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

  • Morimoto N
  • Furushima Y
  • Nagao M
 et al. 
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At Sekisei Reef in southwestern Japan (24 N), coral cover dramati-cally 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 com-mon among high-latitude coral reefs, resulting in difficulties in evaluating them. Therefore, to address these issues, we monitored temperature, salinity, turbid-ity, chlorophyll-a, NO x -N (NO 3 -N þ NO 2 -N), and NH 4 -N concentrations year-round across the lagoon at Sekisei Reef. Turbidity and NO x -N concentra-tion increased with increasing wind velocity, suggesting that variation in tur-bidity 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 sediments and reef frameworks, respectively. In contrast, variation in chlorophyll-a and NH 4 -N concentrations appears to be mainly con-trolled by the seasonality of temperature and irradiance. Long retention time of seawater inside the lagoon seems to have enhanced NH 4 -N assimilation and in-crease of phytoplankton during summer. Inside the lagoon, turbidity, NO x -N, and summer chlorophyll-a concentrations were higher, and variation in temper-ature was larger than outside it. Although water quality appears not to be seri-ously degraded, multiple effects of these water-quality variables might have negatively affected recovery of coral communities inside the lagoon. Recent ex-pansion of land use on nearby islands might have contributed to water-quality degradation inside the lagoon. Many coral reefs are threatened by degra-dation both globally and locally (e.g., Wilkin-son 2004, Knowlton and Jackson 2008). At Sekisei Reef in East Asia (Figure 1), the coral cover has dramatically decreased, probably due to a population outbreak of the coral-eating starfish Acanthaster planci in 1983, and recurrent coral-bleaching events after 1998

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  • Naoko Morimoto

  • Yasuo Furushima

  • Masayuki Nagao

  • Takahiro Irie

  • Akira Iguchi

  • Atsushi Suzuki

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