Water-Quality Variables Across Sekisei Reef, A Large Reef Complex in Southwestern Japan

At Sekisei Reef in southwestern Japan (24° N), coral cover dramatically decreased in the mid-1980s, probably due to a population outbreak of the coral predator Acanthaster planci. Coral communities subsequently recovered well outside the semiclosed lagoon, but recovery has been poor inside it. Hence, water-quality degradation including eutrophication has been a concern inside the lagoon. In addition, temporal variation in eutrophication parameters is common among high-latitude coral reefs, resulting in difficulties in evaluating them. Therefore, to address these issues, we monitored temperature, salinity, turbidity, chlorophyll-a, NO x -N (NO3-N + NO2-N), and NH4-N concentrations year-round across the lagoon at Sekisei Reef. Turbidity and NO x -N concentration increased with increasing wind velocity, suggesting that variation in turbidity and NO x -N concentrations was attributed to resuspension of bottom sediments, and NO x -N release through regeneration processes of micro-organisms from the s...