SPATIAL PATTERNS OF BENTHIC INVERTEBRATES IN REGULATED AND NATURAL RIVERS

Daily fluctuating flows in regulated rivers can lead to extensive areas of substrate that experience drying and wetting. I investigated the longitudinal and lateral patterns of benthic invertebrates in the regulated peaking Magpie River and neighbouring natural rivers. Nearly half of all invertebrates collected in the Magpie River originated from the upstream reservoir. Both lentic and lotic invertebrates, however, decreased in abundance to natural levels 58km downstream. Closest to the dam, lotic invertebrates were twice as abundant as those found in natural rivers. In natural rivers Diptera, Ephemeroptera, Coleoptera and Trichoptera were more common in the shallow and slower areas along transects. The opposite was true in the regulated Magpie where densities increased with water depth and velocity, particularly for the dominant groups Diptera and Trichoptera. The abundance of worm-like organisms (e.g. Enchytraeidae, Lumbricidae, Naididae) and snails (Basommatophora) increased considerably in the varial zone. There were 10 times more Odonata and Plecoptera in the natural rivers, but lateral trends were not evident in either type of river. The influence of sampling location along transects on the interpretation of the community composition can be clearly seen in a resampling of the data. This study illustrates the presence of longitudinal and lateral gradients, and this knowledge needs to be incorporated into the design of research or monitoring programmes. Failing to do so could jeopardize decisions with the management of our flowing waters. Copyright (c) 2011 John Wiley & Sons, Ltd.