In contrast to the northern hemisphere where species of Chironomidae are usually the dominant benthic invertebrates in the coldest upper reaches of glacial streams, mayflies (Deleatidium spp.: Leptophlebiidae) predominate in equivalent conditions in New Zealand. We examined the life histories and annual production of Deleatidium spp. at two sites on the Matukituki River (South Island, New Zealand) and at three sites in its glacier-fed tributary, Rob Roy Stream. Mean annual water temperature at the five sites ranged from 2.1 to 7.0°C. Monthly sampling showed that mayfly populations were poorly synchronised at all sites but were probably univoltine. The large Deleatidium cornutum was the dominant mayfly species found at the upper sites (Sites 1 and 2) on Rob Roy Stream, whereas above the confluence with Matukituki River (Site 3) it co-existed with a complex of smaller species we refer to as D. ''angustum''. Deleatidium ''angustum'' also dominated at the Matukituki sites. Deleatidium production calculated for the five sites, assuming an 11-month nymphal life, ranged from 0.48 g dry weight/m 2 /year (Site 1) to 3.07 g dry weight/m 2 /year (Site 3). The values for D. cornutum at Sites 2 and 3 are high for a species of Deleatidium and reflect its large size. This species appears to be strongly adapted for growth at low temperatures. Climate change scenarios for New Zealand predict the gradual and ultimate loss of small South Island glaciers and a consequent warming of streams as runoff from rainfall and snow melt becomes more dominant in spring. As a result, suitable habitats will be lost for cold-water specialists such as D. cornutum, and they are likely to suffer reductions in their distributional range and local extinction. In contrast, species such as those in the D. ''angustum'' complex may extend their ranges into streams formerly dominated by glacial meltwater.

In comparison to European glacier-fed catchments, the hydroecology of New Zealand glacier-fed rivers has received limited attention, despite the Southern Alps markedly different maritime climate, a more precipitation dominated flow regime and an endemic species assemblage. This study linked longitudinal patterns of the physicochemical habitat and macroinvertebrate community along a New Zealand glacier-fed river and compared and contrasted these with understanding of European alpine river systems. Habitat variables and benthic macroinvertebrates were sampled over two consecutive summer melt periods at three sites along the Rob Roy stream, a tributary of the Matukituki River, in Mt Aspiring National Park near Wanaka, South Island. Although habitat stability increased downstream from the glacier terminus (Site R1), disturbance remained a limiting factor of macroinvertebrate abundance and species richness at the lower sites (R2 and R3). An average water temperature increase of 2°C over 3Ð3 km (from R1 to R3) was observed downstream with habitat heterogeneity and species richness highest at the downstream site (R3). The ephemeropterans Deleatidium cornutum and Nesameletus dominated at the upper two sites with the chironomids Eukiefferiella and Maoridiamesa, but became co-dominant at R3 with Deleatidium angustum, where other plecopteran and trichopteran taxa were also observed. Distributions were predominantly limited by temperature, thought to be due to adaptation to disturbance observed in taxa of New Zealand alpine streams. However, the plecopteran Zelandobius, although tolerant of cool temperatures, appeared limited to areas and periods when channel stability was higher. Modification of the conceptual model of the longitudinal zonation of macroinvertebrate taxa (Milner et al., 2001a) is proposed to account for the different species assemblage of New Zealand glacier-fed rivers.