Impacts of global warming on New Zealand freshwater organisms: a preview and review

Global warming is a reality and there is evidence that some New Zealand freshwaters have already increased by as much as 1.6 C over a 17-year period. Temperature increases may cause a southwards retreat of vulnerable aquatic species in New Zealand as a 3 C temperature increase will shift a derived degree day / latitude regression 670 km southward. Introduced salmonids, stream invertebrates and non-diadromous native fishes may be unable to make compensatory migrations and be extirpated from portions of their range. Diadromous galaxiids may be harmed by egg loss due to increased flood frequency. Existing evidence on genetic heterogeneity of stream insects is ambiguous but some species may have the genetic capacity to cope with increased temperatures. Maintenance or enhancement of riparian vegetation is likely to be important to keep adult stream insects within their temperature tolerance ranges. Higher CO 2 levels may result in reductions in nitrogen levels in leaves that fall into streams, thereby reducing food value to aquatic consumers. Higher temperatures may also produce faster growth rates but smaller adult size in some freshwater invertebrates. An increase in the frequency of extreme climatic events will have the capacity to alter aquatic community structure and higher temperatures may enhance the reproduction of parasites with consequent negative effects on their hosts. Furthermore, warmer stream temperatures could increase the possibility of aquarium escapees establishing