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Research Report 4, Volume II
Bishop KA, Allen SA, Pollard DA & Cook MG
Supervising Scientist 1990
ISSN 0810-9966
ISBN 0 644 12769 4 (Volume 2)
ISBN 0 644 01294 3 (3 Volume Set)
The tropical climate of the Alligator Rivers Region of the Northern Territory has a distinctive Wet-Dry cycle resulting in seasonal flows in the creeks and rivers of its catchments. The present study, begun in August 1978, was aimed at developing an ecological monitoring system that would detect any changes to the freshwater fish communities brought about by recent uranium mining and processing in the lowlands of the ARR.
The key to this development, and the focus of the synecological studies, was a description of spatial and temporal patterns in the community structure of the fish fauna. Interpretation of these patterns was made possible by the collection of detailed environmental data from the study sites.
From the headwaters to the mouths of the creek systems, the habitat-structural and physico-chemical conditions presented a continuous gradient of conditions; cooler, clearer waters over rocky and sandy substrates moving downstream to hotter, more
turbid waters over muddy and clayey substrates with hydrophytes. Distinctive fish communities were present in the upper escarpment and lower reach floodplain zones. These zones contained major refuge habitats for fish.
Superimposed on the longitudinal gradient in environmental conditions, were extensive changes caused by the seasonal nature of water flow. During the Dry season, the conditions were generally heterogeneous between waterbodies, with the most 'favourable' conditions for fish existing in escarpment zone waterbodies and varying 'unfavourable' conditions existing downstream in the lowland and floodplain habitats. This was the season when communities in the upper and lower reaches were most distinctive.
The Early-wet season heralds a rapidly expanding aquatic environment which, by the Wet season, becomes more homogenous between waterbodies, as conditions tend towards and exceeded those recorded in the escarpment zone during the Dry season. In the Wet season, communities in the upper and lower reaches were least distinctive. The community structure recorded during this season in the lowland habitats showed a flux between those recorded in major refuge habitats in the upper and lower reaches.