Sorcha Cronin-O'Reilly

Microtidal estuaries are prone to anthropogenic degradation, with natural features of those in south-western Australia making them more susceptible. However, the benthic ecological health of these systems is rarely assessed, despite the importance of the benthos and frequent application of benthic indices in estuaries elsewhere, particularly macrotidal systems in the northern hemisphere. The aim of this research was to assess the current status of the benthic macroinvertebrate community and its role in the function and management of the microtidal Peel-Harvey Estuary. After accounting for the effects of natural hydrological conditions (e.g. salinity, temperature), the benthic macroinvertebrate community was shown to respond to anthropogenic stress as represented by sediment condition (i.e. oxygenation, organic enrichment, mud content, sulphide presence), demonstrating its potential utility for assessing estuarine health. However, existing benthic indices commonly used in macrotidal estuaries (e.g. the multivariate AZTI Marine Biotic Index) yielded results inconsistent with sediment condition, demonstrating their limitations when applied to highly adaptive, stress-tolerant macroinvertebrate communities that are common in microtidal estuaries. A new multi-metric Estuarine Benthic Community Index was developed, following a multivariate approach to select community metrics that showed greater responses to sediment condition than natural stress. Overall, the benthic macroinvertebrate community in the Peel-Harvey Estuary was typically in good to fair health, with decreased health in the summer and deeper depositional areas. It is largely dominated by small-bodied, opportunistic species, and apparently retained in early succession due to chronic natural and anthropogenic stress. This was further reflected by the community’s limited impacts on solute fluxes of benthic metabolism, nutrient exchange and denitrification, with sediment condition being more influential. These findings demonstrate that these benthic faunal communities do not play a substantial role in estuarine function, with the application of resulting benthic indices restricted to assessing more structural aspects (e.g. diversity) of benthic ecological health.