Abstract
Abundance of bottom organisms in Lewis and Clark Lake was measured by transect—station and randomly—stratified sampling methods. Samples were collected with modified orange—peel and Ekman dredges, and cores were used to delineate bottom types. Hexagenia nymphs have reached an abundance on the inundated flood plain comparable to populations in natural lakes with suitable habitat. Length frequencies of nymphs, plotted at monthly intervals throughout the summer, suggest a 2—yr life cycle for the major portion of the populations. Concentrations of the fall progeny (1—10 mm) indicate that the early distribution of nymphs is a function of imago activity during oviposition. Distribution of the overwintering nymphs differs considerably from that of the previous fall. Evidence of migration from densely populated shore areas was observed. An interaction of depth, wind, current, and bottom type is influential in redistributing the population.
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