Temporal variation of imidacloprid concentration and risk in waterways discharging to the Great Barrier Reef and potential causes.

The widely used neonicotinoid insecticide imidacloprid has emerged as a significant risk to surface waters and the diverse aquatic and terrestrial fauna these ecosystems support. While herbicides have been the focus of research on pesticides in Australia's Great Barrier Reef catchment area, imidacloprid has been monitored in catchments across the region since 2009. This study assessed the spatial and temporal dynamics of imidacloprid in 14 waterways in Queensland, Australia over seven years in relation to land use and concentration trends. Imidacloprid could be quantified (i.e., concentrations were greater than the limit of reporting) in approximately 54% of all samples, but within individual waterways imidacloprid was quantified in 0 to 99.7% of samples. The percent of each catchment used to grow bananas, sugar cane and urban explained approximately 45% of the variation in imidacloprid concentrations and waterway discharge accounted for another 18%. In six waterways there were significant increases in imidacloprid concentrations and the frequency and magnitude of exceedances of aquatic ecosystem protection guidelines over time. Overall, the risk posed by imidacloprid was low with 74% of samples protecting at least 99% of species but it was estimated that upto 42% of aquatic species would experience harmful chronic effects. Potential explanations of the changes in imidacloprid were examined. Not surprisingly, the only plausible explanation of the increases was increased use of imidacloprid. While field-based measurement of the effects of imidacloprid are limited in the Great Barrier Reef Catchment Area (GBRCA) the risk assessment indicates that biological harm to aquatic organisms is highly likely. Action to reduce imidacloprid concentrations in the GBRCA waterways is urgently required to reverse the current trends and mitigate environmental impacts.