A review of the ecological effects of macrophyte managment in soft-bottomed waterways

Report: TR 2013/03

Author: A James (EOS Ecology)

Abstract

Throughout the world, macrophytes can cause problems for humans. For example, they:

• choke irrigation and drainage channels impeding water movement
• hinder recreational activities such as angling, boating, and rowing
• clog hydroelectric and irrigation scheme intakes; and
• some invasive species develop monocultures to the detriment of native flora and fauna.

Thus a suite of usually invasive species often requires intensive management.

New Zealand is no different, and several introduced species are present that require management in lakes, rivers, and artificial or modified drainage waterways. A range of management techniques in use around the world include biological control (such as stock grazing and grass carp introductions), chemical control (such as herbicide use), and mechanical control (such as physical removal or cutting of plants.

All control methods inevitably have some level of ecological and physicochemical impact on the targeted waterway, which is often not desirable. Large tracts of Waikato’s agricultural land are on former wetland areas and large-scale drainage schemes are required to maintain its productivity. As these drains are often the only remnant wetland habitat left they are home to many native freshwater species, some of which have high ecological and cultural value (such as eels and freshwater mussels).

The Waikato Regional Council has concerns that some of the macrophyte management techniques used in these schemes are potentially detrimental to the native flora and fauna. To assist the development of science-informed policy in this area, Waikato Regional Council contracted EOS Ecology to compile a literature review on the ecological and physicochemical impacts of mechanical and chemical macrophyte management in soft-bottomed waterways.

The first step in ensuring that the most appropriate low-impact methods are used is an understanding of the relative impacts of different macrophyte management techniques. This review collates and summarises research on the ecological and physicochemical impacts of mechanical clearance of macrophytes and silt, and chemical spraying for the management of macrophytes. While predominantly aimed at soft-bottomed waterways, literature from lakes and reservoirs is included as these environments have similar characteristics to many drainage channels (e.g., low water velocities and macrophyte problems) and the effects of mechanical and chemical control would be similar. Also included are some laboratory and artificial pond bioassay experiments that give an indication of the relative toxicity of some common macrophyte herbicides.

The second step in identifying the best macrophyte management techniques for minimising ecological impacts is to examine any national or international best practice or guideline documents aimed to minimise the environmental impact of these activities. EOS Ecology have therefore reviewed several such guidelines from New Zealand and abroad.

A review of the ecological effects of macrophyte managment in soft-bottomed waterways [PDF, 2.1 MB]