A test of Tyler's Line - response of chironomids to a pH gradient in Tasmania and their potential as a proxy to infer past changes in pH

摘要

Summary1. Tyler's Line delimits two distinct limnological provinces that reflect differences in climate, geology and vegetation in Tasmania. Lakes west of Tyler's Line are typically acidic and dystrophic with relatively shallow euphotic zones, whereas eastern lakes are circumneutral and oligotrophic or ultra-oligotrophic, allowing deeper penetration of light. Consequently, Tyler's Line defines a boundary where species assemblages change over a relatively short distance.2. A survey of 48 Tasmanian lakes was undertaken to identify indicator taxa of the two limnological provinces and breakpoints along the pH gradient where shifts in taxa occur. Chironomidae (Diptera) were used because they are ideal candidates for lake classification.3. Three independent methods (geographical position, piecewise linear regression, two-way indicator species analysis) verified that chironomids accurately reflect the environmental variables defining Tyler's Line at lake and catchment scales. Chironomid genera are often speciose, and members of the same genus can have markedly different responses to a given environmental variable. Although the types of taxa changed along the pH gradient, richness did not. This finding contrasts with many studies from the northern hemisphere but accords with other studies from Australia.4. Models of pH, developed using both partial least squares and weighted averaging partial least squares, can be used to understand past natural variability of pH in Tasmania and to test hypotheses regarding the timing, magnitude and source of contamination in impacted aquatic ecosystems.