[A-HW32] 水圏生態系における物質輸送と循環：源流から沿岸まで

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

*中屋 眞司¹、永田 翔¹、豊田 政史¹、安元 純²、新谷 哲也³、中村 崇⁴ (1.信州大学工学部水環境・土木工学科、2.琉球大学農学部地域農業工学科、3.首都大学東京都市環境学部 都市基盤環境学科、4.琉球大学理学部海洋自然科学科)

キーワード：海底湧水、サンゴ、与論島、数値シミュレーション

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.