Effects of dam construction on sediment phosphorus variation in a semi-enclosed bay of the Seto Inland Sea, Japan

This study investigated the effects of dam construction on sediment phosphorus concentrations in a semi-enclosed bay in western Japan. Long sediment core samples spanning over 100 years were collected from the bay, and their P fractions were analyzed. Sediment P concentrations and the P accumulation rate in an artificial lake increased after the construction of a coastal dam in 1959. The amount of P accumulated in the 60 years after the dam construction was ∼1.7 times that prior to the dam construction. Moreover, concentrations of mobile forms of P, primarily redox-sensitive P species, were higher in freshwater sediments above the dam than in saline sediments below the dam. The redox-sensitive forms of P in freshwater sediments increased sharply after the dam construction, from 100 to ∼900 μg/g, accompanied by a decrease in chloride (Cl−) concentrations to <2000 mg/l. In the artificial lake, the maximum values of total P (TP) and redox-sensitive P concentrations were ∼1200 μg/g and ∼900 μg/g at depths of 23 cm and 3 cm, respectively. Smaller peaks observed in the TP and redox-sensitive P concentration values likely corresponded to the recycled P released from sediments. The maximum values corresponded to hypereutrophic conditions that were caused by extensive discharges of sewage during the 1970s. The lake has been gradually recovering from these hypereutrophic conditions, as observed from the trophic state index. However, despite a substantial decrease in P loading after the 1980s, the lake still has a high trophic level. The presently high mobile P concentrations in surface sediments may lead to high-magnitude P releases with environmental changes in the future.