School of Geography - Theses

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    Spatial variation in benthic community structure in upland streams: the influence of fine sediment transport
    Bond, Nicholas R. ( 2000)
    Physical factors can play an important role in structuring plant and animal communities. Together with spatial variation in their magnitude, differences in species tolerances to these factors are thought to explain considerable variation in the structure of natural assemblages. This thesis examines spatial variation in fine sediment transport in upland streams, and the effects of this on the structure of benthic invertebrate assemblages in this system. I initially surveyed sediment transport rates (3 times) using box type bedload traps and the faunal assemblages (twice) on individual stones at 12 sites on 3 regulated and 3 unregulated streams in the upper reaches of the Acheron and Yarra catchments, which drain from the Great Dividing Range, southeastern Australia. This study region spans a geologic boundary, and includes areas of acid volcanics in the north, and granite and sandstone to the south. Sediment transport rates, which were measured only at low flow ranged from 5+2 to 821+115 g.day' (mean f 1 SE) at each site. Transport rates between the volcanic and granitic/sandstone catchments accounted for most of the variation between sites, and thus differences in transport rates between the two geologies approached 3 orders of magnitude. The flow diversion weirs on some of the streams were found to negate any differences in sediment transport rates between the two catchment types. Multivariate analysis of macroinvertebrate assemblages clearly distinguished between assemblages from sites in the volcanic and granitic areas. Overlaid on these differences was the effect of flow regulation, in which assemblages at regulated sites in both catchments converged toward one another in terms of overall assemblage structure. The composition of the assemblages at these regulated sites differed from unregulated sites in both catchment types. Notably, although multivariate techniques clearly established the above patterns of difference, commonly measured variables such as species richness, total abundance and the abundance of common taxa showed ambiguous patterns with respect to catchment geology. The multivariate analyses suggest that differences in community structure between granitic and volcanic streams are partly related to the effects of sediment transport. However, I was not able to demonstrate this to be the case. In a field experiment in which sediment transport was excluded across individual patches of the streambed at 6 of the unregulated study sites, no changes were observed in the fauna colonising these patches relative to controls in which sediment transport was maintained at natural levels. This was so in both the volcanic and granite catchments. Nevertheless, multivariate analyses again showed assemblages within the granite and volcanic catchments to differ in a similar fashion as observed in the original survey. In a set of artificial stream channels located adjacent to one of the study streams, changes in the benthic community were monitored in response to short-term and long-term sediment addition. These two experiments considered changes in sediment loads that might occur, in the short term as a result of small spates (a pulse disturbance), and in the long-term as a result of natural variation in sediment loads, or alternatively as a result of human impacts (a press disturbance). In both cases sediment addition caused only minor, but consistent changes in the benthic assemblage. In the case of disturbance caused by spates, it appears that changes in flow are the predominant mechanism disturbing the community during these events. Overall, these communities appear to be resistant to the effects of sediment transport. Presumably this resistance relies on the maintenance of suitable habitat, and thus sedimentation of rocky substrates would probably cause far greater changes than were detected here in response to sediment transport. It is possible that in the granite streams surveyed, the effects of sediment transport are to some degree mediated by the ability of animals to find refuge from sediment induced abrasion, perhaps by seeking shelter on the lee side of rocks, or in dead water zones where scour by sediment transport is minimised. Future research must concentrate on the physiological tolerance to sediment transport of these organisms, and whether behavioural adaptations allow stress to be minimised in heterogeneous stream reaches.