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The general and specific characteristics of the freshwater environment are considered as a setting for the study, in terms of physical, chemical and biological characteristics, and in particular, the problem of utilizing this resource without causing unacceptable deterioration is noted.
The lakes of the central North Island of New Zealand are described in terms of their origins, catchment characteristics and climate, their biota and the nature and development of the problems of aquatic weeds, growth in the lakes, leading to a consideration of aquatic plants as weeds, and the methods available to control them.
The herbicidal chemical, diquat, is described in detail, covering the biochemistry and physiology of its action, factors affecting its herbicidal activity, and its use as a control for aquatic weeds.
An evaluation is made of biological, chemical and radiochemical assay methods for diquat, and residues of diquat in various lake components were measured after spraying. Diquat is measureable in the lake water and macrophytes for only a few days, but some persists in the lake hydrosoil from year to year. The amount of applied diquat entering the target macrophytes is typically much less than 10%. Major losses of the diquat apparently occur through dispersion, microbial degradation, and uptake by phytoplankton or other suspended matter, but eventually residues are incorporated into the hydrosoil.
The herbicidal impact of diquat sprays on macrophyte standing crop is reported fox locations in Lake Rotoiti and Lake Karapiro. The immediate and longer term response to the treatment appears to be determined by the plant species, the diquat dose, and the intensity of natural controlling factors, particularly light intensity, temperature, and wind-induced waves. The development of the latter and their effect on the stability of the weed beds and associated sediments is discussed. Mathematical models are developed to describe the response of the weed beds to the natural controls and to the sprays, leasing to the application of the models to develop various control strategies.
Non-target effects were sought by examination of the records of the local trout fishery, by direct observation of planktonic and benthic organisms, and by monitoring chemical changes in the water. The fishery has shown no changes attributable to the diquat applications, while any changes in other non-target species are minimal. Depression of dissolved oxygen in treated areas would only be of environmental significance in circumstances not normally encountered in these lakes.
Adsorption and diffusion of diquat in the sediments were examined, and the annual sedimentation rate in Lake Rotoiti was established at about 0.5kg.m-2.yr-1. It was concluded that the lake sediments could continue to assimilate the residues from the annual sprays for an indefinite period without allowing measureable concentrations to remain in the water.
The problem of the uptake and toxicity of diquat in a system of competing sinks is considered, and suggestions made for defining and modelling the interactions. The approach is proposed as a prototype for examining the fate and significance of any other pollutant or natural material in aquatic systems.
An appendix lists the significant aquatic weeds of New Zealand, with a resume of the history, spread and biology of the major examples. The environmental interactions of these plants are reviewed, particularly with respect to eutrophication, together with currently available controls using mechanical, chemical and biological measures. |
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