LIFE-HISTORY AND PRODUCTION OF FILTER-FEEDING INSECTS ON STONE DIKES IN THE LOWER MISSISSIPPI-RIVER

Population and production dynamics of two filter-feeding macroinverteb rates, the caddisfly Hydropsyche ol-ris and the midge Rheotanytarsus s p., were examined in the lower Mississippi River Miles 510-515. Sample s were collected from September to November 1987, May to December 1988 , and March to May 1989 from stone dikes that protrude into the river. Developmental synchrony was high among individuals in both population s, and seasonal changes in the relative abundance of instars indicated bivoltine and trivoltine life cycles for H. orris and Rheotanytarsus sp., respectively. The size frequency method was used to estimate prod uction (P) and biomass (B) of each cohort in each population. P/B rati os for spring and fall cohorts of H. orris equaled 4.5 and 3.7. Spring , summer, and fall cohorts of Rheotanytarsus sp. had P/B ratios of 4.0 , 4.7, and 4.1, respectively. Annual production, estimated as the sum of cohort production values, of the H. orris and Rheotanytarsus sp, po pulation equaled 10.1 and 0.359 g m(-2) (dry weight). Annual P/B ratio s equaled 10.8 and 16.1 for the H. orris and Rheotanytarsus sp. popula tions, closely matching values expected for bivoltine and trivoltine p opulations, respectively. Annual production of H. orris is in the low end of the range of published estimates for other lotic hydropsychid p opulations inhabiting coarse substratum in warm, lowland rivers of med ium size or small, trophically enriched streams. Annual production of Rheotanytarsus sp. is the first such estimate based on a simultaneous assessment of life cycle and production. Given the lack of naturally o ccurring gravel and cobble substrate in the lower Mississippi River, s tone dikes are an especially important habitat for N. orris and Rheota nytarsus sp.