LARVAL DYNAMICS OF A RIVERINE METAPOPULATION - IMPLICATIONS FOR ZEBRAMUSSEL RECRUITMENT, DISPERSAL, AND CONTROL IN A LARGE-RIVER SYSTEM

Zebra mussels (Dreissena polymorpha) in the Illinois River are likely to function as a metapopulation (a system of local populations connect ed by dispersal). River currents reduce or eliminate internal recruitm ent by local populations, making them dependent upon external recruitm ent from propagules (larvae) produced by upriver populations. Because of the importance of external recruitment, understanding the populatio n dynamics of riverine zebra mussels requires knowledge of larval flux . In 1994 and 1995, larval drift past a fixed site on the Illinois Riv er was characterized by pulses of similar-sized individuals. These pul ses were composed primarily of D-stage (straight hinge) larvae, not ye t competent to settle. The continual passage of pulses of young larvae suggests that the upriver source population(s) spawned in frequent di stinct bursts throughout the summer months rather than spawning only o nce or twice. Overall larval abundance was a poor predictor of availab ility of competent larvae. Although larvae were frequently present in high numbers, larvae >200 mu m in shell height (competent to settle) w ere present only sporadically at our fixed study site from May to Nove mber in 1994, and present only from June to early July in 1995. Settle ment at a given site on the Illinois River is likely to be sporadic ra ther than constant. Colonization and population growth at a given site will occur only if suitable habitat and environmental conditions are available during the windows when competent larvae are available. Chan ges in mean size as a larval cohort drifted downriver suggested a grow th rate of 6.72 mu m/d. At this growth rate, most larvae probably trav el a minimum of 304.6 km (190.4 miles) before settling. Thus, larvae s pawned in the main channel of the lower 70% of the Illinois River are most likely to settle in the Mississippi River. We suggest that coloni zation and population growth in the upper 70% of the Illinois River ar e strongly dependent upon larvae produced at upriver source sites in L ake Michigan and the Chicago area waterways. Control of zebra mussels in a given stretch of the river will depend upon control of the uprive r source populations.