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

在阿里亚克湾，近年来秋季至初冬期间多次发生大范围赤潮，严重破坏了海藻紫菜养殖。根据自1981年以来有莱克湾在10-12月期间出现红潮的以往记录，自1998年以来，红潮的规模明显增加。然而，在过去的二十年里，通过主要河流从沿海土地装载到有源湾的养殖物数量几乎没有增加。在本研究中，我们于2002年4月至2003年4月期间，每月在有莱克湾的24个采样点监测水质状况。2002年7月、10月至12月，湾内出现了大范围的红潮。在这两个时期，河水的流入冲淡了湾内海水的表层，导致表层水体分层和高度富营养化，导致湾内发生大范围赤潮。1998年以来赤潮规模的显着增加表明，1980年以来潮汐幅值和潮流的逐渐减小不是大型赤潮频发的主要原因之一。研究结果表明，1997年艾萨亚湾堤防合闸改变了阿里亚克湾内侧河口环流，河水流入形成的盐度降低和富营养化的地表水比合闸前更加停滞。从1998年开始，有莱克湾内侧赤潮的规模明显增大，很可能是伊萨亚湾关闭闸门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.