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.
The daily and seasonal movements of the walleye, Stizostedion vitreum, were studied by use of a VHF radio‐tag system for three seasons each year during 1973 and 1974 in Lake Bemidji, a large eutrophic lake in north central Minnesota. The walleye moved parallel to the shore and did not show a diel on‐shore and off‐shore movement pattern. Submerged projections on the lake bottom such as rock bars or spits tended to serve as boundaries of the ranges within which the test fish moved. Large movements of tagged fish coincided with extensive periods of heavy cloud cover and precipitation. Wind speed and direction also appeared to influence fish movement especially when the fetch was large. Marked daily changes in water temperature and dissolved oxygen concentration did not occur, and, thus, had little effect on the movement of test fish. There were slight seasonal changes in the extent of the daily movement of test fish, but the depth ranges shown by the tagged fish did not change greatly for each season.
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