[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-09] Development of a Coupled Watershed-Ocean-Vegetation Model of Busuanga, Palawan, Philippines

*Ayin Modina Tamondong^1,2, Takashi Nakamura¹, Faisal Amri¹, Kazuo Nadaoka¹ (1.Tokyo Institute of Technology, 2.University of the Philippines)

Keywords:COAWST, SWAT, seagrass model, watershed model, ocean model, coastal simulation

The coastal water environment has long been considered as one of the most important natural resources. It is valuable to several essential industries such as transportation, trade, fisheries, aquaculture, and tourism. Unfortunately, with the rapid urbanization, economic development, population growth, and increasing threats of climate change, a need for environmental conservation and proper coastal management arises. Part of this threatened coastal environment is the seagrass ecosystem. Seagrasses are marine flowering plants that commonly grow on soft substrata (mud, sand) in shallow coastal waters. They are one of the most unique and productive ecosystems and they are a vital natural resource. Among the ecosystem services offered by seagrasses are providing coastal protection and serving as water filter. They trap sediment and excessive nutrients which improves the water quality, and they also act as barriers to reduce current velocity. However, excessive sedimentation may cause seagrass dieback. Light attenuation in the water due to reduced water quality has significant impacts on the survival of seagrasses. The rise in coastal developments increases the siltation deposited by rivers to the coastal environment which predicates an unclear future for seagrass meadows. There is a need to monitor the current situation of seagrasses as well as to develop predictive models to determine their possible mortality. This poses a problem because the coastal environment is highly dynamic, diverse, and variable and it is affected by different land-ocean border processes such as waves and currents, sediment transport, chemical and biological modifications, and their interactions with the coastal structure. The complexity and interdependency of the physical processes in the coastal zone make the use of numerical models advantageous. In this study, a coupled watershed-ocean-vegetation model approach was explored to determine the impacts of river runoff on seagrasses in Busuanga, a small tropical island in the Philippines. For the watershed model, the Soil and Water Assessment Tool (SWAT+) was utilized while the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system was employed for the ocean model. Kalra et al. in 2020 developed and added a submerged aquatic vegetation (SAV) growth and mortality model to the open-source COAWST modeling framework. This model was adapted and modified in this research to be suitable and applicable to the tropical environment of seagrasses in Busuanga. The seagrasses in Busuanga were initially assessed using field survey, drone mapping, and satellite remote sensing. Based on observed field and classified drone data, seagrass beds affected by river discharges have lower percent cover than areas far from the river mouth. The seagrass area near the river was also observed to have higher turbidity based on measured water quality data. The simulation results of water temperature, suspended sediment, and current velocity were validated using data from deployed sensors. Based on simulated data, similar to the observed data, the seagrass beds near the river outlet have higher suspended sediment concentration which caused a decrease in seagrass above-ground biomass and plant density. A comparison between simulation results with and without river discharge was performed. Based on correlation values of sensor data and simulation results, improvement was observed from -0.432 to 0.640. In addition, an increase in suspended sediments due to river discharge increased light attenuation and decreased the photosynthetically active radiation (PAR) in the simulation. Results also show that seagrasses reduce suspended sediment and current velocity while increasing the bed thickness which confirms that sediment deposition occurs on seagrass beds.