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Managing wetland water and nutrient levels to conserve the aquatic fauna

Balla, Shirley Anne (1992) Managing wetland water and nutrient levels to conserve the aquatic fauna. PhD thesis, Murdoch University.

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Abstract

Aquatic invertebrates were collected from six wetlands on the Swan Coastal Plain near Perth, Western Australia, every three weeks between August 1988 and September 1989. The aim was to obtain information on the impact of changing wetland water levels and nutrient enrichment on the aquatic invertebrate communities.

The wetlands are surface expressions of a superficial unconfined aquifer and receive inputs from the groundwater and runoff from surface catchments. The mediterranean-type climate, in concert with the existence of the unconfined aquifer, profoundly shapes the character of these wetlands. The seasonal changes from winter wet to summer drought affords a predictable annual cycle; only the length of the wet and dry seasons varies. Water levels peak in spring (September-October) and are lowest in autumn (March-April).

The seasonal wetlands near Perth are dry for four or five months each year when rainfall is average, and up to six or eight months from December to August when rainfall is below average. In years of above average rainfall many of these wetlands do not dry. Water is only consistently present in the seasonal wetlands from August to the end of November; a period of about 120 days. None of the invertebrates were found to have larval phases that were longer than this and most were less than half this period. The timing of drying is determined by the level of the groundwater which is dependent on the quantity of the previous years rainfall. The rate of drying is relatively slow compared to the rate of refill, and drying can occur between December and April. The timing of refill is determined by the beginning of the winter rains. Refill is rapid and can occur between April and July.

The six wetlands represented a small subset of the numerous wetlands on the Swan Coastal Plain and encompassed a range of depths, pH, conductivities, nutrient levels and water colour. As the physical and chemical characteristics of the wetlands changed through the seasons, so did the invertebrate communities. The fauna in the less enriched wetlands differed between wetlands, while the communities in the hypertrophic wetlands were more similar to each other. A total of 176 taxa, over 5.9 million individuals, with a biomass of more than 240kg were collected from these wetlands. High species richness was associated with seasonal drying either of the entire wetland or over a substantial portion. Macroinvertebrate abundance was highest in the presence of either green algal blooms or cyanobacterial blooms. Macroinvertebrate biomass appeared to be associated with nutrient enrichment, however, the larger, heavier macroinvertebrates may require the presence of aquatic macrophytes, since the highest biomass occurred in wetlands with both cyanobacterial blooms and abundant macrophytes.

Three of the wetlands contained fish. The native goby Pseudogobius olorum occurred in North Lake and the introduced mosquitofish Gambusia holbrooki occurred in North Lake, Nowergup Lake and Thomsons Lake. However, the information gathered in this study was insufficient to properly ascertain the -impact of fish on macroinvertebrate communities. Field experiments are recommended to further assess the influence of fish.

Only a few (about 3%) taxa appeared to require permanent wetlands, and all of these were non-mobile (i.e. had no apparent means of active dispersal between wetlands). However, over 27% of the taxa appeared to require water during the period when the seasonal wetlands were dry. These were all mobile animals with short-lived non-aquatic adults, or aquatic adults with no desiccation resistant stage. These results indicated that some permanent wetlands need to be conserved for the maintenance of these taxa. Many of the permanent wetlands in Perth were once seasonal wetlands that have become overfilled and hypertrophic due to urbanization. Rehabilitation of these hypertrophic wetlands would render them more suitable as a habitat for a greater number of species.

About 70% of the fauna appeared to have no requirement for water throughout the summer/autumn drought period because they had either long-lived non-aquatic adults or a desiccation resistant stage. Almost 80% of the taxa are capable of active dispersal and most of these animals probably disperse to the wetter south-west, or alternatively, aestivate until the rains begin, because they were not found in the permanent wetlands when the seasonal wetlands were dry.

Reduction of nutrient inputs from drains and revegetation of wetlands is likely to increase species richness by improving water quality and providing greater habitat complexity and food resources. The high mobility of many species of macroinvertebrates will ensure that they recolonize suitable habitats.

Fringing terrestrial vegetation around wetlands should be conserved as habitat for animals with non-aquatic adults (e.g. odonates) to use for feeding, breeding and shelter. In addition, avian and mammalian fauna are often associated with the terrestrial vegetation surrounding wetlands.

The primary determinant of wetland water levels is rainfall, but groundwater abstraction reduces wetland water levels and urbanization increases water levels (and incidentally, nutrient levels). In addition to lowering maximum and minimum water levels, abstraction also has the potential to reduce the time water is present in the seasonal wetlands and accelerate the rate of drying. The impact of abstraction is presently being moderated by reducing abstraction near wetlands or by pumping water into wetlands to artificially maintain water levels. To conserve the aquatic fauna groundwater abstraction should be managed to ensure that drying of seasonal wetlands does not occur before December and the rate of drying should not exceed 2cm/day. Pumping water into wetlands to compensate for abstraction should occur in spring if peak water levels have not flooded the fringing vegetation, rather than maintaining a shallow pool in autumn.

The major environmental cost of groundwater abstraction and urbanization is loss of wetland habitat. Since approximately 70% of the original wetlands on the Swan Coastal Plain have been lost, it is likely that some species of macroinvertebrates that occur today are remnants of much larger populations. For the larger, predatory taxa where only a few individuals occur in each wetland, the number of wetlands inhabited often determines the size of the population. Further loss of wetlands is likely to reduce population sizes, and may already have led to the loss of species. In addition to conserving the remaining wetlands generally, attention to conserving a mosaic of wetland types is essential for maintenance of species diversity. The less obvious swamps and damplands probably support more aquatic flora and fauna than the lakes and are extremely valuable wetland habitats which should be the focus of future research. While information is being gathered on the range of wetland types in relation to vegetation and aquatic fauna existing on the Swan Coastal Plain, the best medium term approach is to conserve as much wetland habitat as possible. In the long term, conservation of Swan Coastal Plain wetlands requires that limits to population growth in Perth be addressed.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Davis, Jenny
URI: http://researchrepository.murdoch.edu.au/id/eprint/51757
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