SUBSTRATE TYPE AS A FACTOR INFLUENCING SPATIAL-DISTRIBUTION OF CHIRONOMID MIDGES IN AN URBAN FLOOD CONTROL CHANNEL SYSTEM

Spatial distribution of chironomid midges in relation to the various substrate types in a concrete-lined flood control channel system carrying domestic and industrial waste water was studied. Bare concrete surfaces, filamentous algae, detritus, fine sand, and coarse sand and gravel were the 5 types of substrates in this drainage system. A minimum of 10 benthic samples from each of the 5 substrates prevailing in different sections of the channel system were collected, using a modified Surber sampler (0.09 m2 frame and 500 .mu.-pore net). The study was repeated on 4 different occasions. Midges belonging to 5 genera [Chironomus, Cricotopus, Tanypus, Dicrotendipes, Paralauterborniella] were taken. Chironomus spp. and Cricotopus spp. were the 2 most abundant groups forming 97.4% of the total midges taken on any occasion. Chironomus spp. larvae predominated in fine sand; Cricotopus spp. larvae were associated with detritus. On all 4 sampling dates, fine sand contained 42-80% of the total Chironomus spp. larvae taken from all 5 substrates. Similarly, substrate composed of detritus supported 58-78% of the total Cricotopus spp. larvae. Midge larval densities in some parts of the channels exceeded 3000/0.9 m2 on some occasions. During sampling, a number of drift-borne Chironomus spp. and Cricotopus spp. larvae and pupae contributed to the benthic collections.