. In all cases, these parameters were high in summer, and low during the rainy months. Seagrass density and distribution were influenced by depth and substrate, with larger species occupying deeper areas dominated by coarse sand and smaller species restricted in shallow areas dominated by fine sand. Correlation tests revealed that at the periphyton loads observed in this study, periphyton biomass did not have a negative effect on growth and productivity of associated seagrasses. Results further demonstrate that while habitat conditions have important influences on seagrass growth, productivity, density, and distribution, seagrass demography and vegetative dynamics may explain differences in these parameters as a consequence of differences in module longevities, mortality and recruitment patterns in the different species. The faster rhizome elongation rate in Cymodocea rotundata which occupy environmentally changeable areas, and the slow rhizome growth observed in Thalassia hemprichi which predominate in deeper, more stable areas, illustrate the different ecological roles of the small, colonizing species, and the large, climax species