Rapid colonisation of a newly formed lake by zebra mussels and factors affecting juvenile settlement

Invasive non-native species are widespread in freshwaters but their capacity to establish in new lakes has seldom been assessed. In this fouryear study (2006–2009), we used Side-scan Sonar and underwater video to illustrate how a 200 ha urban, amenity lake at Cardiff Bay (Wales, UK) was invaded extensively by zebra mussels Dreissena polymorpha (Pallas) within 2–3 years of creation in 2001. Veliger surveys and artificial substrates were used to assess conditions affecting juvenile settlement. Within 5–8 years of lake formation, all hard substrates at 0.5–7 m held mussel densities of 250–6600 m which, coupled with a crude estimate of habitat available, suggested a lake-wide population of at least 9–31 million adults. Veligers reached 8 (± 2 SE) to 14 (± 4) L during May–September when water temperatures were >14 °C, but densities and juvenile settlement declined at high discharge when lake flushing rates increased: settled densities in a drier year (2007) exceeded those in a wet year (2008) by 120× implying more effective colonisation under low flows and longer residence times. These data illustrate how rapid invasion by non-native species should be appropriately factored into planning and risk assessments for new water bodies, and potential effects on amenity, ecosystem processes and ecosystem services considered. Our data suggest that drought and low flow under future climates could be a particular risk factor affecting Dreissena colonisation.