Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water

Abstract

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.