Concerns over diquat use in Lake Wanaka

REGARDING the use of the herbicide diquat to control Lake Wanaka’s weed problem (ODT letters, 10.5.17). Diquat is not a ‘‘permanent’’ solution as it does not affect the plant’s root system. Diquat, like 1080, can also wipe out nontarget species. Is ‘‘indepth’’ monitoring of the herbicide being carried out? Or is there a fear it may produce inconvenient findings such as the death of kea with every aerial 1080 operation — 12% of local kea on average.

A study of diquat carried out by several US universities (Cornell University et al) indicated it was ‘‘non selective’’ and would attack other aquatic plant species. Their study also indicated diquat dibromide is slightly toxic to fish and its toxicity to fish and food organisms on which fish survive has been reported in many studies. Some species of fish may be harmed but not actually killed by sublethal levels of diquat dibromide.

Regardless of any toxicity to fish, oxygen can become depleted in diquattreated water by decaying aquatic plants. This decreases the amount of oxygen available for fish survival. The US research also indicated that yellow perch suffer significant respiratory stress when herbicide concentrations in the water are similar to those normally present during aquatic vegetation control programmes. I would imagine this is no less the case for Lake Wanaka’s trout.

LC50 is the lethal concentration of a chemical in air or water that kills half of the experimental subjects exposed to it over a specific time period. A Ni was tu dy showed concentrations of 2.9 ppm is the LC50 for juvenile trout. Even if concentrations used are less than this, are there accumulative effects? Apparently no monitoring of fish has been carried out on Lake Wanaka. I would have thought that to be a priority. Regardless of what are touted as ‘‘safe’’ levels, it seems there is a love affair and a whole industry built around the quick and easy blanket use of ‘‘poisons’’ in New Zealand. G. Spearing

Oamaru [Niwa freshwater ecologist Paul Champion replies: ‘‘Diquat is one of two herbicides registered in New Zealand for use in water. Diquat is a shortlived contact herbicide controlling aquatic weeds. The fate and environmental impact of diquat application to aquatic weed beds has been the subject of intensive studies in New Zealand and overseas.

‘‘Diquat is toxic in its concentrated form, but at the concentrations used in the environment (commonly one part to one million parts of water) it has been shown to have minimal impact on aquatic fauna and the New Zealand Environmental Protection Authority has permitted its use for aquatic weed control here.

‘‘G. Spearing quotes a Niwa fact sheet on diquat, which states that 50% of juvenile trout are killed when exposed to a concentration of 2.9 ppm. This test involved keeping fish in water at that concentration for 96 hours. This would not be the case in Lake Wanaka or other water bodies where the herbicide concentration rapidly declines after application. There is no evidence of bioaccumulation of this compound.

‘‘There are concerns that the decomposition of aquatic weeds killed by diquat may deoxygenate water and have harmful effects on fish and other aquatic life, especially in warm water. In the case of deep lakes like Lake Wanaka, the area of weed beds treated are very small compared with the surrounding volume of water and decomposition is generally over a few weeks. Fish tend to avoid waters with low dissolved oxygen.

‘‘Aquatic plant species range in their susceptibility to diquat from tolerant (for example, native characeans) to highly susceptible (for example, oxygen weed species). The selective action of diquat can be utilised to enhance lowgrowing desirable native characean species, while controlling tall nuisance weed beds.

‘‘G. Spearing rightly points out that the roots of lagarosiphon are not killed by diquat, but this herbicide is commonly used where there are extensive weed beds to be controlled and rapid reduction of plant biomass is needed. This method of weed control is generally the most costeffective and efficient. A range of other tools are used in Lake Wanaka in areas where lagarosiphon is not well established and progressively areas are being permanently cleared of this weed (eradicated) in this lake.’’]