Report: TR 2007/32

Author: Zoe Dewson and Russell Death (Institute of Natural Resources-Ecology, Massey University)

Abstract

To assess the influence of water supply intakes on instream habitat and invertebrate communities, we sampled sites upstream and downstream of existing water abstractions on ten Waikato region streams.

Streams were sampled in early summer (December 2006) and again at the end of summer (March 2007) following the time of highest water usage, over the summer holiday period.

Sites downstream of water takes generally had lower water velocity, depth and wetted width than upstream sites at the time of sampling, but this varied considerably among streams.

Decreases in flow had no effect on water temperature, measured continuously over the low flow period, or on spot measurements of conductivity, pH or dissolved oxygen (DO). There was no overall consistent response of chlorophyll a concentrations to water abstraction in either December or March, but variable differences between upstream and downstream sites in individual streams.

Despite decreases in velocity, depth and wetted width for most streams, the invertebrate communities at sites upstream and downstream of water intakes remained similar in terms of community composition, invertebrate density, number of taxa, and the percentages of Ephemeroptera, Plecoptera and Trichoptera (EPT) individuals and taxa (excluding Hydroptilidae).

A comparison of invertebrate communities in December 2006 and March 2007 showed that the high water take period could increase the influence of water abstractions on invertebrate communities in some streams.

The downstream site at Waitete Stream appeared to be the most impacted by water abstraction. There was visibly more sediment accumulation and more algae at the downstream site on this stream and the invertebrate communities at sites upstream and downstream of the water intake were the most dissimilar of all streams in the study both before and after the main water take period. The small size of this stream and the high proportion of flow removed might make this stream more vulnerable to water abstraction than the larger streams in this study.

The Effects of Water Supply Intakes on Macroinvertebrate Communities in the Waikato Region [PDF, 802 KB]