EFFECTS OF ABIOTIC FACTORS ON MACROINVERTEBRATE DRIFT IN THE LOWER MISSISSIPPI RIVER, LOUISIANA

We assessed the effects of abiotic factors on invertebrate drift compo sition in the lower Mississippi River from November 1984 to June 1985. We sampled drift, measured in situ water quality variables and record ed river stage and discharge 1 night each month. Principal component a nalysis (PCA) and standard multiple regression were used to investigat e temporal relationships between drift and abiotic factors in a large river system. Several drifting taxa (Ephemeroptera, Trichoptera) were significantly, negatively related to discharge and positively related to temperature, current and conductivity. In separate analyses, collec tor, engulfing predator and detritivore feeding groups were significan tly, positively related to stage/discharge and negatively related to c urrent velocity. In addition, each trophic component had several prey and one predator highly correlated with it. Seasonal variability in dr ift was a response to both invertebrate predation and fluctuations in the hydrologic cycle. Our study could not distinguish between two main effects of river discharge on drift: dilution and habitat availabilit y. The possibility that drift may be a separate community apart from t he benthos, and thereby affected strongly by seasonal abiotic factors, is discussed.