Integrated submerged aquatic vegetation management in an urban New Zealand river

Abstract

A comprehensive overview of the waterway network in Christchurch City, New Zealand, classified the reaches of three rivers and numerous smaller waterways according to the values for which submerged aquatic vegetation should be managed. The priority values; drainage, recreation, fish habitat and native plants, were assigned according to Council’s historical information on the river and with regard to comments they receive from the general public. The classification recognised that aquatic plants have positive and negative effects on the range of values that the waterway network supports, and that these values can be prioritised. In reaches selected on the basis of their value-based classification, field trials of modified management options were designed to establish seasonal growth patterns and the implications of a reduced level of maintenance. Results from trials that involved harvesting the central strip of deeper reaches of the Avon River, leaving submerged macrophyte habitat on either bank, and selective removal of Potamogeton crispus by hand raking in shallower areas, showed considerable seasonal and interannual variability in species dominance. This has implications for timing and frequency of harvesting. The long term aim is to establish management options that optimise for competing values, minimise maintenance costs and are based on a more comprehensive understanding of growth characteristics of the range of aquatic plant species in the river.