New Zealand Journal of Marine and Freshwater Research abstracts

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Invertebrate recolonisation of previously dry channels in the Rakaia River

P. M. SAGAR

Fisheries Research Divison Ministry of Agriculture and Fisheries Private Bag, Christchurch New Zealand

Abstract Recolonisation of previously dry channels by stream invertebrates was studied in the Rakaia River during winter 1981 and summer 1982. The winter experiment continued for 42 days, with stable low flows, whereas the summer experiment was characterised by fluctuating large flows which caused it to be abandoned after 27 days. The fauna was dominated numerically by Chironomidae, a leptophlebiid mayfly (Deleatidium), and oligo-chaetes during the winter, and by Deleatidium alone during the summer. Recolonisation was considered complete after 33 days in winter and 15 days in summer. Flow fluctuations were the main factor affecting colonisation rates, and it was assumed that drift was the main source of colonising animals. Small freshes during low-flow periods in winter resulted in a rapid increase in total density of invertebrates and number of taxa present and also affected the population structure of Deleatidium larvae in colonisation baskets. Before these freshes numbers had increased steadily over a 27 day period. In summer large floods during high flow periods initially decreased benthic invertebrate numbers in samples but numbers increased rapidly once the flood had passed. This appears to be the first study of its kind on a large unstable river system.

Keywords benthic environment; invertebrates; colonisation; braided rivers; New Zealand; stream flow; rivers; benthos; benthos collecting devices; Rakaia River

New Zealand Journal of Marine and Freshwater Research, 1983, Vol. 17: 377-386 0028-8330/83/1704-0377$2.50/0 © Crown copyright 1983 Received 15 April 1983

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