Seasonal flooding, instream habitat structure and fish assemblages in the Mulgrave River, north-east Queensland: towards a new conceptual framework for understanding fish-habitat dynamics in small tropical rivers
Thomas S. Rayner A C D , Bradley J. Pusey B and Richard G. Pearson AA School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia.
B Australian Rivers Institute, Griffith University, Brisbane, Queensland 4111, Australia.
C Present address: School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
D Corresponding author. Email: thomas.rayner@unsw.edu.au
Marine and Freshwater Research 59(2) 97-116 https://doi.org/10.1071/MF07129
Submitted: 6 July 2007 Accepted: 19 December 2007 Published: 27 February 2008
Abstract
Strong relationships between seasonal flooding, instream habitat structure and fish assemblages have been well documented in large tropical rivers (e.g. the flood pulse concept). However, the mechanics of these relationships are likely to differ substantially in smaller coastal rivers, such as those in Costa Rica, south-east Brazil and Australia’s Wet Tropics. These systems typically feature steep upland streams with short, deeply incised lowland channels and poorly connected floodplains. This hypothesis was investigated by documenting spatial and temporal variation in fish-habitat relationships in the Mulgrave River, north-east Queensland. Sampling was conducted at four lowland sites under a range of flow conditions, from dry-season baseflows to a one-in-ten-year flood. Longitudinal environmental gradients and fine-scale habitat patches were important in regulating fish assemblage structure during the dry season. However, high wet-season flows, constrained by the deep channel, acted as disturbances rather than gentle flood-pulses. In particular, the mobilisation of bed sediments led to scouring of aquatic vegetation and a dramatic reduction in habitat heterogeneity. Seasonal movements of fish led to significant changes in assemblage structure – from a community dominated by Neosilurus ater, Hypseleotris compressa, Awaous acritosus and Redigobius bikolanus during the dry season, to one dominated by Nematalosa erebi, Ambassis agrammus and Glossamia aprion during the wet season. Based on these observations, together with information from the literature, a conceptual model of fish-habitat dynamics is presented that is better suited to small tropical rivers than those developed in larger systems with expansive floodplains.
Acknowledgements
This project was funded by grants from the Cooperative Research Centre for Tropical Rainforest Ecology and Management (Rainforest CRC) and James Cook University (JCU). Mike Steele (JCU), Lee Belbin (Blatant Fabrications) and Mark Kennard (Griffith University) assisted with statistical analyses. Mirjam Maughan (JCU) helped prepare Fig. 1. In-kind support was provided by Queensland Department of Primary Industries – Fisheries. Alan Hooper (Queensland Department of Natural Resources and Mines) provided hydrological data for the Mulgrave River. Field assistance was provided by Colton Perna, Zoe Baker, Paul Thuesen, Paul Godfrey, Amanda Soymonoff, Mo Healy, Anne Gulliard, Megan Barnes, Cameron Crothers-Stomp, Andrew Kaus, Andrew Jones, Rusty Ligon and Michael Pusey. Access to private land and other assistance in the field was provided by the Rossi, Thomasen and Moller families. We would like to thank two anonymous reviewers for their comments on this manuscript.
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