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Quantitative studies of stream drift with particular reference to the McLay model McKone, Warren Douglas

Abstract

This study was concerned with problems of measuring stream drift and in particular attempts to evaluate the McLay (1970) model of the distance drifted by stream invertebrates. During 1971-1973 observations and experiments were conducted at two spawning channels (Jones and Gates Creek) and at the Abbotsford Trout Hatchery, all located in southwestern British Columbia in the Fraser River drainage. Most species of invertebrates drifted in increasing numbers shortly after sunset. Variations occurred in the numbers of various species along the length of Gates Creek Channel, although water flow, depth, temperature, gravel depth and stream cover were similar throughout the channel. The distribution of organisms was related to detrital content which was high in the upper reaches but was replaced by algae in the lower end of the channel. Daily variations in the drift rate occurred for various species although no changes were observed in physical conditions along the channel. The McLay model thus makes assumptions of uniformity of distribution which may not be met in field conditions. Laboratory studies suggested that Baetis drift at a low constant rate until the carrying capacity of the gravel is reached. Carrying capacity is higher during the day than at night, reflecting higher activity at night. When density exceeded carrying capacity, a higher constant drift rate occurred. Some species introduced into artificially induced laminar flow actively moved toward the substrate by swimming, and drifted a shorter distance than that predicted by the McLay model, while by contrast those that were passive drifted farther than predicted. With turbulence, both types obtained sites on the bottom within a short distance. Increasing substrate density of a particular species caused added animals to drift farther before they could find sites for settling. The time required for an introduced pulse to pass downstream was longer than predicted considering stream velocity. Additionally, a pulse of drift causes animals to leave areas downstream. For several species the distance drifted at different velocities was found to be best described by a power function suggested by Elliott (1971b), but modified by the use of mean rather than modal velocity. The rate at which dead Epeorus left the drift was linearly related to distance. Thus, leaving the drift was not a random process for dead animals. Various combinations of disturbance of the substrate and the complete filtration (blockage) of drift, including the maintenance of a blockage for-ten days, were carried out under differing conditions of light intensity and water velocity. The assumption of the McLay model of additivity of pulses of animals was validated but the quantity introduced into the drift at each disturbance varied between and within species. Below a filtration of drifting species, numbers of animals which "put up" into the drift naturally, conformed to the predictions of the model, with each species exhibiting a characteristic rate of leaving the drift for a given water velocity and light intensity. When a blockage was maintained for several days, the observed drift on successive days was a satisfactory fit to that predicted; but on successive days larger numbers entered the drift just below the blockage than was predicted, possibly indicating support for the hypothesis of intergravel upstream movement. The numbers of most species dropped to low levels in the drift by four days, suggesting that there was a density dependent behavioral response whose magnitude was related to the carrying capacity of the substrate for each species. When stream organisms were simultaneously disturbed from the substrate downstream from where they were filtered out of the drift, the numbers at various distances should be constant if the correct width of disturbed area is selected. This tendency was confirmed in field trials. Estimates of the substrate density of organisms can be based on predictions of the McLay model, but only for species and age groups that are actively engaged in drifting. It is concluded that the McLay model provides a simple, holistic conception of the process of stream drift. However, variations in the substrate density, flow conditions, behavior of the various kinds of organisms including drift behavior and upstream migration combine to provide many circumstances in which the model is inadequate.

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