Distribution and Long-term variation of Turbidity in Tokyo Bay

The seawater of Tokyo Bay is very highly turbid due to anthropogenic activities. We examined the distribution, properties, and long-term variations of turbidity in Tokyo Bay. A transmissometer of three wavelengths was used in the investigation of the distribution and the properties of turbidity. The long-term variation of turbidity was analyzed based on a Secchi disc depth data set collected from 1948 to 2006 by the Chiba Prefectural Fisheries Research Center. 1. Beam attenuation coefficient, Secchi disc depth, color of water by the Forel-Ule scale, suspended solid (SS), and chlorophyll a (Chl. a) concentration at the innermost part of Tokyo Bay (off Urayasu) in 2005-2006 indicated 7.5 m(-1), 1.3 m, No. 18, 9.0 mg L-1, and 59.1 mu g L-1 in summer, and 2.3 m(-1), 2.5 m, No. 16, 5.1 mg L-1, and 27.1 mu g L-1 winter, respectively. 2. Contributions to the turbidity caused by dissolved organic matter, particulate organic matter, and particulate inorganic matter in summer reached values of 5%, 85%, and 9% at the innermost part of the bay (off Kawasaki), and 8%, 50%, and 42% at the mouth of the bay (off Kyonan), respectively. 3. The Secchi disc depth of Tokyo Bay decreased rapidly in the 1960s, and indicated the lowest value in 1964. Since then, the Secchi disc depths in summer have not shown marked changes. But the values in winter have increased since the end of the 1960s. The present values in winter indicated measurements close to those observed in the 1950s. 4. If as a future aim of water quality in Tokyo Bay a Secchi disc depth of >1.5 m, that is, a state under which red tide conditions are not generated, the concentrations of Chl. a and SS in summer must be decreased by 34% and 21.5% from the present value, respectively.