UPPER THERMAL TOLERANCES OF 12 NEW-ZEALAND STREAM INVERTEBRATE SPECIES

The upper thermal tolerances of 12 New Zealand freshwater invertebrate taxa were compared using a laboratory lethality testing protocol. Temperatures that were lethal to 50% of the test organisms (LT(50)s), following acclimation to 15 degrees C, generally declined over the 4 day period of the tests. LT(50) values after 48 and 96 h exposure ranged from 24.5 to > 34 degrees C and 22.6 to 32.6 degrees C, respectively, indicating that temperatures that occur in summer in many NZ streams and rivers may limit the distribution and abundance of some of these invertebrate species. Larval insects included both the most sensitive species (a plecopteran and two Ephemeroptera) and the most tolerant (the larvae of the elmid beetle Hydora sp.) in this study. The two molluscs studied (Potamopyrgus antipodarum (Prosobranchia: Hydrobiidae) and Sphaerium novaezelandiae (Bivalvia: Sphaeriidae)) showed high thermal tolerance, whilst two crustaceans (Paratya curvirostris (Atyidae) and Paracalliope fluviatilis (Eusiridae)) were moderately tolerant. The effect of a higher acclimation temperature (20 degrees C) on thermal tolerance was also investigated for the three most sensitive species. This appeared to increase the thermal tolerance of the two Ephemeroptera (Deleatidium spp, and Zephlebia dentata (both Leptophlebiidae)) but to reduce the tolerance of the plecopteran Zelandobius furcillatus (Gripopterygidae).