Seasonal water-quality sampling in estuaries, what can it tell us? a case example of eastern Australian subtropical estuaries

Point-sampling of water is often referred to as an inaccurate and crude method for use in estuaries; however, budgetary, spatial and time constraints often leave it as the only feasible option. The present paper investigates the relationship between spring water quality and diatom sampling of 52 eastern Australian subtropical estuaries and the OzCoasts determined classification of estuarine condition. In general, estuarine health decreases along the total phosphorus (TP), total nitrogen (TN) and the centric to pennate diatom ratio gradients. However, although there is a general trend, results are varied within each class, with some estuaries classed as extensively modified, recording low nutrient conditions. Principal components analysis (PCA) indicated that 55% of the variance among sites is explained by the first and second axes, with TN, TP, TN : TP, latitude and pH having high correlation with PCA Axis 1, and temperature, conductivity and the centric to pennate diatom ratio being correlated with Axis 2. Two diatom species that may have bioindicator value for estuary conditions were identified. We concluded that even though natural variability and accurate characterisation of estuaries is not possible with point-sampling, it can still identify important information on the status of estuarine condition, particularly in relation to total nutrient concentrations where budget and/or time constraints limit environmental monitoring.