Can mussels be used as sentinel organisms for characterisation of pollution in urban water systems?

Urbanisation strongly impacts aquatic ecosystems by decreasing water quality and altering water cycles. Today, much effort is put towards the restoration and conservation of urban waterbodies to enhance ecosystem service provision leading to liveable and sustainable cities. To enable a sustainable management of waterbodies, the quantification of the temporal and spatial variability of pollution levels and biogeochemical processes is essential. Stable isotopes have widely been used to identify sources of pollution in ecosystems. For example, increased nitrogen levels in waterbodies are often accompanied with a higher nitrogen stable isotope signature (δ15N), which can then be detected in higher trophic levels such as mussels. The main aim of this study was to assess the suitability of nitrogen stable isotope as measured in mussels, as an indicator able to resolve spatial and temporal variability of nutrient pollution in an urban, tidally influenced estuary (Swan River estuary; Western Australia). Our results showed a trend by which sites with higher nitrates concentrations yielded higher nitrate δ15N values; however, nitrogen concentrations and nitrogen stable isotope signature of nitrate throughout the estuary were well within natural values, indicating groundwater inflow rather than pollution by human activity was responsible for differences between sites. The δ15N signature in mussels was very stable over time within each site which allowed for the detection of spatial difference and indicated that mussels can be used as time-integrated sentinel organism in urban systems. In addition, our study indicates that the nature of the relationship between δ15N in the mussels and the nitrate in the water can provide insights into site specific biogeochemical transformation of nutrients. We suggest that mussels and other sentinel organisms can become a robust tool for the detection and characterization of the dynamics of a number of emerging anthropogenic pollutants of concern in urban water systems.