δ15N of estuarine fishes as a quantitative indicator of urbanization

Nitrogen stable isotope values (δ¹⁵N) have commonly been used as a qualitative indicator of catchment urbanization in estuaries, but no quantitative relationship has to date been established between δ¹⁵N and degree of urbanization. We sampled five species of common estuarine fish (Mugil cephalus, Acanthopagrus australis, Sillago ciliata, Gerres subfasciatus and Herklotsichthys castelnaui) of different trophic levels from six estuaries in Southeast Queensland, Australia, to test if a quantitative relationship exists between fish δ¹⁵N and urbanization in the catchment. Degree of urbanization (urban %) in the catchment adjacent to the tidal estuary was measured using polygons constructed in Google Earth images, and verified using data from the detailed Queensland Land Use Mapping Project (QLUMP). Significant linear relationships exist between fish δ¹⁵N and urban % in most cases irrespective of the section of the estuary (upper, middle and lower) where the fish were caught, with no difference in slope and elevation between lines established for different sections. Inclusion of additional fish δ¹⁵N data from other Australian estuaries in a combined multiple regression model adding trophic level as a predictor generated a significant relationship predicting increase in fish δ¹⁵N values with increasing urban % and trophic level. There is therefore a generic quantitative relationship between estuarine fish δ¹⁵N values and degree of urbanization applicable to different fish species, geographic location or feeding habit. While the generality of this relationship needs to be tested further in other geographic locations, our findings potentially improve the applicability of fish δ¹⁵N values as a quantitative indicator of urban influence in estuaries.