As consumers, waterfowl play an important role in the regulation of benthic community structure and nutrient dynamics of coastal ecosystems. In this study, the amount of benthic organisms consumed by birds in a brackish lake, Lake Akkeshi in northeastern Japan, was estimated from direct observation and from energy requirement equations. The 19 avian species observed foraging in the study site were categorized into four guilds: predators foraging on other avian species, piscivores, invertebrate feeders foraging mainly on bivalves, and herbivores consuming seagrass and seaweed. Each guild comprised from one to three tropho-species, three of which were investigated in detail. The dominant piscivore, the grey heron, consumed about 100 kg of fish per day in spring and summer. The dominant invertebrate feeders, diving ducks, consumed about 6500 kg and 2000 kg of clams per day in autumn and winter, respectively. Herbivorous ducks consumed about 9000 kg of seagrass and seaweed per day in autumn and about 900 kg in winter. Nearly all of these amounts estimated from the observations were very similar to those estimated indirectly from energy requirement equations. Even when the agreement was very similar, however, some of the assumptions used for the energy requirement equations did not correspond to the actual energetic requirements of birds in the field. We conclude, therefore, that although the amount of benthic organisms consumed by birds can be estimated using energetic requirement equations, selection of a particular energetic requirement equation must take into account the field condition of the birds.

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A field study was conducted on the spatial distribution and individual activity range of the potamidid gastropod Cerithidea rhizophorarum in the estuary of the Kushida River, Mie Prefecture, central Japan. Large snails occurred widely in various habitat conditions in and around the salt marsh, whereas small snails were confined to the non-vegetated area, especially a puddle, around the salt marsh. Cross-shore distribution of the snails showed peak abundance in the lower vegetated to upper non-vegetated area in all four seasons, but small snails occurred only in the non-vegetated area. Reproductive activities such as mounting and spawning were observed in both the lower vegetated and upper non-vegetated areas. A tracking survey of marked large snails for 72 days revealed that the individual activity range was confined to 1-3 m in the vegetated area and 2-5 m in the non-vegetated area.

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The inner part of Isahaya Bay was isolated from the rest of Ariake Bay by a reclamation dike in April, 1997. Thereafter, several changes in environmental conditions, such as a drastic increase in red tides and hypoxic water, were observed in Ariake Bay. We periodically monitored the number of species and mean densities of bivalves in two regions: (1) 20 fixed stations around the mouth of Isahaya Bay from June, 1997, to November, 2003; and (2) 24 fixed stations around the mouth of the Mizunashi River, Shimabara, located about 20 km south of the mouth of Isahaya Bay, from November, 1995, to December, 2001. In June, 1997, hypoxic water masses appeared in both the Isahaya and Shimabara regions, and most of the bivalve species distributed near the hypoxic regions decreased rapidly from 1997 to 1999. The number of species and mean density of bivalves reached a minimum in June, 2000, in the Isahaya region and in December, 1999, in the Shimabara region. Thereafter, since 2000, these values have been increasing in both regions. In the Shimabara region, most of the species dominant before 1999 recovered by December, 2001. In the Isahaya region, however, hypoxic water also recurred in June of both 1999 and 2001; there, only a few bivalve species increased rapidly at first and then subsequently decreased in abundance until November, 2003. In conclusion, if there is no amelioration of the environmental changes that have been occurring in Ariake Bay since 1997, we predict that a few opportunistic species of bivalve will continue to increase rapidly while other species disappear completely from the Isahaya region.

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