Persistence and non-target impact of imazapyr associated with smooth cordgrass control in an estuary

摘要

The herbicide (+/--2-[4,5-dihydro-4-methyl-4-(I-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-p yridinecarboxylic acid) (imazapyr) has shown potential to control smooth cordgrass (Spartina alterniflora Loisel), a noxious weed in many estuaries throughout the world. Research was conducted under tidal estuary conditions in Willapa Bay, Washington, to determine imazapyr's persistence and aquatic risk and impact to non-target estuary species. Persistence of imazapyr in water and sediment followed an exponential decay. The maximum levels of imazapyr found in water and sediment samples after application were 3.4 mug/ml and 5.4 mug/g, respectively which are 5 to 6 orders of magnitude lower than levels needed to affect aquatic invertebrates and fish. Imazapyr in water and sediment samples approached the zero asymptote at 40 and 400 hours respectively, with half-lives of <0.5 and 1.6 days, respectively. Water collected 6 or 30 in from outside the spray zone with the first incoming tide was 99% lower than the maximum water concentration collected at the edge of the spray zone. In comparison to imazapyr applied to bare mud, an application to a smooth cordgrass canopy resulted in 75% less herbicide in the underlying sediment. The amount of imazapyr found in the interstitial water within the sediment was slightly less than that found in sediment. Japanese eelgrass (Zostera japonica Aschers and Graebn) was killed by imazapyr when it was applied to a dry canopy, but sites were reinfested within 1.0 year of treatment. Applications of imazapyr to native eelgrass (Zostera manna L.) and Japanese eelgrass covered by a thin film of tidal water had no effect. Smooth cordgrass treated with imazapyr was colonized by Japanese eelgrass and Salicornia (Salicornia virginica L.) within 12 to 20 months MAT. The osmoregulatory capacity of Chinook salmon (Oncorhynchus tshawytscha (WAL-BAUM)) smolts based on plasma sodium level and gill ATPase was not affected by imazapyr at concentrations up to 1600 mug/l. Based on these studies, imazapyr had a short half-life in estuary conditions and there was a very large margin of safety between the maximum concentration of imazapyr that occurs following application and the concentrations that could result in toxicity to invertebrates and fish. There was no direct or indirect evidence of long-term persistence or impact following its use in an estuary. It is unlikely that imazapyr used to control smooth cordgrass will pose a risk to non-target estuarine organisms.