1999 Volume 54 Pages 36-43
The Yatsu Tidal Flat is a small tidal wetland of approximately 40 ha, located in the middle of landfill along the northern shore of Tokyo Bay. The flat is connected to the bay through two narrow tidal waterways. Originally, extensive tidal flats rimmed the entire coast of Tokyo Bay. Almost all of these, however, have been landfilled, and the Yatsu Tidal Flat was left stranded in the center of the landfill. This small flat is now the most important feeding habitat for shorebirds along the northern bay. The flat was registered under the Ramsar Convention in 1983. Macrobenthic animals are vital prey for shorebirds, and they are also important elements in the tidal flat ecosystem. Observations of the macrobenthic fauna at the Yatsu Tidal Flat were conducted in 1974-75, one year after the flat was enclosed by the surrounding landfill, by a team of researchers. A similar survey was later implemented in 1994-95 by the Japanese Environmental Agency and local governments. Environmental conditions of the flat, such as particle size distribution of the sediment and nutrient load, changed greatly between these surveys. The authors also conducted observations at the Yatsu Tidal Flat during the winter of 1986-87. Both the horizontal distribution of macrobenthic animals and their vertical distribution in the sediment column were observed. The results were correlated with sediment conditions (redox potential and particle size). The macrobenthic fauna was dominated by the polychaetes Tylorrhynchus heterochaetus and Neanthes succinea. Tylorrhynchus heterochaetus was restricted to the margin of the flat, which consisted of fine-sandy aerobic sediments; N. succinea was found in the central part of the flat, which consisted of reduced muddy sediments. In general, the number of species tended to decrease in more reduced sediments, but the largest number of species was found in an area where the sediment was heavily reduced in its deeper layers but aerobic near the surface. In addition, oligochaetes and several species of small polychaetes, such as Sigambra sp. and Capitella sp., occurred in very reduced sediment (Eh less than-200mV). The results of our observations indicate that aerobic conditions near the surface are important in maintaining the macrobenthic population in an organically polluted tidal wetland. A comparison of the results of the three surveys that have been conducted during the 20 years since the flat was enclosed shows that the diversity of the macrobenthos increased gradually, but that the size of the nereidid polychaete population, which provides the most important food source for shorebirds, has decreased in recent years. This decrease may be due to a reduction in the loads of organic matter and inorganic nutrients entering the flat.
