THE VERTICAL DISTRIBUTION OF THE INVERTEBRATE FAUNA IN THE BED OF A STREAM 1

A type of sampler is described that permits the collection of benthic fauna to a depth of 30 cm in the beds of streams. It depends on the colonization of natural substratum placed into the streambed, and invasion can occur both horizontally and vertically downward. When fully colonized, these samplers collect many times the number of animals taken by vigorous stirring of the substratum upstream of a net. Sets of samplers in which only one of four possible horizontal layers about 7.5 cm deep was available for colonization were emplanted in an Ontario stream and lifted from 1 to 28 days later. The total numbers of many types of animals increased steadily with time in all 4 layers; more than 28 days are probably needed for full colonization. When all the catches are considered together only about 20% of the total was in the top layer, about 26% was in the bottom layer, and the rest was about evenly distributed in the two middle layers. This type of distribution was found for all the groups of animals present. Only Simulium appeared to be normally confined to the surface of the substratum. It is suggested that significant numbers of animals occur deep within the substrata of stony streams, and that even samples collected down to 30 cm do not adequately represent the fauna.