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

近年来，有明湾在秋季至初冬期间多次发生大规模赤潮，严重损害了紫菜养殖业。根据1981年以来有明湾10 ~ 12月赤潮发生的历史记录，1998年以后赤潮规模明显增大。然而，在过去的二十年里，从沿海土地通过主要河流装载到有明湾的营养物质的数量几乎没有增加。在这项研究中，我们在2002年4月至2003年4月每月监测24个采样站在有明湾的水质状况。2002年7月、10月至12月，湾内发生了大规模赤潮。在这两个时期，河水的流入稀释了湾内海水表层，造成了海水的分层和表层的高度富营养化，导致了湾内大规模赤潮的发生。1998年以来赤潮规模明显增大，说明1980年以来潮幅和潮流的逐渐减小并不是导致大规模赤潮频繁发生的主要原因之一。研究结果表明，1997年伊沙亚湾堤坝关闭后，有明湾内部的河口环流发生了变化，河水流入形成的低盐富营养化的表层水比关闭前更加停滞。从1998年开始有明湾内部赤潮规模的显著增加很可能是伊沙亚湾闸门关闭引起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.