UBC Theses and Dissertations

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UBC Theses and Dissertations

Tributary junction, what’s your function? : linking catchment processes to habitat alteration and testing mechanisms for community responses at stream confluences Tavernini, David Aaron


Biotic communities are shaped by both regional and local processes. Locally, communities can be influenced by the quality, quantity, and arrangement of habitat structure and resources. At the same time, landscape processes act to arrange structural components and resources in space. In streams, catchment processes can strongly control habitat attributes. This relationship between catchment processes and resulting effects on in-stream communities has been studied extensively in the context of uninterrupted stream reaches. Many processes are influenced by the size of the stream, which has important consequences for river net- works. Streams of differing size and characteristics join to form tributary junctions, which have received relatively little attention in stream ecology. Tributary junctions are hypothesized to be biological hotspots due to high habitat heterogeneity and possibly unique niche space. However, observational studies show mixed support suggesting the need for more of a mechanistic understanding. In this thesis, I link physicochemical processes to habitat attributes and test two mechanisms for community responses at tributary junctions. I conducted an observational study to test whether stream size explains tributary exports of habitat structural components and resources and the resulting effects on mainstem habi- tat attributes. I found that tributaries do alter the habitat attributes in the mainstem, but tributary size was rarely an accurate predictor, except for the concentration of coarse particulate organic matter (CPOM) and nutrient supply from the tributary relative to the mainstem. Additionally, many attributes varied strongly over time. Using a field-based experiment, I tested whether CPOM addition and substrate coarsening were impor- tant mechanisms for macroinvertebrate community responses at tributary junctions. I found that community structure was highly variable with little explanation due to tributary inflow. Experimental treatments and environmental covariates explained little variance, except for effects on taxa-specific abundances with the supply of CPOM. Overall, this thesis shows that tributaries are important agents for altering habitat structure and resources in mainstem channels, but effects on communities are highly site specific and may instead be driven by dispersal processes. The results of this work suggest the need to investigate other specific processes and mechanisms for community responses at tributary junctions.

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