Ja. Stoeckel et al., LARVAL DYNAMICS OF A RIVERINE METAPOPULATION - IMPLICATIONS FOR ZEBRAMUSSEL RECRUITMENT, DISPERSAL, AND CONTROL IN A LARGE-RIVER SYSTEM, Journal of the North American Benthological Society, 16(3), 1997, pp. 586-601
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.