HR: 0830h
AN: H31C-0486    [PDF]
TI: Environmental change and human impact on mollusk assemblages in brackish Lake Nakaumi, Japan
AU: Seto, K
EM: seto@soc.shimane-u.ac.jp
AF: Koji Seto, Shimane Univ. Reserach Center of Coastal Lagoon environments 1060 Nishikawatu-sho, Matue, 690-8504 Japan
AU: Sato, T
AF: Takaharu Sato, Hiroshima Univ. Faculty of Integrated Arts and Science Kagamiyama 1-7-1, Higashihiroshima, 739-8521 Japan
AB: Biofacies of brackish environments are sensitive to environmental change because they are affected by the mixing characteristics of marine and fresh-water masses in addition to changes in climate. We examined the diversity of bivalves in a brackish lagoonal lake with complex topography in order to evaluate the effect of human modification of flow in the lake. Three sediment cores (X, N1 and N2) were collected from Lake Nakaumi, one of the largest brackish lakes in Japan, to analyze shell assemblages in three different settings. Core X was collected near an inflowing river from another brackish lake (Lake Shinji), N1 was collected from the central part of the lake, N2 was located off a delta created by the largest river that flows into the lake. Core samples contain a large number of fossil shells, and bivalve assemblages were analyzed at 5 cm intervals. In each sample, the numbers of shells are counted; bivalves having hinges were counted as one individual. Bivalve fossils assemblages were then analyzed by using Morisita diversity index (Morisita, 1987) that describes the diversity of the assemblage. High amplitude fluctuation in the index value was observed from the core X, implying frequent disturbance associated with Lake Shinji. The lower sections of both N1 and N2 are characterized by smaller variation in the index value, followed by high amplitude fluctuation in the upper portions. The amplitude is larger in the upper part of N2 than in N1. The distinct change in the index value between the lower and the upper part of N1 and N2 is most likely associated with artificial river course change. Only Theora fragilis, which has a high tolerance of environmental change, is found through the cored intervals, implying that this river diversion had a great impact on the lakeOs mollusk community. Benthic surveys have shown that the diversity of the mollusk fauna in Lake Nakaumi has dropped significantly in comparison to the fauna represented by core samples. Both core data and recent shell assemblage data indicate that other factors have affected mollusk diversity in addition to the river diversions of 1665 and 1840.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 9320 Asia
SC: Hydrology [H]
MN: 2003 Fall Meeting