Can the Nitrogen and Carbon Stable Isotopes of the Pygmy Mussel, Xenostrobus securis, Indicate Catchment Disturbance for Estuaries in Northern New South Wales, Australia?

The nitrogen and carbon stable isotope ratios (dN and dC) of the pygmy mussel, Xenostrobus securis, were determined for three estuaries with varying levels of catchment disturbance in northern New South Wales, Australia. The lower Manning River catchment supported the highest human population densities and 3% residential development with some livestock agriculture (41%); the Wallamba River catchment was mostly livestock agriculture (56%) while the Wallingat River catchment was mostly vegetated (79%). Mussels, estuarine particulate organic matter (POM), and livestock and humanderived waste were collected in two stages during the austral summers of 2001–2002 and 2002–2003 for dual carbon-nitrogen stable isotope analysis. The disturbed Manning and Wallamba River catchment mussels were enriched in N by an average of 3.2% and 1.5%, respectively, compared to the vegetated Wallingat River mussels. Mussel dC values ranged from224.8% to 230.3% and showed an estuarine gradient becoming enriched with distance downstream within estuaries, but were unable to distinguish patterns in catchment disturbance between estuaries. The dN and dC values of POM showed a similar pattern to mussels, indicating a direct link between them within each estuary. A multiple regression model of mussel dN using the fractions of land used for livestock agriculture and residential development within 5 km zones from river networks to a distance equivalent to a tidal ellipse from sites explained 67% of the variation in mussel dN with 95% of the differences lying within 1.6% of observed values. Increasing fractions of land used for livestock agriculture depleted mussel dN values estimated by the regression equation, indicating the use of cow manure as a nutrient source with a value of 2.0%. Increasing fractions of land used for residential development enriched estimated mussel dN, indicating the use of human-derived waste with a value of 20.8%. Pygmy mussels are a useful long-term bio-indicator for the effects of anthropogenic catchment disturbance and nutrient enrichment in estuaries.