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Evaluating indicators of ecological health for estuaries in southwest Australia

Deeley, D.M. (2001) Evaluating indicators of ecological health for estuaries in southwest Australia. PhD thesis, Murdoch University.

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A broad range of environmental indicators were evaluated to determine their suitability for describing the early onset of nutrient enrichment in south-west Australian estuaries. The southwest of Australia experiences a Mediterranean climate with hot dry summers and cool wet winters. Most of the south-west estuarine catchments have been extensively cleared for agricultural and urban development and have been loosing unacceptably high loads of nutrients. Symptoms of nutrient enrichment have been observed in many south-west estuaries.

An evaluation of fifty years of historical water quality data defined the range of normal behaviour for these systems together with the nature of rarer extreme events. It was found that south-west Australian estuaries are highly variable in space and time and appear to be more susceptible to nutrient enrichment than those observed elsewhere in Australia.

While there may be significant fluctuations in physico-chemical conditions and the structure of biological communities in ecosystems subjected to natural variability and anthropogenic stressors, it is possible that the junction of biological communities in these situations is less affected by these types of perturbations. Several indicators of biological function (rates and processes) were developed and evaluated to determine whether they offered a greater degree of diagnostic precision (early warning) than measures of stocks and status (inventory).

In evaluating various environmental indicators, it was found that no single indicator was able to unambiguously define the interactions between physic-chemical and biological processes and the response of these systems to anthropogenic and natural stressors. It has been concluded that a broad range of potential indicators must be evaluated simultaneously, in order to define baseline conditions, measurement endpoints and trends to inform catchment and estuarine management and restoration. Increased confidence in the selected indicator suite can flow from an evaluation of the monotonicity of correlated indicators, especially when assessments show consistent patterns for physico-chemical measures and measures of biotic community structure across several trophic groups.

Traditional physic-chemical indicators have provided reliable information in me past, but problems have arisen when relating these measures to biological endpoints, particularly for estuaries with significant seasonal and inter-annual variability. In the absence of biological data for estuarine ecosystems experiencing significant seasonal and inter-annual variability, socio--economic indicators of catchment land use practices may be the only option. Paleolimnological investigations may also provide additional insight into patterns of natural variability over the longer term, but the degree of taxonomic resolution required and requirements for supporting stable isotope analysis, may consume considerable resources.

Autotrophic protistans (periphyton, phytoplankton), appear to be useful for describing nutrient enrichment, salinity and other physico-chemical conditions, but complicating factors such as the nature of coupling of secondary predation need to be identified. Autecology of local indicator species also needs to be defined. Zooplankton appear to be limited as environmental indicators, but because of their potential role in grazing and materials transfer, they may be useful as elements of biotic indices across several trophic groups. One of the major impediments to using planktonic organisms for inferring the condition of estuarine health is the considerable vertical, horizontal and temporal heterogeneity displayed by these organisms in both disturbed and undisturbed systems.

More recently, benthic macro-invertebrates have been successfully used to describe the nature and magnitude of organic enrichment of estuaries. Community structure, biomass and relative abundance of functional groups and indicator species have also been developed and used as environmental indicators. Problems may occur in the use of these organisms to infer health in south-west estuaries because of the presence of naturally immature communities and variable colonization dynamics where there is significant seasonal and inter-annual variability.

Inventory measures of community structure have problems because of a lack of information about exchange pathways connecting system components and unknown interactions between diversity, stability and resilience of the ecosystem. Species richness, diversity indices and measures of biomass have probably been the most widely used indicators in the majority of published works, but generally without appropriate critical analysis of their utility. Biomass appears to have less inter-annual variability than do other measures of community structure.

A myriad of biotic indices (ratios of functional groups) within and across trophic levels have been described in the international literature. There are problems in defining weightings and concordance methods for elements contributing to biotic indices and the loss of valuable information during these types of data reduction limit their potential. Detailed autecology of members of functional groups are required for biotic indices and this type of information is potentially available for some cosmopolitan species, but generally lacking for endemic species which may describe important nuances of the local environment.

As with biotic indices, there is a range of combined metrics described in the literature. Metrics generally combine physico-chemical elements, and may include some biological information. Many of the problems with biotic indices apply equally to metrics, but when calibrated for a particular local situation, they offer considerable discriminatory power.

Assessment of early colonisation dynamics and the responsiveness of periphyton communities to nutrient additions provided insights into some key processes in south-west estuaries. Periphyton communities in estuaries with a past history of nutrient enrichment responded well to nutrient additions while communities in less disturbed estuaries did not respond as readily. This infers that opportunistic species able to respond rapidly to nutrient additions may become established in estuaries having a nutrient enrichment history while they may be less able to maintain a viable presence in less enriched estuaries. While showing promise, additional testing and refinement of these process indicators would better define their potential as early warning indicators of nutrient enrichment for south-west estuaries.

For describing the ecological health of south-west Australian estuaries, physico-chemical indicators of catchment and estuarine water quality and socio-economic measures of catchment land use history may be of use. If assumptions about the linearity of interactions between the diversity of biotic communities and the stability and resilience of ecosystem function are valid, then conventional measures of community structure will also provide useful insights.

The ongoing selection, evaluation and refinement of environmental indicators for assessing the ecological health of south-west Australian estuaries, needs to proceed as a close partnership between land and waterway managers and scientific specialists.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Paling, Eric
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