The relationship between the distribution of floating objects and water mass distribution in Ariake sea, Kyushu, Japan is reported. According to the visual observation conducted onboard Ariake ferry in July and October 2001, there frequently exists a convergence line of debris and foams off Taira and Nagasu ports. It suggests the existence of coastal fronts or streaks at that position. CTD observation was also carried out in the same season and a low temperature, low salinity water mass intrusion along the coast of Shimabara peninsula was found in October. It implies that it enhances the surface water convergence.


The behavior of floating objects (e.g., debris of plants, sea weeds, manmade objects, foam etc.) is determined by surface current. In particular, surface convergence results in the concentration of floating objects. Concentration of floating objects is often found along fronts. Garrett and Loader (1981) showed how the surface convergence is formed along the tidal fronts. Officer (1976), Bowman and Iverson (1977) showed how the surface convergence is formed along the estuarine fronts. Surface convergence is also formed even without fronts. In this case the area of surface convergence is called ‘streak’. Pingree et al. (1977) shows that the surface convergence is formed in association with flow around headlands, promontories, banks, shoals and islands. Fronts and streaks play important role in the physical environment of coastal region and the distribution of floating objects may give us useful information about the position and strength of the fronts and streaks. Ariake sea (Fig. 1) is a partially enclosed bay located in the center of Kyushu Island, Japan. It has area of 1700km2 (Coastal Oceanography Research Committee, 1985) and is well known by the large amplitude of tidal sea elevation, strong tidal currents and wide tidal flat.

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