EFFECTS OF LIFE-HISTORY ATTRIBUTES AND STREAM DISCHARGE ON FILTER-FEEDER COLONIZATION

Considerable annual variation in precipitation and stream discharge occurred during a 5-yr study of the abundance and size-structure of Simulium spp., Hydropsyche betteni, and Cheumatopsyche spp. larvae. All three taxa exhibited dramatic increases in density associated with increased precipitation and elevated stream discharge. However, Simulium spp. and the two hydropsychid taxa exhibited different seasonal occurrences of early larval instars and peak larval abundance. Simulium spp. produced larval propagules as early as May and continued the production of new larval cohorts throughout the summer, with highest density occurring in May-June. In contrast, H. betteni and Cheumatopsyche spp. produced only one cohort of larval propagules during summer, beginning in June and July, with the highest density in July-August. Short-term (24-d) experimental manipulations of flow regime in a riffle during spring (May-June) and summer (July-August) showed that elevated stream discharge had a strong, positive effect on filter-feeder colonization during both seasons. Taxonomic composition and temporal variation in colonization of filter feeders differed dramatically, however, in spring and summer. In spring, only Simulium spp. colonized rock surfaces; because of their rapid development, pupation, and emergence, they exhibited large temporal variation in abundance, particularly under elevated discharge. In summer, Simulium spp., H. betteni, and Cheumatopsyche spp. all colonized rock surfaces. However, H. betteni was the predominant colonist and its relatively slow developmental rate resulted in reduced temporal variation in filter-feeder abundance. These results indicate the colonization, temporal persistence, and taxonomic composition of filter feeders in riffles is strongly influenced by the life history characteristics of the taxa, particularly their timing of propagule production and subsequent developmental rate, and the temporal distribution and magnitude of discharge in the stream.