Influences of multiple stressors on biodiversity and ecosystem function in constructed urban wetlands

Abstract

Constructed wetlands remove nutrients, contaminants and pollutants from stormwater runoff, while also providing a habitat for wildlife such as macroinvertebrates and fish. Sampling constructed wetlands along a gradient of catchment urbanisation in Melbourne’s west and south east, my overall aim was to determine if increasing catchment imperviousness was related to wetland condition. To address this, I sampled macroinvertebrates to quantify biodiversity, tested fish tissue for heavy metal concentrations to ascertain if bioaccumulation was occurring, used stable isotope analysis to construct food webs, and evaluated ecosystem function using leaf packs to assess rate of decomposition. There was a significant negative relationship between total imperviousness (TI), an indicator of catchment urbanisation, and the abundance of aquatic invertebrates detected for sites in the west, but not in the south east. However, macroinvertebrate communities were relatively homogenous both within and between all study wetlands. Chironomidae (non-biting midges) was the most abundant family recorded at the majority of sites. Chironomids are able to tolerate a wide array of environmental conditions, including eutrophic and anoxic conditions. Their prevalence suggests that water quality is impaired in these systems, regardless of degree of urbanisation, although the causal mechanism is unclear. There was no relationship between TI and water and sediment concentrations of heavy metals with the exception of Zn. Concentrations of metals in fish tissues were highest in benthic species but levels declined with increasing body size and trophic level. Metabolic activity can differ between smaller and larger fish; however, smaller fish may also be feeding on a different food source. My results suggest that metals in these wetlands are not undergoing bioaccumulation. I found that as TI increased, there was a significant decrease in the abundance and diversity of macroinvertebrates and a significant increase in the values of δ13C and δ15N recorded for fishes and macroinvertebrates. An increase in TI was associated with a decrease in the mean trophic position of fishes and an increase in the mean trophic position of macroinvertebrates. My results suggest that sources of carbon differed between sites of low and high catchment imperviousness. A significant positive relationship existed between TI and rate of leaf litter breakdown in the south eastern wetlands. These results suggest that other parameters besides exposure to contaminants are likely to affect leaf litter breakdown. The overall findings of this study demonstrate that local conditions in wetlands such as morphology and vegetation, have at least as important a role in determining wetland biodiversity and ecosystem function as the degree of urbanisation of the catchment.