Wetlands for the future

Southwestern Australia experiences a Mediterranean climate and its estuaries are at risk from eutrophication because of a limited oceanic tidal range, constricted or barred ocean entrances, shallow estuarine basins, extensive agricultural clearing and draining in catchments, fertilized sandy coastal plain soils with limited phosphorus sorption capacity, large inland catchments with little vegetative cover at the onset of winter rains and widespread erosion. Elevated nutrient and sediment loads to southwest estuaries have caused disruption of ecosystem processes in some estuaries including seagrass loss, macroalgal and phytoplankton nuisance, and anoxia. To investigate estuarine health in the southwest, five sites in each of eight estuaries and five sites at a marine reference location (45 sites), were investigated using a range of physical, chemical and biological indicators. Water and sediment nutrient concentrations were determined at each site, together with an assessment of the community structure of phytoplankton, and benthic macroinvertebrates. Historical water quality information was evaluated to provide perspective. Despite some past analytical uncertainty, there appears to have been a significant increase in median total phosphorus concentrations in both the upper Swan (12 to 90 J..Lg L -1 ), and Peel-Harvey estuaries (15 to 70 J..Lg L-1), from 1945 to 1995. Surface salinity transects in summer 1995 typified classical, lagoonal and reverse estuaries, depending on basin morphology, marine contact and runoff volumes. Summer N, P and chlorophyll a concentrations in estuarine waters were correlated and were consistent with the extent of agricultural development particularly for sandy coastal catchments. Phytoplankton species diversity (evenness) was inversely related to total cell densities and estuarine nutrient status. Macro-invertebrate diversity (evenness) was inversely related to abundance. The proportion of cyanophyte and dinoflagellate cells in phytoplankton communities was high for some diverse pristine sites with low levels of chlorophyll a, low for all sites with moderate levels of chlorophyll a, and high for some sites with high levels of chlorophyll a. The suite of environmental indicators evaluated here collectively provided a useful indicator of estuarine health. Introduction Many estuaries in southwestern Australia have been experiencing symptoms of eutrophication including increased frequency, intensity and duration of phytoplankton blooms (Kinhill, 1988; Hosja and Deeley, 1993; Thompson and Hosja, 1996), nuisance 258 Wetlands for the Future accumulations of macroalgae (Lukatelich et al., 1987; Lavery et al., 1991; McComb and Humphries, 1992; Hodgkin and Hamilton, 1993), a loss of seagrasses (D' Adamo et al., 1992; Walker and McComb, 1992), and benthic hypoxia and anoxia (Douglas et al., 1996). Few studies have estimated the mass flux of nutrients to southwest estuaries (Rochford, 1951; Kinhill, 1988; Bott, 1993), and consequently it has been difficult to draw conclusions about the onset and progression of eutrophication. The Mediterranean climate experienced by southwestern Australia, results in highly seasonal streamflow with considerable inter-annual variability. The coefficient of variability (Cv) of annual runoff for the southwest (Cv > 0.5), is greater than that of European (Cv = 0.28) and North American (Cv = 0.36) rivers (Finlayson and McMahon, 1988; Eyre, 1997), and the variability increases as annual rainfall decreases. Time delays of years between increased nutrient inputs and the appearance of symptoms of eutrophication (Hodgkin and Hamilton, 1993), together with seasonal variability in runoff inputs and primary production in estuaries, makes it difficult to establish causal links. The difficulty of defining estuarine response to historical nutrient loadings and in estimating permissible loadings, highlights the need for diagnostic indicators of estuarine health. Environmental indicators of ecosystem disfunction need to be consistent with the common symptoms of degradation (Roux et al., 1993), and to differentiate between the presence, early onset or absence of stress-related impacts (Cairns Jr et al., 1993). Characteristics ascribed to ecosystems under stress may include; increased circulation of contaminants (Cairns Jr et al., 1993), reduced species richness and/or diversity (Rapport, 1995a), reductions in the biotic size spectrum to favour smaller life forms (Rapport, 1995b), simplification of food webs (Havens, 1994), increased primary productivity (Amir and Hyman, 1993), increased dominance by undesirable exotic species (Havens et al., 1996), increased prevalence of disease (Cairns Jr et al., 1993), or morphological aberrations (Clarke, 1994; Tracy and Hough, 1995), and reduced population stability (Keough and Quinn, 1991; Fairweather, 1993; Rapport, 1995b). This paper reports on the first year ofrange-fmding investigations into the health of estuaries in southwestern Australia. Environmental indicators investigated included; (i) time series of TP concentrations to quantify changes in the circulation of contaminants, (ii) relationships between nutrients and chlorophyll a to assess the level of primary productivity, (iii) assessment of species richness and diversity of key trophic groups in impacted and pristine locations, and (iv) screening for the presence of potentially harmful phytoplankton. Materials and methods To investigate estuarine health in the southwest, eight estuaries, and a marine reference location were investigated (Table 1) using a range of physical, chemical and biological indicators.