Changes in red tide occurrence and organisms responsible for declining eutrophic level in hyper-eutrophic Dokai Bay, Japan

¹Department of Environmental Science, Fukuoka Women's University, Higashi-ku, Fukuoka 813-8529, ²Department of Life and Environment Engineering, The University of Kitakyushu, Kitakyushu, Fukuoka 808-0135, ³Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido 060-0810 Japan

Water quality and red tide occurrence were monitored monthly at Dokai Bay, Kitakyushu City, Japan, for 28 years (1980∼2007). Although total nitrogen (TN) and total phosphorus (TP) concentrations decreased significantly over this period, red tides formed during the warm season every year. A comparison of the community of red-tide organisms between periods of high concentrations of TN (9.1 mg L⁻¹) and TP (0.170 mg L⁻¹) (1988∼1992) and low concentrations of TN (2.0 mg L⁻¹) and TP (0.121 mg L⁻¹) (2003∼2007) showed that the diatoms Skeletonema spp. were dominant during both periods, but their cell density and frequency of forming red tides were significantly lower during the period of low TN and TP. Furthermore, small-sized diatoms, including Thalassiosira spp., Chaetoceros spp., Cyclotella sp., and the raphidophyte Heterosigma akashiwo, increased their predominance of red tide component organisms and occurrence frequency of their cell densities to form monospecific red tides during the low TN and low TP period. These findings suggest that the decrease in TN and TP concentrations in Dokai Bay did not affect the seasonality or the frequency of red tide formation, but did result in increased species diversity of red-tide organisms.

  [BACK]  [TOP]