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

[ACG40-P04] A coupled watershed-ocean-ecosystem model for assessing the turbidity and nutrient reduction ability of seagrass meadows

*Yuta Alexander Takagi¹, Takashi Nakamura¹, Dominika Wara Christiana^1,2, Faisal Amri¹, Masaya Yoshikai¹, Novi Susetyo Adi³, Kazuo Nadaoka¹ (1.Department of Transdisciplinary Science and Engineering, School of Environment and Society, Tokyo Institute of Technology, Japan, 2.World Resources Institute, Indonesia, 3.Research Centre for Marine and Coastal Resources, Ministry of Marine Affairs and Fisheries, Indonesia)

Keywords:Computational modeling, Seagrass, Coral reef

Coral reef and seagrass ecosystem models were developed and incorporated into the Coupled-Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) Modeling System to computationally model the interplay of coastal ecosystems with the physical and chemical ocean environment. Terrestrial watershed inputs were modeled using the Soil and Water Assessment Tool Plus (SWAT+) and coupled as river input for the COAWST Modeling System. The area of focus for this work is the Berau Continental Shelf region off the coast of the Berau Regency, East Kalimantan, Indonesia, but the model and computational techniques are developed to be applicable and transferrable to other tropical ocean regions. The computational domain for the physical environmental modeling is triply nested from a coarser regional Coral Triangle domain (1/12º x 1/12º) to an intermediate simulation covering the Makassar Strait between the Indonesian islands of Borneo and Sulawesi (0.0152º x 0.0153º), to the high-resolution domain covering the Berau Continental Shelf region of interest (0.0036º x 0.0040º). The ecosystem model is then applied to this high-resolution domain. Water flow, sediment, and nutrient loads introduced by the Berau River to the coastal ocean system were computationally modeled using a SWAT+ simulation of the Berau River watershed and coupled as inputs to the COAWST-ecosystem simulation. Excess nutrients and suspended sediments introduced by the Berau river pose a threat to the nearby pristine barrier reef ecosystems. Such nutrient stressors can be expected to worsen in the coming years with the planned move of the Indonesian capital city to Borneo Island and the accompanying growth of human impact on the island. Seagrasses are known to act as nutrient sinks, trap sediment, and reduce sediment resuspension in some coastal marine systems providing a potential avenue for ecological remediation of the excess anthropogenic nutrient runoff from the river and turbid water. While the shallow river plume area does not currently host seagrass meadows, the species Halodule uninervis, Thalassia hemprichii, and Enhalus acoroides are native to the region and could be used to establish a seagrass meadow buffer ecosystem between the river and the coral reefs. Such meadows would also provide other ecosystem services such as providing a nursery habitat for fish and a food source for green sea turtles. However, the viability of establishing seagrass meadows is uncertain because of their environmental needs. Our ecosystem biogeochemical models coupled with the SWAT+ and COAWST Modeling Systems can be used to help determine the viability and effectiveness of ecosystem management techniques such as establishing a seagrass buffer ecosystem to protect the fragile coral reef ecosystem from anthropogenic river runoff impacts.