Effect of the herbicide triclopyr on the abundance and species composition of benthic aquatic macroinvertebrates in the Ahuriri River,New Zealand

Abundance and species composition of aquatic benthic macroinvertebrates were compared between an area sprayed with the herbicide Grazon (active ingredient triclopyr), and an upstream control site (no triclopyr). Five Surber samples were collected from each of three riffles in control and treatment sites on eight occasions over a 1‐month period. Aquatic invertebrate species composition was similar in treatment and control sites, and did not change over time. The five taxa that made up 91–95% of all invertebrates by abundance did not vary significantly in treatment compared to control riffles. Abundance of three of the 15 most common taxa (>10 individuals per riffle) differed significantly between treatment and control sites over time. However, none of these fluctuations correspond to the presence of known concentrations of triclopyr in water samples, and it is unlikely that the declines resulted from triclopyr. These results are discussed with reference to known lethal concentrations of triclopyr for some invertebrates, and to the effect of floods on invertebrate populations.