Effect of Light Intensity on the Drift of Stream Invertebrates

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Various experiments were conducted in a small Minnesota stream to determine the effect of light intensity on the circadian rhythms in drift of two aquatic invertebrates, the nymph of the mayfly Baetis vagans McDunnough and the amphipod Gammarus pseudolimnaeus Bousfield. Experiments included 1) artifical light in an exclosure which insured that experimental conditions were applied only to the organisms on the stream bottom area in the exclosure, 2) artificial light in the open stream, and 3) artificially produced darkness in an exclosure. A threshold of light intensity which, decreasing, initiated high drift rates and which, increasing, caused cessation of drift, was about 0.1 ft—c (1 lux) for both species. Continuous artificial light above the threshold level for an entire night period, and also for 4 consecutive days, depressed the normal high nocturnal drift rates in the exclosure to near daytime levels; on the fifth night in the exclosure, in darkness, drift was much higher than normal. Continuous artifical light for an entire night period on an open stream riffle also greatly depressed noctural drift. In another open stream experiment, in which organisms drifted from darkness onto a lighted riffle, some settled to the bottom as evidenced by depressed nocturnal drift rates, while other drifted through the light and off the riffle. The effect of shortened and lengthened “night” periods, produced by artificial light and darkness respectively, was to shift the circadian pattersn of drift correspondingly. Rapidly alternating periods (15 min) of light and darkness in the exclosure produced correspondingly alternating drift, with drift rates high in darkness and low in light. When the normal day—night cycle of light was reversed with artificial light and darkness, the phases of the circadian rhythm in drift were also reversed. Observed circadian rhythms in drift appear exogenously controlled; if an endogenous rhythm exists at all, it is very weak and is influenced strongly by environmental light conditions.