Assessing changes between two years in macrobenthic invertebrate communities in four sub-tropical Australian estuaries

Estuaries provide critical ecosystem services but are threatened by natural and human pressures. Assessing their ecological condition is essential to evaluate future change. This study aimed to determine the biodiversity of macrobenthic invertebrates and potential environmental drivers in four estuaries between two timepoints. Following the significant 2022 flood events on the north coast of NSW Australia, we used benthic cores to sample the macrofauna in 2023, replicating a baseline survey undertaken in 2007. Four replicate samples were taken at three tidal heights (all <1 m) on sand banks in the mid-estuary zone at Brunswick, Sandon, Corindi and Bellinger Rivers. Benthic macroinvertebrates (>1 mm) were sorted, identified, counted and categorised into functional groups. The abundance and species richness was significantly lower in 2023 compared to 2007. Macrobenthic diversity and community composition showed significant variation between years, among estuaries. The bivalve Mysella vitrea (Lasaeidae) was the most abundant species, primarily explaining the dissimilarity between years. Deposit feeders dominated the functional groups, but their percentage was lower in 2023 compared to 2007. Differences in the environmental condition at the sites between two years reflect higher rain run-off in 2023, including significantly lower salinity and higher turbidity. Distance-based linear modelling showed that turbidity correlated significantly with the changes in community structure at the family level. The BEST model included a mix of physicochemical parameters, turbidity and % organic matter and explained 94.8 % of the variation in community structure, suggesting that freshwater and sediment run-off may influence macrobenthic communities in these estuaries. Results from this study support previous findings highlighting the problem of lack of data to document shifting baselines and the need for regular monitoring of estuarine ecosystems. • Macrobenthic communities in four estuaries were compared in 2007 and 2023. • Invertebrate abundance and species richness declined in 2023. • Mysella vitrea was identified as an indicator species for estuarine communities. • Water quality and organic matter explained most of the community variation. • This study highlights the problem of shifting baselines.