Braided river springs: distribution, benthic ecology, and role in the landscape
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Alluvial springs are an interesting feature of braided river flood plains. The aim of this study was to quantify and characterise the fauna and physicochemistry of these springs. The distribution and occurrence of springs was assessed using a GIS analysis in 20 South Island braided rivers. The greatest number of springs was found in east coast inter-montane valleys, 65% of which were associated with obstructions in the river valley, such as alluvial fans, bluffs or flood retention works. A comparison between springs, hill slope streams and main channels in the upper Waimakariri River revealed physically disparate environments, which were reflected by their invertebrate communities. Springs contained the greatest number of unique taxa, which was attributed to high stability and the presence of phreatic taxa. In spring-sources the presence of aquatic plants were found to be a powerful structuring influence on benthic invertebrate communities. The generalist, asynchronous nature of New Zealand benthic invertebrate fauna, and the high diversity and evenness of predatory taxa are also thought to be influential on the high levels of taxonomic richness at spring-sources. Away from the spring-source taxonomic richness decreased due to the decline in stability and habitat complexity. Spring age, or time since major disturbance, was important in structuring benthic invertebrate communities. Older springs were dominated by non-insect taxa, whereas younger springs, were dominated by insects which are rapid dispersers. A macrophyte manipulation experiment was conducted in four spring-sources. The removal of macrophytes from treatment plots resulted in a decrease in taxa abundance, but an increase in evenness. Mayflies and caddisflies increased, replacing mollusca and diptera. This shift in community composition may have been due to changes in available living space, food resources, flow characteristics and physical structure between the cobble and macrophyte habitats. Springs contributed a major proportion of the benthic invertebrate biodiversity to the upper Waimakariri River and this study confirms that they deserve commensurate consideration in regional river management planning.