Zebra mussel (Dreissena polymorpha) seasonal colonization patterns in a sub-tropical floodplain river

Zebra mussel (Dreissena polymorpha) seasonal colonization patterns, growth and habitat preferences were determined in a sub-tropical floodplain river at the southern edge of its distribution in North America during 1995-96 (Atchafalaya River Basin, Louisiana). Zebra mussel movement into subtropical areas represents a major frontier for this species worldwide. The onset of adult zebra mussel colonies occurred when the minimum daily temperature dropped below 31°C and dissolved oxygen levels rose above 6.5 mg 1^-1 in the fall. By mid-winter, mussel populations were established at lateral distances > 10 km from the main river channel. Mussel growth occurred throughout the winter with an increase in growth in April and May. Adult mortality occurred during May-August as dissolved oxygen levels declined and minimum daily temperature warmed above 29°C in the floodplain and 32.5°C in riverine sites. Limiting factors responsible for the seasonal pattern include temperature and dissolved oxygen tolerances experienced during summer months in the ARB. Summer water conditions apparently preclude establishment of resident zebra mussel populations in the Atchafalaya floodplain. Naturally occurring seasonal patterns in temperature and dissolved oxygen in floodplain rivers may have implications for the expansion of this exotic mollusk in warmwater systems with source colonies restricted to mainstem rivers and seasonal sinks in floodplain regions.