Change of water structure and benthic ecosystem in Ariake Bay and their causes

有明湾近年来秋冬初冬频繁发生大规模赤潮，严重破坏了海苔（一种红藻，紫菜）养殖。根据1981年以来有明湾10月至12月赤潮发生的记录，赤潮规模从1998年开始明显增加。然而，在过去的二十年里，从沿海土地通过主要河流流入有明湾的营养物质的数量几乎没有增加。本研究于2002年4月至2003年4月对有明湾24个采样站的水质状况进行了逐月监测。2002年7月和10月至12月，海湾内部发生了大规模赤潮。在这两个时期，河水的流入稀释了海湾内部海水的表层，造成了海水的分层和表层高度富营养化的情况，导致海湾内部发生了大规模的赤潮。1998年以来赤潮规模的显著增加表明，1980年以来潮幅和潮流的逐渐减小并不是大赤潮频繁发生的主要原因之一。研究结果表明，1997年艾萨哈雅湾决堤关闭改变了有明湾内部的河口环流，河水流入形成的盐度降低的地表水比决堤关闭前更加停滞，富营养化状况明显改善。1998年以来，有明湾内部赤潮规模的显著增加很可能是伊沙哈亚湾闸门关闭引发的一系列现象的结果之一。

In Ariake Bay, large scale red tides have repeatedly occurred from autumn to early winter in recent years, and seriously damaged the nori (a red alga, Porphyra) culture. According to the previous records on the occurrence of red tide in Ariake Bay between October and December since 1981, the scale of red tide markedly increased from 1998. However, the amounts of nutrients loaded on Ariake Bay from the coastal lands through the major rivers have increased little in the past two decades. In this study, we monitored the water conditions at 24 sampling stations in Ariake Bay monthly from April 2002 to April 2003. The large scale red tides occurred in the inner part of the bay in July and October to December, 2002. In these two periods, the inflow of the river water diluted the surface layer of the sea water in the inner part of the bay, caused the stratification of the water and highly eutrophicated conditions in the surface layer, and resulted in the occurrence of large scales of red tides in the inner part of the bay. The marked increase of the scale of red tide from 1998 indicates that the gradual decrease of the tidal amplitude and current since 1980 i s not one of the main causes of the frequent occurrence of the large red tides. The results of this study suggest that the estuary circulation of the inner part of Ariake Bay was changed by the closure of the gates of the dike in Isahaya Bay in 1997, and the surface water with lowered salinity and eutrophic conditions formed by the inflow of the river water became more stagnant than before the closure of the gates. It is very likely that the marked increase in the scale of the red tide in the inner part of Ariake Bay from 1998 is one of the results of a series of phenomena originated from the closure of the gates in Isahaya Bay.