[A-HW32] Material transportation and cycling in aquatic ecosystems; from headwaters to coastal areas

[AHW32-17] Impact of terrestrial nutrients in groundwater on coral reefs via submarine groundwater discharge around Yoron Island of the Southwest Islands, Japan

*Shinji Nakaya¹, Sho Nagata¹, Masashi Toyota¹, Jun Yasumoto², Tetsuya Shintani³, Takashi Nakamura⁴ (1.Department of Water Environment and Civil Engineering, Faculty of Engineering, Shinshu University, 2.Department of Regional Agriculture Engineering, University of the Rykuyus, Japan, 3.Department of Civil and Environmental Engineering, Tokyo Metropolitan University, 4.Biology Program, Faculty of Science, University of the Ryukyus)

Keywords:submarine groundwater discharge, coral, Yoron Island, numerical simulation

Anthropogenic activities often have significant environmental impact. On tropical to subtropical islands without rivers, terrestrial nutrients from agricultural activity and waste water are transported from land to ocean via submarine groundwater discharge (SGD). It has been reported phosphorus inhibits skeletal development of corals in laboratory experiments. In general, nitrogen transported from land breeds algae. In Yoron Island, phosphorus and nitrogen concentrations in SGD from Ryukyu limestone-aquifer at east coast were 2.3 mg/L and 3.5 mg/L, respectively. Corals in shallow lagoon have not recovered since mass-scale coral bleaching caused by seawater temperature rise in 1998. However, the causal relationship between terrestrial nutrients in SGD and delay in coral recovery is unknown.

We investigated the effect of nutrients in groundwater on coral reefs via SGD around the Yoron Island of the Southwest Islands, Japan, from numerical simulation in spread of submarine groundwater due to tidal current in coral reef lagoon leading to the open sea. Results are summarized:

1. Submarine groundwater spreads to north part of coral reef lagoon in surface and bottom, reaches the reef habitat, and tend to stay blocked by the outer reef structure.
2. Strong spread zone of submarine groundwater is coincident with low coverage zone of corals, suggesting that terrestrial anthropogenic activities are affecting marine corals via SGD.