Spatiotemporal changes in distributions of the two dominant bivalve species in an estuarine river connecting two brackish lakes, Japan

Spatiotemporal changes in the biomass and abundance of the two dominant bivalve species <i>Corbicula japonica</i> and <i>Arcuatula senhousia</i> were investigated at 14 stations positioned along the Ohashi River from November 2005 to August 2007. <i>Corbicula japonica</i> predominantly inhabited the upstream section of the primary branch and Kensaki stream of the Ohashi River, whereas <i>A. senhousia</i> predominantly inhabited the downstream section of the Ohashi River. However, the distributions were highly variable among stations and seasons. Intrusions of saline water from Lake Nakaumi varied with astronomical and climatological factors, and consequently, the halocline of the Ohashi River is subject to spatiotemporal changes. Changes in distributions of <i>C. japonica</i> and <i>A. senhousia</i> occurred in relation to a flood event, indicating that environmental disturbances affect the distributions of the two bivalve species. Responses of the two species to continual saline water intrusion were also observed during the period of normal flow conditions. We suggest that the distribution of the two species is closely related to the dynamics of the ambient saline water, which is the most important factor correlated to changes in the distributions of <i>C. japonica</i> and <i>A. senhousia</i> in the Ohashi River.