The invasive freshwater gastropods, Physa acuta and Potamopyrgus antipodarum : distribution in urban and non-urban streams in the Georges River catchment

Urban catchments have high coverage of impervious surfaces with modified stream hydrology and water chemistry. In urban areas, it is well documented that opportunistic invasive species colonise and displace many native species once the habitat is degraded. The freshwater gastropods Physa acuta (Physidae) and Potamopyrgus antipodarum (Hydrobiidae) are invasive species (from Europe and New Zealand respectively) and have become widespread throughout the waterways of south-east Australia. The aim of this study was to investigate the distribution of these two gastropod species (P. acuta and P. antipodarium) and the pollution sensitive mayfly family (Leptophlebiidae) in streams with three different levels of urban development (non-urban, peri-urban and urban) in the Georges River catchment, Sydney, NSW. Quantitative sampling of Leptophlebiidae, Physidae and Hydrobiidae was undertaken from a total of 17 sites across the catchment. We found Physa acuta and Potamopyrgus antipodarum were absent from non-urban catchments, whereas Leptophlebiidae nymphs were abundant. The non-urban streams had low pH (7) and higher salinity (>400 μS/cm). Additionally, Leptophlebiidae were absent from highly urbanised catchments. A puzzling finding was that invasive gastropods have an apparent intolerance of the water chemistry of the cleanest non-urban streams. We suspect that the naturally low pH and scarcity of some minerals may be protecting these streams from colonisation by the invasive snails. We question whether the ANZECC pH guideline (minimum pH of 6.5) is prudent for protecting freshwater ecosystems from invasive gastropod species. Further field and laboratory investigation is required to detect and measure other factors that are contributing to the invasive success of both exotic gastropods