An Assessment of the Herbicide Fluridone as an Invasive Aquatic Plant Treatment Option in Alaska (USA) Wetlands

Abstract

The Copper River Delta of south-central Alaska contains ecologically important wetlands that are threatened by climate change and human activities that have facilitated the establishment of the invasive aquatic plant species, Elodea canadensis, also known as Canadian waterweed.cElodea is extremely hardy and difficult to eradicate, and as a result, the herbicide fluridone, marketed under the trade names of Sonar, Avast, and Whitecap, has been proposed as a potential control method for infested water bodies. In this study, the effects of fluridone application on water column dissolved organic carbon (DOC), total nitrogen (TN), and chlorophyll-a (chla) were examined, as they are important indicators of water quality and ecosystem productivity. I hypothesized that concentrations of DOC and TN would initially increase as a result of plant death and then would decrease as fewer plants persist in the water body. Further, chla should increase as the fluridone kills plants because this increases the light in the water column needed for algal growth. The study took place in two invaded pond systems in the Eyak River and Alaganik River drainages of the Copper River Delta (CRD) near Cordova, Alaska. The first system (Cannery) had one pond divided by a barrier into reference and treatment ponds (WCW and WCE) as well as an uninvaded reference pond (EYS) monitored in case of contamination. The second system (Wrongway-Wooded) had separate reference and treatment ponds (WD and WW). Study data were collected from May to October for four consecutive years for the Cannery system (2016-2019) and six consecutive years in the WW/WD system (2016-2021). Fluridone treatments in both systems occurred for three years (2016-2018 for Cannery, 2019-2021 for WW/WD). Fluridone treatment had no significant effect on DOC or TN in either system but had a small negative effect on chla in the WW/WD system. These results suggest that fluridone treatments may have relatively minor impacts on water quality in CRD ponds, although further research may be needed to understand long-term impacts on water chemistry along with other community and ecosystem parameters.

Introduction

As the largest contiguous wetland on the Pacific coast of the United States, the Copper River Delta (CRD) of south-central Alaska provides important habitat for many native species, including Pacific salmon (Oncorhynchus spp.) and migratory birds (Mohlenbrock, 2006; Cline, 2005). This region is part of the Pacific Flyway and provides breeding grounds for vulnerable bird species such as the Dusky Canada Goose (Branta canadensis occidentalis) and Red Knot (Calidris canutus). The CRD is one of the few places in North America that maintains high biodiversity and relatively low human influence (Cline, 2005) and serves as an example of the previously pristine arctic and subarctic regions (Carey et al., 2016). However, this region is increasingly threatened by human development, climate change, and invasive species (Luizza et al., 2016). One particular challenge is the invasion of several CRD water bodies by the aquatic plant Elodea canadensis, also known as Canadian waterweed and hereafter Elodea (Carey et al., 2016). Elodea was first discovered in Alaska in 1982 and has expanded its distribution across the state since that time. Models have shown that favorable conditions for Elodea will reach farther into interior and western Alaska in the coming years (Luizza et al., 2016).

Elodea is successful as an invasive species because of its unique functional traits. Elodea can spread and grow from small fragments, easily attaching to humans and organisms and traveling to new water bodies (Carey et al., 2016). In addition to transport by biological vectors and water movement (e.g., river flow), several human factors have been identified as potential pathways of spread for Elodea, including fishing gear, boats, and most notably floatplanes (Carey et al., 2016). Additionally, Elodea differs from many native plant species in that it can survive in a wide range of conditions, often photosynthesizing more efficiently than native plants (Jones et al., 1996). As a result, when water chemistry changes, either because of human activities, climate change, or Elodea itself, the plant can continue to grow and reproduce (Schindler and Smol, 2006; Rahel and Olden, 2008; Carey et al., 2016). Similarly, Elodea can survive in water bodies with various levels of productivity (Carey et al., 2016) and sequester nutrients for long periods, allowing it to establish itself earlier during spring than other plants (Thiébaut, 2005). Due to these traits, Elodea can grow rapidly and dominate vegetation communities, thereby impeding boat traffic and human activities (Mjelde et al., 2012; Carey et al., 2016). The high growth rate of Elodea can also lead to increases in nutrients and subsequent eutrophication (Carey et al., 2016). This community dominance, in turn, can reduce water quality, decrease dissolved oxygen content, and increase water pH (Carey et al., 2016), resulting in impacts on native organisms such as Chinook salmon (Oncorhynchus tshawytscha) (Luizza et al., 2016).

Because Elodea has become established in some Alaska waters and is actively invading others, appropriate management strategies are needed. One potential management strategy is physical removal, which includes mowing plant heads or pulling out plants (Carey et al., 2016). However, this method has proven to be very ineffective for the eradication of Elodea (Carey et al., 2016; Zehnsdorf et al., 2015) because Elodea can grow from fragments, therefore cutting can fail to eradicate established populations and increase dispersal (Carey et al., 2016). Another potential management strategy is habitat modification, such as altering water levels to discourage Elodea growth (Carey et al., 2016). This approach, however, has had mixed results and could impact other organisms that may already be stressed by Elodea (Carey et al., 2016). Further, some researchers have suggested biological control methods such as introducing herbivorous fish such as rudd (Scardinius erythrophthalmus) and grass carp (Ctenopharyngodon idella) (Zehnsdorf et al., 2015). While biological control may be affable, these introduced species could also consume native plants (Carey et al., 2016, Zehnsdorf et al., 2015) as well as become invasive themselves (Carey et al., 2016).

Due to uncertainty surrounding the efficacy and logistical challenges of physical and biological control methods, management agencies have an interest in chemical methods of control such as with herbicides. One common herbicide is diquat, but diquat is a general herbicide, meaning it kills all plants in the area sprayed (Morton et al., 2014). Because of this, the use of a targeted herbicide has been suggested for Elodea control, especially in sensitive areas like pristine Alaska wetlands (Morton et al., 2014). Fluridone, which is marketed as a targeted herbicide under the trade names Sonar, Avast, and Whitecap, has been offered as a possible method for eradicating Elodea (Morton et al., 2014). Fluridone acts on plant metabolism by blocking the creation of carotene, which makes the plant extremely sensitive to sunlight and unable to photosynthesize properly (Morton et al., 2014). Fluridone has been shown to be effective at eradicating the desired targets at low concentrations (Schmitz et al., 1987), but the impacts of fluridone on the surrounding ecosystem remain unclear. An early study found that fluridone did not affect phytoplankton, zooplankton, benthic organisms, fish, or water quality and that dissolved oxygen increased after application, likely due to the decrease in night-time oxygen demand as the plants are killed (Arnold, 1979). However, the apparent effects of fluridone on non-target organisms are mixed, with some studies finding an impact on non-target species (Carey et al., 2016; Farone & McNabb, 1993; Netherland et al., 1997). Because fluridone treatments could cause unintended effects on aquatic ecosystems, its effects on the whole system must be understood (Bremigan et al., 2005; Netherland et al., 1997). Therefore, further research on the unintended effects of fluridone on aquatic ecosystems is needed including on water chemistry that governs many other ecosystem properties.

In this study, I examined the effects of fluridone application on water-column dissolved organic carbon (DOC), total nitrogen (TN), and chlorophyll-a (chla) of CRD ponds, as these variables are important indicators of water quality and ecosystem productivity. I hypothesized that fluridone would have a measurable effect on water quality because of its potential to disrupt the cellular metabolism of aquatic plants, which could cascade into effects on water chemistry. Specifically, I expected to see an increase in DOC and TN initially as a result of plant death, followed by a decrease over time as plant biomass declined. I also expected to see an increase in chla as the removal of macrophytes including Elodea should increase light in the water column which would encourage phytoplankton production. My specific objectives were to determine the effects of fluridone on water quality across time and space and explore the economic and political implications of invasive aquatic plant management.