The ecology of freshwater communities of stock water races on the Canterbury Plains

Agricultural intensification on the Canterbury Plains in New Zealand has lead to the degradation of natural streams and rivers through lowering of water quality and significant reduction of surface flows from the use of ground and surface water resources. However, this same agricultural expansion has led to the development of a network of permanently flowing open water races to supply stock water to farms across the Canterbury Plains. Stock water races form an extensive network, with approximately 6,500 km of races. Initially I surveyed 62 water races and compared habitat characteristics, water quality, benthic invertebrate and fish communities with nearby natural streams. Races are characterised physically by straight, narrow and shallow channels, and small, uniform substrate. Water races are more turbid than natural streams, and can have high summer temperatures. The benthic macroinvertebrate communities of water races contained a range of taxa, including some not found in natural streams, but communities were less diverse than communities in natural streams, and tended to be dominated by a limited set of generalist taxa. A longitudinal study of three water races showed gradients in physical characteristics of races, including a downstream decrease in channel width, water depth, current velocity and substrate size. However, few strong longitudinal changes to community structure were found, as the generalist taxa commonly occurring in water races were able to tolerate conditions throughout the race network. To test if macroinvertebrate communities were limited by the homogeneous habitat of water races, I conducted a substrate manipulation experiment, where large cobbles and small boulders were added to reaches in five water races. Despite an increase in substrate and current heterogeneity, there were few significant changes to the macroinvertebrate communities over the four months of the manipulation. This outcome does not eliminate low habitat heterogeneity as a limiting factor for water race communities. Rather, the benthic invertebrate community throughout the water race network is a product of the homogeneous habitat, which limits the availability of colonists of taxa that would benefit from increased habitat diversity. A survey of the fish assemblages of water races found races had a depauperate fish community. Only two species were commonly found in water races, and the average species richness of races was 1.5. By contrast natural streams had a higher diversity of fish species (mean 4 three species), and contained representatives of a greater number of species that are typical of streams and rivers on the Canterbury Plains. My research has shown that stock water races provide an important source of aquatic biodiversity on the plains, both in addition to natural streams and in their own right. However, the biodiversity value of stock water races could be improved with enhancement of in-stream habitat.