Evaluation of Dredged Sediment as a Silt and Clay Source for Artificial Tidal Flats

Abstract

This research was carried out to investigate the feasibility of using dredged sediment (DS) as the additive of silt and clay for artificial tidal flats. A series of experiments conducted in the real seashore and in a tidal flat simulator demonstrated that DS could be used for artificial tidal flats. Furthermore, the tidal flat simulator experiment showed that the macrobenthos population increased with the DS addition. However, use of conditioners made of paper sludge and poly-aluminum chloride for DS granulation treatment was associated with a time lag in the growth of the macrobenthos: these agents may be released during the course of the experimental period. Possible reasons for the increased polychaete and gastropod abundances in the artificial tidal flats might be an increase in supplied organic matter and stimulated benthic microalgae growth due to the DS addition. Finally, a growth test was carried out for the short neck clam Ruditapes philippinarum, which showed that it can grow in artificial tidal flats to which DS has been added. Too much DS, however, may suppress its growth.Keywords: artificial tidal flat, benthic ecosystem, dredged sediment. INTRODUCTIONIn the 1940s, the total area of natural tidal flats in Japan was approximately 82,600 ha. By the 1980s, however, about 40% had unfortunately disappeared (Kimura, 1994). In recent years, useful functions of tidal flats such as biological production, water quality clarification and recreation have been recognized more and more (Miyoshi et al., 1991;Kimura et al., 1992;Imamura, 1997;Evans et al., 1998;Ishii et al., 2001;Morrison et al., 2002;Tiner et al., 2003;Sakamaki et al., 2006;Magni and Montani, 2006). For example, Kimura et al., (1992) compared water quality at the seashore where an artificial tidal flat existed, and its offing region. They found that COD in the seashore was significantly lower than that in the offing region. This difference in the water quality in the investigated area was accounted for by increased capability of the artificial tidal flats for degrading organic matter. Benthic organisms have been investigated as the cause of the water clarification capability of artificial and natural tidal flats (Miyoshi et al., 1991, Imamura, 1997. Because such useful functions have highlighted the importance of tidal flats, in 2003 the Japanese government enacted a law for the promotion of nature restoration, which charges those engaged in development activities in coastal areas with mitigation of damage to or lost of tidal flats.