Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water
Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water
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DOI：
10.3800/pbr.1.3
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发表时间：
2006-02
影响因子：
0.6
通讯作者：
H. Tsutsumi
中科院分区：
文献类型：
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作者：
H. Tsutsumi
有明湾位于日本西部九州岛西海岸外，是一个面积约1700平方公里的典型封闭性近海海域。在大潮时，湾内部分区域的潮差可达约6米，因此在退潮时，湾岸沿线会出现约2万公顷的潮滩。潮滩以及潮滩外的浅水区是极具生产力的区域，被用于各种渔业活动。然而，有明湾的生态系统如今正处于危急状态。湾内沙质潮滩上的底栖动物数量已显著减少。这一现象以面向有明湾的熊本县沙质潮滩上最受欢迎的菲律宾蛤仔捕捞量急剧下降为代表。1980年之前，沙质潮滩的年捕捞量超过5万吨，但在1995年至1997年间下降到仅约400吨。在蛤仔密集种群崩溃的沙质潮滩上，大型底栖动物极少出现。自20世纪60年代以来，大量的沙石从流入有明湾的河流中被采集。很明显，沙质潮滩上新沉积的沙子显著减少是菲律宾蛤仔密集种群崩溃以及沙质潮滩上其他大型底栖动物数量显著减少的主要原因之一，因此，恢复沙质潮滩底栖生物群落的唯一有效措施似乎是用从湾外海底采集的天然沙子覆盖沙层。在有明湾的潮下带区域，自1998年以来，浮游植物秋季水华的规模显著增大，赤潮几乎覆盖了整个内湾区域，尽管过去三十年来通过主要河流从陆地输入到湾内的营养物质一直稳定波动。紫菜养殖因两次大规模秋季赤潮（2000年和2002年）的发生而受到严重破坏。至少从2001年起，在初夏时节，湾内部分区域出现了缺氧的底层水，对底栖生态系统造成了严重的环境干扰。这两种现象通常都发生在水体因表层盐度降低而分层的时候。这些事实表明，从20世纪90年代末起，湾内水体分层加剧。在有明湾西侧的内湾——谏早湾，自1986年起开展了一项大规模的潮滩围垦项目，以建造陆地和一个总面积约3500公顷的淡水池塘，1997年4月堤坝的闸门关闭以降低堤坝内的水位用于围垦。最近在湾西海岸的一些区域观测到潮流减弱。很可能是谏早湾地形改变所导致的水文条件变化，在陆地营养物质输入没有增加的情况下，引发了有明湾内湾区域大规模秋季赤潮的出现以及夏季缺氧底层水的产生，进而导致了有明湾内湾区域水体分层的加剧。
Ariake Bay is located off the west coast of Kyushu, western Japan, and is a typical enclosed coastal sea of approximately 1,700 km2 in area. The tidal amplitude reaches approximately 6 m in the inner parts of the bay at the spring tide, and consequently approximately 20,000 ha of tidal flats appear along the coast of the bay at ebb tide. The tidal flats and the shallow waters offshore from the tidal flats are extremely productive areas, and utilized for various fisheries. However, the ecosystem of Ariake Bay is now in critical condition. The abundance of benthic animals has decreased markedly on the sandy tidal flats in the bay. This phenomenon is represented by the drastic decrease in the harvest of the most popular short neck clam, Ruditapes philippinarum, on the sandy tidal flats of Kumatoto Prefecture facing Ariake Bay. The annual harvest on the sandy tidal flats exceeded 50,000 tons until 1980, but decreased to only around 400 tons between 1995 to 1997. On the sandy tidal flats where the dense clam population collapsed, very few macro-benthic animals occurred. Since the 1960s, a large amount of sand and gravel has been collected from the rivers that flow into Ariake Bay. It is apparent that the marked decrease in newly deposited sand on the sandy tidal flats is one of the main causes of the collapse of the dense population of the short neck clam and marked decrease of other macro-benthic animals on the sandy tidal flats, and thus it appears that the only effective measure to recover the benthic communities on the sandy tidal flats is creating sand covers with natural sands collected from the sea floor offshore of the bay.In the subtidal areas of the bay, the scale of the autumn bloom of phytoplankton has increased markedly and red tides have covered almost the entire inner bay area since 1998, although the nutrient input from the land to the bay through the major rivers has fluctuated stably over the past three decades. The nori culture was seriously damaged by the occurrence of a large-scale autumn red tide on two occasions (2000 and 2002). In early summer, hypoxic bottom water has occurred in the wide areas of the inner parts of the bay since at least 2001, causing a serious environmental disturbance to the benthic ecosystem. Both of these phenomena commonly occurred when the water was stratified mainly due to the decrease of salinity in the surface layers. These facts indicate that the stratification of the water was enforced in the inner parts of the bay from the end of the 1990s. In Isahaya Bay, an inner bay of the west side of Ariake Bay, a big reclamation project of the tidal flats to create land and a fresh water pond of approximately 3,500 ha in total area has been conducted since 1986, and the gates of the dike were closed to lower the water levels inside the dike for reclamation in April 1997. A decrease in the tidal current was recently observed in some parts of the west coast of the bay. It is very likely that the changes of the hydrographical conditions caused by the modification of the topography of Isahaya Bay are responsible for the development of the water stratification in the inner parts of Ariake Bay, following the occurrence of the large-scale autumn red tides without any increase in the nutrient input from the land and the hypoxic bottom water in the summer.