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Forestry activity and fine sediment: Influences on benthic invertebrate communities and stream processes

Trayler, Kerry Mandy (1998) Forestry activity and fine sediment: Influences on benthic invertebrate communities and stream processes. PhD thesis, Murdoch University.

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The effects of forestry activities on the invertebrate communities of streams in the karri {Eucalyptus diversicolor) forest were investigated, with emphasis on the role of fine sediment. Clearfell logging and road construction adjacent to stream lines were examined with the view to determining whether these activities caused changes to stream invertebrate communities and if so, whether increased deposition of fine sediment was responsible for these changes. A further objective of this study was to determine whether increased sediment deposition would reduce the rate of leaf decomposition and to quantify the concentration of deposited sediment that might cause this.

Bed structure and the vertical distribution of invertebrates (>53|im) in two low gradient sandy streams affected by clearfell logging were compared with those of two undisturbed streams. Particular attention was paid to whether differences could be attributed to increases in deposited sediments associated with clearfelling. Invertebrate community structure was substantially altered in the logged streams with fewer taxa collected than from undisturbed streams. Invertebrate densities in the logged and undisturbed streams did not differ at the surface of the bed, but below this, densities were significantly lower in the logged streams. Increased sedimentation did not appear to be responsible for the differences in community structure between logged and undisturbed streams. The logged streams exhibited greater depth, current velocity, temperature and conductivity than the undisturbed streams and all of these factors may have contributed to the differences in invertebrate community structure.

The epigean macrofauna and sediment deposition were compared in four streams before and after the construction of a forestry road across one stream. The physical and chemical conditions of all streams were similar throughout, except for a thin layer of fine sediment observed on the bed of the stream below the road. This was thought to have been deposited as a result of road runoff after a high rainfall event and was not observed in the controls. The construction of the road had no detectable impact upon macroinvertebrate richness, density or community composition. Sediment deposition associated with the road runoff was thought to be within the predictable range for these streams. Possibly, the invertebrate communities of these streams are adapted to regularly altered distributions of fine sediment within the substratum.

Decomposition of flooded gum, Eucalyptus rudis, leaves were measured under different concentrations of deposited sediment (0, 250, 1000, 4000 g.m'2) in order to determine whether increased sediment deposition would disrupt leaf processing and to determine the concentration of sediment at which this might occur. This was performed under semicontrolled conditions using artificial channels constructed in a reach of a lowland river. Leaf pack processing did not appear to be influenced by increased sedimentation. Leaf processing rates were similar between treatments and considered ‘slow’ overall. Although sediment deposition did not reach the expected concentrations, the highest treatment remained above 1000 g.m2. The absence of an effect may have been the result of the combined effects of reduced shredder and microbial activity in this system.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Davis, Jenny
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