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Long-term changes in Secchi disk transparency in Lake Nakaumi, Japan, from 1932 to the present, which includes the periods before and after the loss of eelgrass (Zostera marina L.) beds, were compiled from previous reports. During the fi rst (July 1932-February 1934 and the second (January 1949-November 1950) periods, the mean transparency was greater than 3 m at all observed stations. Data during the third period (July 1954-March 1956 are only available for the station at the center of the lake. The mean transparency during the third period was signifi cantly lower than that of the second period but higher than the mean during the fourth period. This observation suggests that the decrease of transparency occurred during the third period. Because the decline of eelgrass beds in Lake Nakaumi occurred in the mid-1950s, the decrease of transparency most likely resulted from the shift in primary producers from submerged macrophytes to phytoplankton. Although the maximum transparency sometimes exceeded 3 m, mean transparencies during the fourth period (May 1975-December 2003 were less than 2 m at all stations, significantly lower than those during the fi rst and second periods. The shift in the chief primary producer, from benthic macrophytes to phytoplankton, caused a subsequent shift in secondary producers. The opportunistic fi lter-feeding bivalve Musculista senhousia, regarded as a biofouling species of local fi sheries, increased in Lake Nakaumi. The longterm monitoring data of transparency suggested that restoration of submerged aquatic vegetation may be indispensable for the remediation of the lake environment in this shallow eutrophic lagoon.

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Musculista senhousia is a marine mytilid mussel native to Japan that constructs byssal thread mats (hereafter ''mats'') that destroy local fisheries, and so it is regarded as a biofouling species. To elucidate the factors that prevent the formation of mats of M. senhousia, we first observed the seasonal changes in the populations of M. senhousia in two rivers with similar salinities. Then we compared the condition of Lake Nakaumi, which is covered with dense mats at present, with the condition of the same lake before the decline of the eelgrass beds. The results showed that a particular set of conditions (in terms of salinity, dissolved oxygen concentration and sediment type) did not lead to the dense habitation of M. senhousia in one river, while mat construction was observed in another river with a similar set of conditions. Together with the past description of Lake Nakaumi, we conclude that larvae of M. senhousia head upstream all year round, and can settle as far upstream as the salinity remains within its range of tolerance. Whether it constructs mats depends on the presence of its predators.