Diel and Seasonal Drift of Zooplankton in a Headwater Stream

Abstract

Diel periodicity of zooplankton drift was examined in a third order reach of the Illinois River, Arkansas. Quantitative samples were taken every 2 h for a 24-h period during June and January, to determine if zooplankton drifted in greater numbers at night. Significantly more zooplankton (P < 0.05, Wilcoxon) were collected at night during both sampling periods. Day-night densities also were significant for total Copepoda and nauplii in June and January, Euchlanis dilatata in June, and Rotatoria and immature Cladocera (Bosmina and Daphnia) in January. Apparent but nonsignificant differences were observed for Cladocera in June and January, and Rotatoria in June. Maximum densities during June were recorded from 2000-2200 h for all major groups. Maximum rotifer density was strongly influenced by distinct peaks of bdelloids and E. dilatata at 2000 and 2200 h, respectively. January samples were more variable, but maximum densities for Copepoda and total zooplankton were recorded from 2100-0100 h, and Rotatoria from 2300-0100 h. Cladocera maximum densities occurred at 1900 and 0300 h coinciding with distinct peaks for Bosmina longirostris. Drift of Keratella cochlearis, an abundant rotifer, peaked at 2300 h. Greater densities (ca. 7 x) of drifting zooplankton were recorded in January than in June. Diel pulses of drifting zooplankton at night increased the animal component of the seston. Zooplankton may significantly influence the trophic dynamics and activity patterns of filterfeeding macroinvertebrates and planktivorous fish in headwater stream communities. INTRODUCTION Although the literature is seemingly replete with studies of downstream drift by stream invertebrates (e.g., see Waters, 1965, 1972; Elliot, 1967, 1973; Muller, 1974; O'Hop and Wallace, 1983; Allan et al., 1986; and literature cited therein), relatively few have reported occurrence of drifting zooplankton in headwater reaches (but see Williams and Hynes, 1976; Williams, 1977). Yet, zooplankton (defined sensu Pennak, 1966) may play a pivotal role in the trophic dynamics and community structure of stream ecosystems by regulating the flow of essential biochemicals through the food chain. Most stream macroinvertebrates and fish drift downstream at sometime during their life history with distinct diel periodicity with far greater numbers drifting at night (Waters, 1972; Brown and Armstrong, 1985). Although other plausible explanations for the nocturnal drift pattern have been presented (e.g., Wiley and Kohler, 1980, 1984; Allan et al., 1986), some invertebrates and fish may drift downstream as a result of their feeding activities (Ploskey and Brown, 1980; Armstrong and Brown, 1983). Zooplankton in lakes and reservoirs exhibit a similar rhythmic diel activity pattern by migrating nearer the surface at night (Pennak, 1944, 1949; Cushing, 1951; McNaught and Hasler, 1963; George and Fernando, 1969; Short, 1977). Similar vertical migration periodicity has been observed for zooplankton in large rivers (e.g., Volga River; see Hynes, 1970). If zooplankton in headwater streams exhibit the same diel migratory behavior, as suggested by Brown et al. (1989), then I Present address: Department of Biological Sciences, Murray State University, Murray, Kentucky 42071 2 Present address: Department of Wildlife and Fisheries, P.O. Drawer LW, Mississippi State University, Mississippi State, Mississippi 39762