Date of Award

2017

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Science (Environmental Management)

School

School of Science

First Supervisor

Professor Pierre Horwitz

Second Supervisor

Dr Neil Pettit

Third Supervisor

Professor Ray Froend

Abstract

Perched, seasonal claypans of southwestern Australia are poorly understood in terms of their ecological character, such as relationship between hydrology and their biota. An example is Little Darkin Swamp, located on the Darling Plateau in southwestern Australia. The overall aim of this thesis was to describe its ecological character, to understand what drives this claypan system and how its ephemeral nature affects wetland processes and functions.

This study first comprised a detailed characterisation of the wetland’s attributes, following the geomorphic-hydrological approach proposed by Semeniuk and Semeniuk (2011). This revealed that its hydrology is highly dependent on rainfall, that it is an endorheic system, with a basin that is structurally spatially heterogeneous with distinct vegetation zones, and that surface waters have nutrient levels that are similar to oligotrophic systems. These features make it similar to other claypan wetlands of southwestern Australia and vernal pools of California, USA.

Continuous high-frequency dissolved oxygen data during the hydroperiod showed that there are large temporal and spatial variations in ecosystem metabolism, and that the trophic status of the wetland is finely balanced, fluctuating between auto- and heterotrophy due to its ephemeral nature. Due to its oligotrophic nature, rates of gross primary production (GPP) and respiration (R) were overall low, and the wetland was overall slightly autotrophic over the study period. Furthermore, dual isotope analysis of δ13C and δ15N of sources and consumers revealed that aquatic macrophytes make a higher contribution to the food web compared to other sampled sources. However, the food web was also supported by sources of carbon that were not sampled, probably filamentous algae and methanotrophic bacteria.

Experimental re-hydration of dried sediments emphasized that the seasonality of the water regime, and the shallow bathymetry of the basin, influences organic matter content, nutrient levels, oxygen consumption, plant growth and macroinvertebrate richness, differently between the centre of the wetland versus the edges. These results confirmed that there are at least two distinct zones in the wetland in terms of biotic response following rewetting, caused by the differences in duration and frequency of inundation of the sediments.

The outcomes of this study showed that the ephemerality (i.e. seasonal drying and wetting) of Little Darkin Swamp drives important internal ecosystem processes, such as ecosystem metabolism, nutrient cycling, and primary production, which in turn determine the trophic status and distribution of biotic communities in the wetland. Therefore, any changes to the hydrological regime will greatly affect how these system functions and can potentially negatively impact such unique shallow, seasonal perched systems of southwestern Australia.

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