Identifying the Influence of Channel Morphology on Physical Habitat Availability for Native Fish: Application to the Two-spined Blackfish (Gadopsis bispinosus) in the Cotter River, Australia.

The impact of channel morphology and flow on physical habitat availability for the two-spined blackfish (Gadopsis bispinosus) was assessed in the Cotter River, ACT, Australia. Physical habitat requirements for three life stages were identified based on previous field sampling in the Cotter River. Two sites were selected with contrasting channel morphology: Spur Hole, with a moderate gradient and runs and glides, and Vanity’s Crossing, with a steeper gradient, rapids, and fast-flowing pools. Physical Habitat Simulation System (PHABSIM) was used to simulate the flow v. physical habitat availability relationship for each life stage at both sites. Clear differences were apparent between sites, with Spur Hole characterised by increasing habitat with increasing flow and Vanity’s Crossing showing the opposite relationship. The nature of the channel morphology determined this difference, with Spur Hole characterised by marginal zones becoming inundated at higher flows and providing additional suitable physical habitat as discharge increases. Vanity’s Crossing does not contain similar marginal zones. Further analysis demonstrated that high water velocity was the most important factor limiting physical habitat availability at both sites. This approach demonstrates the importance of channel morphology in determining physical habitat availability and an alternative use of PHABSIM to highlight limiting factors for target species.

The Influence of Geology and Season on Macroinvertebrates in Belizean Streams: Implications for Tropical Bioassessment

Considerable attention has been paid to the potentially confounding effects of geological and seasonal variation on outputs from bioassessments in temperate streams, but our understanding about these influences is limited for many tropical systems. We explored variation in macroinvertebrate assemblage composition and the environmental characteristics of 3rd- to 5th-order streams in a geologically heterogeneous tropical landscape in the wet and dry seasons. Study streams drained catchments with land cover ranging from predominantly forested to agricultural land, but data indicated that distinct water-chemistry and substratum conditions associated with predominantly calcareous and silicate geologies were key determinants of macroinvertebrate assemblage composition. Most notably, calcareous streams were characterized by a relatively abundant noninsect fauna, particularly a pachychilid gastropod snail. The association between geological variation and assemblage composition was apparent during both seasons, but significant temporal variation in compositional characteristics was detected only in calcareous streams, possibly because of limited statistical power to detect change at silicate sites, or the limited extent of our temporal data. We discuss the implications of our findings for tropical bioassessment programs. Our key findings suggest that geology can be an important determinant of macroinvertebrate assemblages in tropical streams and that geological heterogeneity may influence the scale of temporal response in characteristic macroinvertebrate assemblages.