SURVIVAL, MORPHOLOGY AND REPRODUCTION OF ENHALUS ACOROIDES (L.f.) ROYLE IN THE BOLINAO-ANDA REEF SYSTEM, NW PHILIPPINES: EFFECTS OF VARYING LIGHT CONDITIONS

Seagrasses play a critical role in coastal ecosystems worldwide, providing various ecosystem services based on their region and genus. In Southeast Asia, where seagrass biodiversity and extents are at their highest, the livelihoods and food security of many coastal communities depend on these plants. Despite their ecological and economic importance, seagrasses face global threats from human activities such as pollution and land use changes. Enhalus acoroides, a widely distributed seagrass species in the tropical Indo-Pacific region, is particularly valuable for coastal management and conservation efforts due to its size and provision of various ecosystem services. Although previous research has indicated that it is less sensitive to environmental changes than other tropical seagrass species, recent reports highlight its vulnerability to siltation and eutrophication. This dissertation aimed to examine how Enhalus responds and adapts to changes in light availability, taking into account both morphological adaptation and phenotypic plasticity. Field surveys, reciprocal transplantation field experiments, and investigations of sexual reproductive effort were conducted in the Bolinao-Anda Reef system (NW Philippines) to evaluate the impact of long-term environmental changes on Enhalus populations. The findings of this study revealed that Enhalus has the capacity to adapt its traits and survive changes in depth, light gradients, and different habitat types. This is evidenced by larger shoots in low-light environments, which is apparently a response to the reduction in light availability, as evidenced in both in situ and experimental setups. Larger leaf surface area in light-reduced setups also had higher concentration of chlorophylls a and b pigments. Transplants from light-reduced environments, although morphologically large, appeared more vulnerable (with low survival values) to environmental changes associated with translocation. Being morphologically large is therefore likely a stress response to light reduction, allocating more energy on light harvesting than sexual reproduction. Reciprocal transplantation experiments indicated a high survival rate, suggesting the potential of Enhalus for use in rehabilitation. However, despite having wider plasticity to adapt to light-limitation, they can be wiped out when threshold is reached. This thesis underscores the need for further research on Enhalus' response to stressors, genetic variation, and adaptive capacity to address conservation and management challenges