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Are glyphosate-based herbicides harmful to aquatic organisms?

Category: Environment and Wildlife

In forestry scenarios, the risk to most aquatic organisms, including aquatic plants, is strongly mitigated by the use of protective buffer zones which are designed to minimize potential exposure levels. Risk analyses show that aquatic plants and algae are relatively more sensitive to glyphosate herbicides than aquatic animals. Among aquatic animals, fish and larval amphibians (tadpoles) are particularly sensitive to products containing POEA (a surfactant mixture used in some glyphosate based herbicides). Amphibians associated with small, shallow wetlands within targeted spray site are considered to be at relatively great risk. However, several field studies examining the effects of glyphosate and POEA demonstrate no significant effects on amphibians under realistic exposure levels and environmental conditions.

Small, shallow wetlands which are not mapped or easily observed from the air present a special case of potentially higher risk, particularly to amphibians which frequent these habitats. Although laboratory studies clearly demonstrate that fish and amphibian larvae (i.e. tadpoles) are quite sensitive to formulated glyphosate products containing the POEA surfactant, several field studies show no significant effects under realistic exposure scenarios. Some species of algae and aquatic plants are also very sensitive to glyphosate-based herbicides. Field study results support the conclusions drawn by several independent reviews which suggest that the use of glyphosate-based herbicides in accordance with product labels and as typically used in Canadian forest vegetation management do not pose a significant risk to aquatic organisms. This is particularly true given the routine application of buffers specifically designed to protect sensitive aquatic systems.

An extensive number of studies have focused on the potential effects of glyphosate-based herbicides to aquatic organisms including zooplankton, fish and amphibians and these studies have been reviewed by several authors (Giesy et al 2000, Solomon and Thompson 2003, Durkin et al. 2003; Tatum 2004) all of whom conclude that he likelihood of direct acute toxic effects to aquatic organisms are unlikely. Fish and amphibian larvae (tadpoles) are known to be highly sensitive to glyphosate-based herbicide formulations, particularly those containing the polyethoxylated tallow amine (POEA) surfactant when exposed under laboratory conditions (e.g. Folmar et al. 1979; Wan et al. 1989; Howe et al. 2004; Edginton et al. 2004). As part of a watershed level investigation on the effects of a glyphosate-based herbicide applied to a western Canadian coastal forest system (Holtby and Bailey 1989) , temporary stress effects and minor mortality (2.6%) were observed in caged coho salmon fingerlings held in an experimentally over-sprayed tributary and the main stream below the sprayed area. However, no acute mortality, changes in over-winter mortality, growth rate or probability of using the tributary were observed for resident fingerlings. Similarly, several subsequent studies confirm the general sensitivity of amphibian larvae to these herbicide products when exposed under laboratory or mesocososm conditions (e.g. Chen et al. 2004; Relyea et al. 2005; Williams and Semlitsch 2009).In general, the lowest reported concentrations resulting in 50% or greater mortality in amphibian larvae after 96 hrs exposure under lab conditions approximates 0.8 mg a.e./L (Edginton et al. 2004; Relyea and Jones 2099), although Williams and Semlitsch have reported >80% mortality in 1 of 3 amphibian larval species following exposure to the Roundup WeatherMax formulation at an equivalent of 0.6 mg a.e./L. Coincidentally, the 0.8 mg a.e./L value is also considered to be threshold concentration below which all aquatic organisms would be protected irrespective of exposure period (CCME 2012). Given the demonstrated sensitivity of amphibian larvae and the potential for direct overspray or off-target drift inputs to small ephemeral wetlands under typical forest-use scenarios, raised legitimate questions with respect to potential risk to amphibians under typical forest-use scenarios (Thompson et al. 2004; Govindarajulu 2008). To address this issue directly an extensive hierarchical program of research was conducted including laboratory, in-situ mesocosm, whole wetland and operational monitoring. Based on operational monitoring studies of typical aerial spray programs in Ontario (Thompson et al. 2004), the maximum concentration expected in such wetlands would be less than 0.55 mg/L (ppm) 99 times out of 100 (i.e. below the threshold concentration for significant acute effects). No significant differences were observed in mortality of two different amphibian species that were variously exposed in buffered, adjacent and oversprayed wetlands. Several other field studies have confirmed no significant acute effects of formulated glyphosate herbicide products on larval amphibian survival, growth or development even at levels considered to represent a maximum worst case in small wetlands (Wojtaszek et al. 2004; Edge et al. 2014; Edge et al. 2012). Similarly, in situ enclosure studies conducted in naturalized wetlands showed no significant effects on juvenile frogs directly exposed to a formulated glyphosate-based herbicide even following direct exposure at the maximum permissible label rates (Edge et al. 2011; Edge et al. 2013). The differences between laboratory and field study results can be generally explained by sediment sorption and degradation process that are active in natural shallow wetland ecosystems and which limit exposure magnitude and duration to both glyphosate and the POEA surfactant (Wojtasek et al. 2004; Edge et al. 2012; Wang et al. 2005; Rodriguez Gil 2015 Personal communication) as compared to laboratory studies where these factor are either not included or minimized in standardized testing protocols. Overall, results of these field studies confirm that the use of glyphosate-based herbicides in accordance with product labels and as typically employed in Canadian forest vegetation management do not pose a significant risk to amphibians or other aquatic organisms.

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