Environmental Effects of The Invasive Macrophyte Eurasian Watermilfoil

Abstract

Eurasian watermilfoil, Myriophyllum spicatum, is an invasive aquatic weed that first appeared in the United States in the early 1940s. Since its discovery, it has come to plague thousands of bodies of water all across North America. To combat the spread and eradicate existing colonies, a number of methods have been exploited, some with better results than others. The plant can be excised mechanically via machinery or manually by uprooting the plant at its base. More common methods for counteracting the plant growth include chemical compounds designed to not only kill off the invasive species, but also to promote the biological stability of less invasive, native species. Such chemical applications have experienced mixed results. Lastly, a number of studies have discussed the impacts of introducing enemies native to the invasive species, such as milfoil weevil and certain crayfish, in attempts to contain the rampant proliferation of Eurasian watermilfoil. This latter method is often considered to be the most effective and also impose the least negative ecological consequences on the stability of the ecosystem. This study attempts to further the area of containment as relates to artificially populating aquatic ecosystems with natural predators of Myriophyllum spicatum. A previously unstudied species of crayfish, Oconectus immunis, was studied with regards to its ability to affect the biomass of Myriophyllum spicatum. Results after 10 weeks of observation concluded that there was a significant reduction in the overall biomass of populations of Eurasian watermilfoil exposed to both medium densities of crayfish and high densities of crayfish. In line with other studies, impacts on Eurasian milfoil associated with low densities of crayfish were negligible. While the results of this study may have implications in the recognition of additional species to reduce invasive Eurasian milfoil populations, further inquiry is necessary to more fully understand the consequences of such treatments on the macrophyte in question as well as on the aquatic ecosystem affected.

Environmental Effects of The Invasive Macrophyte Eurasian Watermilfoil

The aquatic plant Myriophyllum spicatum, better known as Eurasian watermilfoil, is a pesky waterborne weed that afflicts lacustrine ecosystems all over the world. In the United States, the first confirmed specimen was identified in Washington D.C. in 1942 (Buchan & Padilla, 2000, p. 1442). Since then, it has reached the aquatic environments of lakes in 46 states. More and more, it has become a problem not only to the stability of ecosystems, but also to regions that depend on the integrity of their lakes to attract tourists. Many such communities experience revenue-related difficulties when macrophytes like Eurasian watermilfoil hit their waters because it can create problems for boating and fishing enthusiasts.

Across the country, communities are working to curb the spread of invasive non-native species like Myriophyllum spicatum. Local governments, Green schools, and environmental education institutions are continually working in tandem in an effort to contain the spread of such malicious macrophytes. The Ojibwe Reservation in Wisconsin, in a joint collaboration with the LCO Ojibwe Community College Extension, has recently earned a grant to help curtail the spread of this menacing migratory nuisance (LCO, 2005). Myriophyllum spicatum has become such a widespread problem in fact, that in 2006 “the Idaho legislature approved $12 million over three years to eradicate Eurasian watermilfoil” (Control, 2008, p. 54-55). Still, amidst much discussion about the negative aspects surrounding the introduction of such invasive species into lacustrine habitats, some point out that the aquatic does actually provide some benefit to marine environments, if only to a very limited extent.

One of the benefits of Eurasian watermilfoil, indeed it appears there are few, is its ability to cleanse the surrounding environment of toxic chemicals. Because of heavy, though largely mishandled and inappropriately disposed of, commercial and civilian use of trinitrotoluene, or TNT, there has been significant groundwater and surface water pollution near such sites. To combat this, one study reported the “rapid depletion of TNT from M. spicatum culture media was observed and the TNT disappearance rate was a function of both TNT and plant concentration” (Jacobson et al., 1998, p. 2272). Furthermore, the study reported that such disappearances were in fact the result of biological activity, and not simply chance, given that the addition of sodium azide inhibited the transformation of TNT (p. 2272). For drinking water. it's imperative that human exposure to chemicals like TNT be eliminated.  And the aquatic weed’s cleansing abilities concerning TNT seem to be only a single aspect of the macrophyte’s beneficial properties.

Another purported benefit of M. spicatum is the apparent inhibition of the growth of Microcystis aeruginosa, another macrophyte that can be rather detrimental to overall water quality. One study from Nikai, Yamada, and Hosomi (2005) concluded that Eurasian watermilfoil actually benefits the water quality by inhibiting the growth of this macrophyte, as M. aeruginos is one of the more problematic and toxic plants that plague aquatic systems. However, the general consensus among the scientific community is still of the opinion that any Eurasian watermilfoil colonization results in more harm than good.

To further determine which characteristics of Eurasian watermilfoil are the strongest, the beneficial or the detrimental, it is necessary to comprehensively examine the current body of literature. Though much research has been performed that describes the basal effects of the introduction of such a macrophyte into new aquatic ecosystems, it is also necessary to consider ancillary implications surrounding such infestations. Furthermore, both positive and negative effects of various methods of treatment must be considered in determining to what extent Myriophyllum spicatum indeed helps, or hinders, the current stability of the marine life in a given system. Finally, tertiary consequences surrounding the economic impact of such macrophyte colonizations, as well as the actions taken to counteract migrations of such exotic non-native species, should receive equitable investigation as pertains to the aforementioned primary and secondary inquisitions.

Literature Review

The presence of such exotic non-native species alone can be the source of much instability in aquatic ecosystems around the world. One study asserts “exotic species are one of the major causes of loss of biodiversity in aquatic ecosystems in North America” (Boylen et al., 2006, p. 243). Considering the fact that a confluence of multi-million dollar policies are in place attempting to combat the spread of this macrophyte, and the reality that programs are generally only implemented after a contamination has been discovered, one can see why it might seem like we, as a nation, have been spinning our wheels in our attempts at containment. This in addition to the fact that early determination of such specimens can be rather difficult without substantial investments of time or money, is another reason why containing the spread of this macrophyte remains difficult. Additionally, this purported loss in biodiversity is often attributed to “public boat launching areas as the introduction of M. spicatum” (Boylen et al., 2006, p. 247). As such, it appears that enthusiasts who enjoy taking their boat to the lake for a day of water skiing or inner tubing might actually be promoting the spread of one of the most pervasive, resilient species of macrophyte to ever come in contact with North America. The convergence of all of these factors has culminated to transform Eurasian watermilfoil into one of the most prevalent, pestilent species to ever afflict the lacustrine ecosystems of this continent.

There are a number of factors that contribute to the demise of native species as a result of Eurasian watermilfoil having been introduced into aquatic ecosystems. Some of the most damaging aspects to native species pertaining to the invasion of the macrophyte Myriophyllum spicatum, are the means in which the weed impedes native species’ access to light, as well as its natural advantage for acquiring nutrients. The study from Eiswerth, Yen and Van Kooten (2011) explain, “because of its higher efficiency in nutrient uptake and photosynthesis (Grace and Wetzel, 1978), milfoil’s large canopy reduces the sunlight available to native plants (Madsen et al., 1991)” (p. 1673). Additionally, when exotic invasive species infest lacustrine systems there are also adverse effects on native fish populations and marine organisms. Eiswerth, Yen and Van Kooten (2011) also noted that “because aquatic invasive weeds alter the composition of native aquatic plant communities, they can adversely impact animals that depend on those communities (Madsen, 1997)”, and further expounded by describing how such marine life might be affected, “such as large mouth bass (Micropterus salmoides) via deceased predation success (Engel, 1987), and salmonids (Salmonidae spp) via reduced spawning success (Newroth, 1985)” (p. 1673). Such consequences are merely an introduction into the widespread negative aspects of exotic invasive species inhabiting ecosystems. The numerous methods available via which to combat the spread of such exotic species is an important component in determining the overall value or cost associated with Eurasian watermilfoil’s migratory propensities. Lastly, the effects of mechanical and biological treatments also comprise a significant portion of the investigation into the dynamics of non-native invasive species’ colonizations.

Detrimental Effects of Eurasian Watermilfoil Invasion

A recent study that focused on four distinct bodies of water in the regions of Quebec and New York had some interesting results concerning the concentration, abundance, and diversity of various macrophytes. The study from Wilson and Ricciardi (2009) went as far as to conclude that their findings were in direct contradiction to one “invasional meltdown hypothesis, [which] postulates that previously established exotic species facilitate the invasion or proliferation of other exotic species (Simberloff and Von Holle 1999)” (p. 28). Such theories essentially posit that an already well-established presence of aquatic invertebrates will lend itself to a more conducive environment in which foreign species may populate. Naturally then, Eurasian watermilfoil, or Myriophyllum spicatum, would theoretically allow for the accelerated proliferation of other exotic species of invertebrates in its given setting. However, the study reported that the “abundance of the Eurasian snail Bithynia tentaculata was not significantly different on native and exotic macrophytes”, (Wilson & Ricciardi, 2009, p. 28). As such, one is faced with the possibility that an existing population of exotic macrophytes, such as Myriophyllum spicatum, not only has little effect on the growth and population of other exotic species, but that its presence may in fact hinder such cultivation among other species. The study concludes by stating “the establishment of M. spicatum can reduce the density of many other species of macrophytes, including native milfoils” (p. 28). In all, it seems that these findings, based in the research conducted at the Richelieu River, the Lac St-Louis River, Chateaugay Lake, and Saranac Lake, demonstrated Eurasian watermilfoil’s apparent propensity for interference with other macrophyte species’ attempts to spread. The implications for such discoveries are significant as the abundance or scarcity of different species may directly affect the homeostasis of that region. In addition to the effects that Eurasian watermilfoil has on other exotic species, it can also significantly deter the populations of native species.

The loss of native aquatic macrophytes is one of the most serious consequences of colonization of the invasive exotic species Eurasian watermilfoil into lacustrine systems. As Myriophyllum spicatum proliferates a newly inhabited area, other native species are left without significant exposure to light or nutrients as a result of the Eurasian milfoil’s superiority in nutrient uptake and photosynthetic capabilities. One study tracked the expansion of a newly discovered bed of Eurasian watermilfoil by placing a 6m2X6m2 grid directly over its center and frequently monitoring the macrophyte’s progress. Noting the effects of Eurasian watermilfoil expansion over the course of nearly a decade, the study concluded that, regarding native species, “within three years (1989), the formerly native communities were reduced to only a few stems of native plants under a dense canopy of M. spicatum” (Boylen, Eichler, & Madsen, 1999, p. 209). The growth of Eurasian watermilfoil was so accelerated in fact, that after two years “percent cover in the grid was almost 80%, and approached 100% by 1989” (p. 208). For a newly inhabited aquatic weed to achieve such astounding areal coverage in the matter of only a few years is simply astounding, but perhaps even more surprising was the manner in which Myriophyllum spicatum displaced the native species there previously residing. The authors also commented that the overall biodiversity of the region monitored significantly declined, stating “species richness declined from an average of 5.5 species per quadrat in 1987 to slightly over two in 1989 (of which one species was always M. spicatum)” (p. 209). A reduction in biological diversity is not only significant as it relates to fewer species inhabiting ecosystems, but such reductions also affect food chain stabilities of aquatic ecosystems since primary sources of nutrition for one or more organisms may no longer exist, or thrive, in that system.

Effects on Food Chains

Further effects surrounding the invasion of non-native Eurasian watermilfoil into heretofore-unaffected systems include the degradation of basal food chain plants essential to life in such aquatic environments. One study corroborates such claims in stating “introduction of non-native plants may alter the complex interactions occurring in this habitat....[and] Dense stands of non-native plants are often responsible for reduction in oxygen exchange, depletion of dissolved oxygen, increases in water temperatures, and internal nutrient loading” (Madsen et al., 2008, p. 97-98). Not significantly dissimilar to human beings, plant life can suffer tremendously as a result of changes to ambient temperatures. Additionally, a reduction in available oxygen can negatively impact the native macrophytes of the region. Further noting the decline in native macrophyte species, the authors of this study explain, “monotypic stands of Eurasian watermilfoil directly reduce native-plant species richness and diversity, and also indirectly reduce habitat complexity resulting in reduced macroinvertebrate abundance” (Madsen et al., 2008, p. 98). Due to some of the unique characteristics of Eurasian watermilfoil, the study found that there was a preponderance of the plant at various depths throughout the New York lakes that were studied, and that the invasive species had all but replaced the native species indigenous to the region (p. 105). As might seem clear at this point, there is substantial evidence conceding the negative outcomes that arise as a result of invasive colonizations of the plant, Myriophyllum spicatum.

As has already been mentioned, research into the negative effects of Eurasian watermilfoil and other invasive macrophytes has determined that the introduction of these species may significantly alter the dynamics of the food-chains of affected systems. Another study from Kovalenko and Dibble (2011) found that among systems replete with Eurasian watermilfoil, “isotopic niche width was significantly different in watermilfoil dominated lakes” (p. 171). The study found that isotopic niche was actually greater in areas affected by dominate populations of Myriophyllum spicatum, resulting in assumptions which postulate, “some invasive plants are not used by the fish in the same way as native plants “ (p. 171). Additionally, while the introduction of species like Eurasian watermilfoil into unaffected systems might result in changes pertaining to the trophic diversity, such introductions seem to have little effect on the trophic position as relates to those whom are second in line in the milfoil consumption food chain. In other words, macrophytes like Eurasian watermilfoil might alter the variety of nutrition available to marine life in a system, but their presence does not significantly change the order in which a particular system’s food-chain operates. Nonetheless, recognition of the effects of Myriophyllum spicatum concerning a system’s trophic diversity must be considered since, unlike humans, a majority of organisms in nature maintain a fairly homogenous diet. As such, any change in the availability of certain nutrients might negatively affect the stability of one of more organisms whose livelihood is dependent on such nutrients. Additionally, substantial economic troubles can arise as a result of Eurasian watermilfoil invasion.

Economic Impacts

Eurasian watermilfoil can source of a vast array of problems in regards to the economics of communities with revenues derived from lake-based attractions. For example, the sale of boating permits would certainly be affected in the event that a lake became infested with Eurasian watermilfoil. Additionally, as a source of state revenue, fishing license sales might also experience volatility in the event that invasive aquatic weeds colonize a previously uninhabited lake or other body of water. One study, examining the potential economic impacts of Eurasian watermilfoil that might arise from the colonization of a major tourist attraction location such as Lake Tahoe in California, observed “that even a relatively small percent impact [to the percentage of people who travel to Lake Tahoe for recreation] could be significant in absolute terms”, explaining that “a 1% decrease below an annual baseline of $50 million is on the order of [a loss of] $500,000/year” (Eiswerth, Donaldson, & Johnson, 2000, p. 516). Additionally, it is important to recognize that such annual baselines, in other words the annual net economic benefits of the entity, were calculated very conservatively (p. 515), so dollar amounts double or even triple the reported numbers would not be outlandish. Furthermore, assuming such a miniscule reduction in the annual baseline reflects extreme conservatism, as it is much more likely that a widespread colonization of Myriophyllum spicatum would result in reductions to this figure significantly greater than a mere one percent. All things considered, this study represents yet another example of the devastating impact that invasive aquatic species can affect on otherwise stable lacustrine habitats. Still, Myriophyllum spicatum can incite economic disturbance that not only affects lacustrine communities, but also the housing markets situated in the vicinity of such communities as well.

One of the more ancillary adverse effects of the introduction of Eurasian watermilfoil into previously unaffected systems are the negative economic impacts the weed imposes on housing markets. With some estimates placing the economic cost of worldwide invasive species at more than $1.4 trillion, it is no surprise that lakefront properties exposed to Myriophyllum spicatum constitute a large percentage of those losses (Zhang & Boyle, 2010, p. 394). This same study fro Zhang and Boyle (2010) concluded “Eurasian watermilfoil significantly and substantially affects lakefront property values as the primary component of total aquatic macrophyte growth in a lake (p. 403). Furthermore, the study relates how “if milfoil infestation levels increase from 4 (41%-60% coverage), to 5 (61%-80% coverage), the marginal change can have a 6.4% reduction in property values….an aggregate property value loss of about $6.4 million” for 1000 lakefront properties with an average value of $100,000 (p. 403-404). One might reasonably have assumed, after some introductory reading on the matter, that the introduction of Eurasian watermilfoil can certainly have negative effects on the stability of the affected ecosystem. However, it may not be widely understood that such infestations are often accompanied by concrete economic detriment. Reiterating the magnitude of such findings, it might serve to mention that another study from Colautti et al. (2006) found “nuisance non-indigenous species result in an ‘invisible tax’ that ranges between CDN $13.3-34.5 billion per year” (Frid et al, 2010, p. 413). That such substantial economic losses might stem from the presence of an aquatic weed should represent adequate motivation for local governments to bolster efforts of eradicating this malicious macrophyte.

Controlling The Spread

There is an abundance of research explicating the numerous unfavorable outcomes associated with the non-native invasion of exotic species into otherwise stable aquatic environments. Given the fairly unanimous consensus as to the question of whether or not Eurasian watermilfoil is ultimately beneficial or harmful, it is now necessary to examine the collection of research that pertains to various methods of treatment for eradicating Myriophyllum spicatum infestations before formulating a method for this particular study. Among the methods that have been extensively tested are those that focus on the manual excavation of the weed, those that resort to chemical applications, and those that purport the artificial population of the macrophyte’s natural enemies.

Mechanical and Manual Approaches

While it may require a more concerted physical effort, mechanically removing Eurasian watermilfoil may significantly increase the life span of native aquatic organisms, as well as boost economies with revenue streams that arise as a result of lacustrine activities. Because of the many detriments brought upon lacustrine systems as a result of the infestation of Myriophyllum spicatum, its removal can substantially impact the overall health of marine life. Commenting on the survival rate of bluegill bass, one study noted that “after the vegetation removal, natural mortality declined for blue gills ages 4-6” (Unmuth & Hansen, 2001, p. 448). Furthermore, the authors stated that addressing the Eurasian milfoil infestation also resulted in the harvesting of bluegills that were significantly larger (p. 448) which might indicate an overall healthier aquatic environment in which for marine life to thrive. Additionally, ridding lacustrine environments of Eurasian watermilfoil may produce monetary benefits that otherwise might not have been present. The same study noted that, following the removal of the macrophyte, Myriophyllum spicatum, “…the number of boat trips in summer increased significantly” (p. 448). Since boaters and anglers both need permission to engage in activities related to local lakes, the increased sale of permits and day passes as a result of removing Eurasian watermilfoil indicates economic advantages relative to the treatment of this invasive species that might have been heretofore scarcely considered. This particular study indirectly sheds light on the economics of Eurasian watermilfoil invasion and certainly necessitates further investigation into the matter. Additionally, Unmuth and Hansen illustrate that regardless of the particular containment method, e.g. mechanical or biological, ridding an aquatic ecosystem of invasive species can significantly benefit the quality of life of native marine organisms. One method that is not quite as clear-cut concerning the benefits to native marine life, however, is the implementation of benthic barriers.

Another approach to combating the existing populations of Eurasian watermilfoil is with the use of benthic barriers. Benthic barriers are essentially large tarps spread out across the top of the water and then dragged to the bottom of the lake to physically confine the plant between the floor of the lake and the tarp. Once in place, these mats are covered with weights to keep them from drifting or otherwise shifting around. The purpose of benthic barriers is to block incoming sunlight that would otherwise facilitate the growth and proliferation of aquatic weeds like Myriophyllum spicatum. There has been considerable success on behalf of those employing such eradication methods. One study from Laitala et al. (2012) conducted experiments where interspersed populations of Eurasian watermilfoil were covered for periods of 4, 8, 10, or 12 weeks, respectively, and found that “the 4-wk duration reduced Eurasian watermilfoil biomass 75%, and all other duration treatments reduced Eurasian watermilfoil biomass 100%”. The authors further explicate how “the 4-wk treatment had no effect on native plant biomass” (p. 170). The efficacy of this method of treatment might only be matched by its inherent simplicity, considering the fact that if hiring professionals to treat a surge in Eurasian watermilfoil is out of the question financially, such an apparatus can be made ‘from scratch’ for a fraction of the commercial cost. During the longer treatment durations of between 8-12 weeks, the study noted that there were some adverse effects on the native plant population, citing “other treatments reduced native plant biomass 79 to 93%”, and also noting that “At the conclusion of the 12-wk study, Eurasian watermilfoil biomass had increased in the 4-wk treatment but did not reestablish within treatment plots of longer duration” (Laitala et al., 2012, p. 170). Ultimately, the study concluded in reporting “the 8-wk duration is sufficient for removal of Eurasian watermilfoil while allowing regrowth of native aquatic plants” (p. 170). The results seem to indicate a very viable approach to combating the spread of this particular macrophyte. While other studies have experienced erratic success contingent on weather conditions and seasonal variances, it appears that the success of benthic barriers is due in large part to their design. In design, it should be understood that there is relatively little as pertains to such devices, conceding that the simplistic nature of separating light from plant is achieved in nothing more than the implementation of a common physical barrier between the two. As such, this method may in fact stand as the single most viable treatment option for curtailing not only the spread, but also the existing populations of Eurasian watermilfoil.

Introducing Natural Enemies

Until recently, one reason for the plant’s seemingly invincible reputation may have arisen from milfoil treatments with poor efficacy. For many years, topical agents were applied to the visible portions of Eurasian watermilfoil, though often with little success. However, one article reported on the studies of researchers out of the University of California at Davis, stating “a better option for the future may be to apply the herbicide triclopyr below the water surface” (Strand & Weisner, 2001, p. 16). Still, some scientists out of UC Davis suggest that the best treatment method may in fact lie in the artificial population of herbivorous insects into infected lakes. In fact, the study “showed that the relative growth rate of a weevil that eats a water weed called hydrilla increased by 50% when the weevil was fed plant material with 3.5% nitrogen, as compared to plant material with only 2% nitrogen” (Strand & Weisner, 2001, p. 17). Implementing such measures would have exponentially increasing benefits when one considers that the more milfoil these weevils consumed, the larger they would become, and as a result of their growth would consume even more milfoil, and so on. However, the fact that Eurasian watermilfoil has lower nitrogen contents than some other macrophytes leads a number of scientists to assert artificial population control is still the most effective method for curtailing the spread of this aquatic weed. Still, there is much evidence to that supports the effectiveness of biocontrol methods when properly executed. However, some explanations as pertain to the effectiveness of certain methods are less obvious than others.

Another complexity surrounding the extensive colonizations of Eurasian watermilfoil is not centered on whether or not the plant is attractive to macroinvertebrate species from a eutrophic standpoint, but whether the concrete physical structure of the plant is any more appealing than the structure of other macrophytes. Investigations in this respect, while initially somewhat trivial to some, serve a purpose in further explicating the circumstances surrounding invasions of non-native macrophytes. Determining factors that contribute to the consumption of various macrophytes on behalf of macroinvertebrates might shed light on different approaches to combating the spread of Eurasian watermilfoil and other invasive species. In fact one study from Hansen, Sagermon, and Wikström (2010) concluded “the species identity of large-structuring soft-bottom plants can influence the small-scale distribution of plant-associated macroinvertebrates” (p. 2152). This study was conducted using a combination of live plants and artificial plants, and “found support for higher invertebrate abundance and biomass…in morphologically more complex plant habitats than in simpler plant habitats” (p. 2152). Ultimately, this study seems to have determined that various invertebrate organisms are not only drawn to non-native macrophytes like Myriophyllum spicatum for their trophic attributes, but perhaps as a direct result of the structure of the plant. As such, attempts to eradicate this aquatic weed may need to incorporate the artificial population of not only fish, but certain insects as well.

Herbivores

The availability or deprivation of certain macrophytes in aquatic environments may dictate the predatory patterns of marine life within the system. Specifically concerning the consumption of watermilfoil herbivores by lake-fish, one study concluded “predatory sunfish reduced the density of milfoil herbivores” (Ward & Newman, 2006, p. 1055). These findings may constitute a milestone in the efforts of scientists to curtail the spread of Eurasian watermilfoil in lakes around the globe. In many aquatic environments, there is an overabundance of Eurasian watermilfoil as a result of overpopulated fish species. Not directly, of course, but many herbivorous invertebrates whose diets consist largely of various species of macrophytes become a staple in the diets of indigenous fish. As a result, the invertebrates responsible for curbing the milfoil epidemic are to a large extent eradicated, thus leaving species like Myriophyllum spicatum to thrive in the absence of predators. However, maintaining or increasing the populations of these herbivorous life forms might be the key to finally slowing the spread of Eurasian watermilfoil in aquatic environments. The study itself notes that in order to curb the populations of this macrophyte, “adequate densities of herbivores must be sustained over the season to control the plant” (Ward & Newman, 2006, p. 1056). It must further be noted that such hypotheses are not strictly theoretical, as scenarios where lakes containing such concentrations of herbivores have already been observed as significantly suppressing milfoil populations. While the artificial population of various species of fish into littoral habitats may not serve as the most effective or even practical approach to combating the proliferation of Eurasian watermilfoil, it is certainly a realistic and viable option to solving the problem.

The milfoil weevil, Euhrychiopsis lecontei, is another herbivore that may affect additional difficulties on Eurasian watermilfoil not only by means of direct consumption, but also indirectly as a lasting detriment to the plant’s ability to survive. A study from Newman et al. (1996) whereby milfoil weevils, in numbers of 0, 6, 12, or 24 were placed inside containers with Eurasian watermilfoil and monitored, reported “weevil stocking density resulted in a significant decline in watermilfoil biomass” (p. 131). What is important to understand concerning these findings is the manner in which such reductions were achieved. The study further explicates, “total stock (g per plant) of sugars, starch and total nonstructural carbohydrates was reduced in the roots”, resulting in the researchers conclusions that “Herbivory by weevils may have long term effects via the disruption of plant carbohydrate stores that are essential for over-winter survival and subsequent growth” (p. 131). The significance of these findings lies in the understanding of milfoil weevils’ effect on the biological integrity of Eurasian watermilfoil. Affecting not only the physical structure of the plant by means of consumption, Euhrychiopsis lecontei also manages to disrupt the chemical structure of the macrophyte in such a way as to threaten the overall stability of the organism going forward.

Yet another organism that exhibits a propensity for the degradation of conditions conducive to the survival and proliferation of Eurasian watermilfoil is the algivorous snail. In a study that tested the growth of Eurasian watermilfoil in relation to eutrophic conditions of the host system, as well as in relation to the presence of snails, the researchers concluded, “that M. spicatum thrives in habitats with high nutrient inputs and few algivorous consumers (e.g., snails)” (Chase & Knight, 2006, p. 1646). Furthermore, the study found that “in the presence of snails, increasing levels of nutrients had little effect on the biomass of epiphytic and filamentous algae, M. spicatum, or native macrophytes, but large increases in the biomass of snails (p. 1646). It must be noted that absent any snails, Eurasian watermilfoil and other native microalgae experienced considerable growth in biomass, and only the introduction of snails to the system hindered such reactions to nutrient enrichment (p. 1646). As such, an interesting discovery was observed over the course of this study that may in fact have important implications in alleviating the presence of Eurasian watermilfoil in many lacustrine ecosystems. Given that macrophytes like Myriophyllum spicatum thrive in environments plush with nutrients, but only marginally so when snails are introduced into identical conditions might be indicative of certain biocontrol characteristics innate to snails. Similar to experiments involving the milfoil weevil, it appears that snails may represent a previously unconsidered avenue via which to pursue the containment of Eurasian watermilfoil. In addition to a variety of herbivorous organisms, the introduction of crayfish has also been employed to control populations of Eurasian watermilfoil.

Crayfish

Further attempts at limiting the spread of Myriophyllum spicatum involve investigations into the interactions of the plant with various other aquatic life and how those interactions diminish or proliferate the plant’s propagation. One study in particular focused on the relationship between Eurasian watermilfoil and the rusty crayfish, specifically in a number of lakes located in the western regions of Quebec, Canada. In the study, cages were places atop areas of dense congregations of milfoil and then artificially populated with either low, medium, or high concentrations of crayfish. The study, from Maezo, Fournier, and Beisner (2010), concluded “only in high-crayfish treatments was milfoil biomass significantly reduced (by 68%) at the end of the experiment compared with the absent crayfish treatment” (p. 689). Even still, that significant reductions in milfoil populations were reported indicates a potentially dramatically effective measure for reducing colonizations of Eurasian watermilfoil. Furthermore, the study reported “…macrophytes encountered at low densities under the milfoil canopy…were completely eliminated in the high-crayfish treatment and almost eliminated in the medium-crayfish treatment” (Maezo, Fournier & Beisner, 2010, p. 689). These findings are important as, despite the researchers’ lack of enthusiasm concerning the outcome of the experiment, namely that ‘only’ high-density populations of crayfish resulted in the reduction of milfoil populations, the 68% reduction that was observed was rather significant. If populations of crayfish were maintained at these high levels, less than one percent of the original milfoil population would remain inside of a year. Again, while researchers continue the hunt for methods of prevention and eradication that are both economically viable and which can be realistically implemented, identifying a method with solid rates of reduction is a grand feat in and of itself.

The damaging effects of infestation by invasive non-native species into regions previously uninhabited are quite substantial regarding the stability of the system affected, so the integration and development of programs designed to control the spread of such invasives is paramount to maintaining a firm grasp on future colonizations. Recent studies have shown that the benefits of implementing policies to control the spread of these waterborne weeds far exceeds any costs associated with such programs. One study from Frid et al. (2013) concluded, “despite large upfront investment costs required in the development of a biocontrol programme…the potential economic returns of such a programme are significant” (p. 429). The methods of prevention used in this study were both mechanical and biological, either using a rototiller to physically destroy the plant at its base, or a chemical spray applied externally. While such programs entail the maintenance and continual upkeep of various regions from both an aquatic and terrestrial standpoint, the study demonstrates the benefits of keeping Eurasian watermilfoil at bay. The study also asserts that the proposed prevention program is suitable for this particular type of macrophyte, stating “because of its rapid spread rate and persistence in the environment, watermilfoil may be another invasive that warrants the development of biocontrol” (p. 429). Although rototilling lakeshores may constitute another method of prevention somewhat more viable to controlling the spread of Eurasian watermilfoil, the biological treatment of such outbreaks might be even more effective.

Chemical Applications

The seeming invincibility of Eurasian watermilfoil is made manifest in the weed’s apparent resilience to various herbicides. That the organism shows virtually no signs of being affected when administered such chemicals is certainly one of the reasons it has experienced such unchecked proliferation throughout many of the world’s lacustrine waters. At least that was the case twenty years ago. Now, however, a recent study has demonstrated that in fact Myriophyllum spicatum is susceptible to certain forms of herbicide, and not only was the plant affected, but it experienced an 88% reduction of its total population from the beginning of the study to the end of it (Wersal et al., 2010, p. 5). Even more astonishing were the results showing no significant impact on the well being of native species. Furthermore, the study indicated that “the use of triclopyr resulted in 91% control of Eurasian watermilfoil at 5 WAT…[and] similar results were achieved in the Pend Oerille River, Washington, where overall biomass of Eurasian watermilfoil was reduced by 99% within the year of treatment and maintained 99% and 72% control in the treated areas at 1 year after treatment” (Wersal et al., 2010, p. 8). The spectacular results achieved during the course of this study might indicate a turning point in the efforts to eradicate this particular macrophyte. Also, while complete eradication of Myriophyllum spicatum has yet to be attained with any treatment, a reduction of 99% in non-native concentrations of exotic species is most definitely a step in the right direction towards containing this waterborne epidemic.

A Resilient Macrophyte

One reason the aquatic weed known as Eurasian watermilfoil has come to populate so many lacustrine environments may be due to its inherent resilience regarding external stimuli. As has already been noted, Myriophyllum spicatum is one of the underwater macrophytes that manages to live throughout the months of winter, where other macrophytes primarily die off. Additionally, there is evidence that Eurasian watermilfoil also possess strong defenses for combating intense UV exposure. When exposed to abundant amounts of UV-B (ultraviolet B radiation), plants have defense mechanisms that produce a kind of compound that absorbs the UV-B while simultaneously acting as a filtering screen. In conducting their experiment, the authors of one study found that “the increase in the amount of these compounds under enhanced UV-B radiation was significant in C. demersum, but negligible in M. spicatum, while the total amount produced was much higher in M. spicatum” (Germ et al., 2006, p. 49). In other words, the study found that while one macrophyte specimen exhibited significant increases in defensive processes as a result of increased exposure to UV-B radiation, Eurasian watermilfoil, despite showing an insignificant increase in the production of protective substances to UV-B radiation still demonstrated more overall production of the substance. These findings are further explicated in the study as the authors explain that such is “possibly the consequence of the fact that the genus Myriophyllum is phylogenetically younger…and the complexity of UV-B absorbing compounds has been increased during evolution” (p. 49). Considering the potential evolutionary advantage of Eurasian watermilfoil, it is little wonder how the organism has managed to become one of the dominant aquatic macrophytes in lacustrine systems around the world.

Other detrimental effects of non-native, invasive macrophyte species introduced into aquatic systems are the ensuing disadvantages imposed on native plant species. There is much evidence pointing to Eurasian watermilfoil as a dominant species of macrophyte, one that generally contends the proliferation of other species while simultaneously promoting its own. One explanation for the abundance of Myriophyllum spicatum in marine environments is due to its photosynthetic properties, namely the disparity of such functions as compared to other macrophytes. One study from Santos et al. (2012) concluded that “the adaptive value of having a facultative C4-like photosynthesis allows plants to colonize environments that C3-only plants cannot utilize or utilize less efficiently, such as the high-light and temperature conditions of shallower waters” (p. 692). Eurasian watermilfoil is a species unique in a variety of aspects, but specifically as pertains to photosynthetic effects, Van et al. (1976) concluded that it is a plant that has both “C3 and C4-like photosynthetic pathways” (p. 692). As such, the plant is uniquely equipped to survive in a wide variety of environments in which other marine plant-life simply cannot thrive, resulting in the accelerated population of the species into virtually any system that it inhabits. This of course, occurs at the expense of the survival and proliferation of other native species that ultimately cannot compete with the dominant reproductive and survival capabilities of the non-native species, especially when confronted with the unmatched colonization ability of Eurasian watermilfoil.

Additional resilient attributes of Myriophyllum spicatum not only involve defensive actions against ultraviolet radiation, but certain reactions that occur at the cellular level. One study notes “it is conceivable that this submerged macrophyte uses resources, especially light, as a proximate factor to react to biotic stressors, especially competing photoautotrophs” (Gross, 2003, p. 503). However, not only stressing the plants photosynthetic capabilities, the author continues by saying, “M. spicatum polyphenols may additionally serve as antioxidants”, and further expounds on the plant’s apparent invincibility by stating that it may in fact have the ability to thrive in shade as well (p. 503). That an organism has been endowed with such resilient design is only further explanation as to its widespread colonization of lakes around the globe. This, of course, naturally results in the title of what many consider to be the most pestilent aquatic weed in recent history.

Hybrids

There are dangers associated with the colonization of invasive non-native species that are slightly more opaque than damaged ecosystems or reduced numbers in native aquatic life. A phenomenon that remains relatively new to many researchers is the hybridization of invasive species with species already present in the region. In other words, it seems that the introduction of invasive species like Eurasian watermilfoil into environments already hosting other species of watermilfoil may result in genetic super-strains between the two. One study from Moody and Les (2002) commenting on the apparent molecular divergence of certain strains of macrophytes reported “M. spicatum and M. sibiricum are sister species phylogenetically (Fig. 1), yet they showed an even higher degree of divergence…[indicating that] the pattern of distinctive nucleotide divergence between parental species made elucidation of hybrids straightforward” (p. 14869). According to the authors, there is clear evidence that in fact the prolonged cohabitation of invasive species and native species of macrophytes, specifically the convergence of Myriophyllum spicatum and Myriophyllum sibiricum, resulted in a hybrid watermilfoil both similar to and distinct from the two parental species. The study notes one interesting observation concerning hybridized species in that they are “relatively benign within their native range, yet become invasive upon entering new territories where hybridization with related congeners is possible (42)” (Moody & Les, 2002, p. 14870). This is a significant finding because it demonstrates the viral effects that one invasive species can have in the perpetuity of invasive species as a whole. As such, it is imperative that the scientific communities continue to formula methods of treatment to more precisely and effectively combat this destructive macrophyte.

Before moving on, another interesting development concerning the widespread introduction of Myriophyllum spicatum must be addressed. Specifically, the manifestation of hybrid species of macrophytes with characteristics more resilient than either of the two parent species alone necessitates further investigation. Certain species of milfoil are more resistant to nuisances like milfoil weevil, such as the hybridized watermilfoil, the product of natural hybridization between Northern watermilfoil and Eurasian watermilfoil. The milfoil weevil, Euhrychiopsis lecontei, has generally inhabited Northern watermilfoil, Myriophyllum sibiricum, but has recently been found to host the more exotic Eurasian watermilfoil (Roley & Newman, 2006, p. 121). That such a parasite would prefer an exotic host in place of its historically preferred host, the Northern watermilfoil, suggests a possible solution to the Eurasian watermilfoil dilemma. However, as if in response to a potential threat, Eurasian and Northern watermilfoils have been hybridized and the resulting offspring was quite astounding. The study from Roley and Newman (2006) found that even more resistant to E. lecontei, the milfoil weevil, than Eurasian watermilfoil and Northern watermilfoil, the hybrid watermilfoil displayed the greatest degree of resistance to the aquatic parasite (p. 126). The implications of these findings are far reaching to be sure, as it would appear that the introduction of one pestilent aquatic weed, Myriophyllum spicatum, has ushered in a new era of resilience concerning the plant’s ability to survive the hosting stages of E. lecontei. Taking this hybrid’s increased survival mechanisms into consideration, it is logical to suppose that this amalgamation between Northern and Eurasian watermilfoil may represent an even more infectious strain of macrophyte with even stronger propensities toward lacustrine invasion. Such conclusions imply that hybrid watermilfoil species might represent the direst consequence of invasive exotic species.

Unintended Consequences

It is important to mind the surroundings of treatment application sites because organisms or pathogens harmful to the macrophyte Myriophyllum spicatum may also be detrimental to native, non-invasive species. As such, constructing a systematic approach that alleviates the concentration of Eurasian watermilfoil without substantially affecting the population of native macrophytes can be a complicated undertaking. However, a study published in 2008 from Nelson and Shearer demonstrated that specific concentrations and combinations of chemicals could enact containment on Eurasian watermilfoil without detrimentally affecting native plant populations (p. 337). Specifically, the study employed a fungal pathogen Mycoleptodiscus terrestris and the herbicide triclopyr in varying concentrations and reported, “although M. terrestris at 0.08 ml/L did not significantly reduce shoot or root biomass and 0.15 mg/L triclopyr provided only 53% control of plants, combining both agents at these rates reduced Eurasian watermilfoil by 90%” (p. 337). These findings represent a rather significant understanding of the nature of Eurasian watermilfoil and its relation to native aquatic species. In considering the results of this study, it is important to note that the authors also commented on the fact that while larger doses of either chemical affected significant reduction in biomass of M. spicatum, lower doses of either chemical in isolation had only negligible effects on the macrophyte under study (p. 340). Considering now the impacts of the study’s findings, a chemical combination reducing the population of Eurasian watermilfoil while simultaneously maintaining the density of native species is quite astounding to say the least. Still, there are numerous other methods of containment that many might say are not only more natural, but more effective as well. Specifically, there are some trying to stay ahead of the spread rather than maintaining a reactive approach to the problem.

Furthermore, while many experts support the efforts of those intent on eliminating invasive macrophyte populations in lacustrine systems, it is important to consider the implications of altering aquatic systems that have over the course of many years potentially become accustomed to such invasions. A primary concern surrounding any notion of immediate and total decimation of an invasive macrophyte such as Myriophyllum spicatum is that such eradication may be to the detriment of the ecosystem unless previously native species can quickly repopulate the system (Kovalenko, Dibble & Slade, 2010, p. 319). Nevertheless, given the numerous studies citing the detrimental effects of Eurasian watermilfoil, there is still a strong presence of those determined to contain its spread by virtually any means necessary. Interestingly, the study from Kovalenko, Dibble & Slade (2010) concluded that “it is possible to selectively control an invasive macrophyte with minimal effects on the underwater habitat: total macrophyte abundance and habitat complexity did not change whereas watermilfoil abundance decreased significantly” (p. 322). Though the study suggests that this may have been the result of fortuitous timing, citing that the periods of herbicide application coincided with times when native macrophytes are relatively dormant and Eurasian watermilfoil remain in a stage of active growth, many would argue that the findings are still quite significant. In other words, while some might conclude such treatments are only effective during specific times of the year, thus suggesting an inherent weakness in the treatment method, it is considerably more likely that the focus will rest on the study with findings that indicate a distinctly effective method for combating the spread of Eurasian watermilfoil. At any rate, there are certainly consequences to instantaneously eradicating a primary source of food in an ecosystem.

On Insect Populations

A related complication associated with Eurasian watermilfoil is not directly concerned with the macrophyte per se but rather with some of the often-overlooked consequences of administering biological control agents into aquatic ecosystems. Not only can certain herbicides and other fungal pathogens be harmful to native plant populations, but there is evidence that the harmful effects may percolate deep enough into the food chain to negatively impact certain terrestrial insects. One study from Carvalheiro et al. (2008) concluded “…the use of a highly specific biocontrol agent can be significantly associated with the local loss of native species” (p. 698). This particular study, which focused on Mesoclanis polana, a biocontrol agent for Bitou, further explained the dangers of biocontrol methods, stating “M. polana abundance is significantly negatively associated with the abundance and species richness of communities of insects reared from native plants” (Carvalheiro et al., 2008, p. 698). While there are some studies that touch on the composition of certain chemical compounds that can in fact contribute to the detriment of invasive species while simultaneously promoting the proliferation of other species, the fact of the matter is that there are other widely used compounds employed to counteract invasive species that simply do not have that ability. That invasive aquatic species are not only hindering the growth of native plant species but also contributing to the collapse of native populations of aquatic life orders of magnitude higher up the food chain is a testament to the destructive forces that Eurasian watermilfoil can affect on lacustrine ecosystems.

Another concern surrounding the mass eradication of Eurasian watermilfoil is what effects an eradication of that magnitude would affect on other marine life in the system. Native fish, for example, might be met with a number of problems as concern their daily requirements for food. As one study commenting on the removal of Eurasian watermilfoil from lacustrine systems notes, “It’s removal leads to increased diversity and abundance of native plants…[but] nonetheless, abrupt changes in macrophyte communities may be detrimental for fish foraging and food availability because macrophytes provide an important source of food and structure for epiphytic macroinvertebrates” (Kovalenko, Dibble & Fugi, 2009, p. 306). Such situations are exacerbated if the repopulation of native species is too long delayed, as native fish species will be deprived of vital nutrition, however indirectly. Otherwise, this study reported “that fish feeding is not negatively affected by invasive macrophyte control efforts when treatment is sufficiently selective and is followed by immediate native plant recolonization” (Kovalenko, Dibble & Fugi, 2009, p. 310). The results echo logic when one considers that negative outcomes can generally be achieved in any endeavor when sufficient thought is incorporated into the plan of action. Still, other unique attributes of Eurasian watermilfoil, such as its reluctance to die during winter months, as well as the depth at which it grows, may still pose a threat to the stability of lacustrine systems.

Thoughts Going Forward

Another component to curbing the colonization of Eurasian watermilfoil into previously uninhabited bodies of water is taking measures to ensure that uninfected environments are protected against exposure. Such measures require ensuring that organisms present in sites already colonized cannot leave, which contributes greatly to the preservation of sites not presently contaminated with the particular milfoil. Additionally, there are many experts attempting to not only contain the colonization of certain forms of milfoil, but to stay one step ahead by attempting to predict the future regions to which certain macrophytes might spread. This however, is easier said than done and may not currently stand as the most viable method for controlling the Eurasian watermilfoil population. With regards to the current gravity models of prediction, one study notes “the ability of gravity models to predict the sites of new colonizations is more limited than recognized previously”, and continues in stating that, using such methods, “[they] predicted significantly more colonizations would occur in the 200 lakes predicted to have the highest probability of colonization than were observed” (Rothlisberger & Lodge, 2011, p. 69-70). Of course, attempting to predict the outcome of such biological migrations presents certain unique challenges, namely that the results of such events are nearly always the product of extremely stochastic circumstances. Furthermore, the study notes that predicting future colonizations of Eurasian watermilfoil is next to impossible using gravity models because even minor colonizations rarely result in long-term, self-sustaining populations (p. 70). To be sure, until researchers develop a more precise method via which to predict the spread of Eurasian watermilfoil it appears that mechanical eradication of milfoil, biological decimation of milfoil, and the artificial control of native fish populations may still be the most effective option to stop future colonizations of Eurasian watermilfoil.

Materials and Methods

After a thorough review of the current literature surrounding all of the various effects that Eurasian watermilfoil has on aquatic ecosystems, there were still some questions that remained unanswered. Foremost among them was, given the apparent abundance of species similar to the natural enemies of Eurasian watermilfoil, was it likely that sister species of the specimens employed in population control experiments would have similar effects in the reduction of Myriophyllum spicatum. After much deliberation, it was determined that a species with which there was relatively ample access to was Oronectus immunis, a cousin of the rusty crayfish whose effects on Eurasian watermilfoil were examined in the experimental studies conducted by Maezo, Fournier and Beisner (2010). The primary purpose in conducting studies that involved the use of subjects with close ties to previous experimentation was to determine, in the event of a potential contamination, whether or not potentially native species not explicitly determined to be enemies of Eurasian watermilfoil could contribute at all to controlling its spread. This was a reasonable question as it is logical to presume that future sites of colonization may not necessarily be home to the plant’s native predators, but might potentially host related species, however distant. While it is beyond the scope of this study to examine all of the distant relations of the various natural enemies of Myriophyllum spicatum, this study will focus on Oconectus immunis, the papershell crayfish.

The papershell crayfish is indigenous to a number of water bodies throughout the state of Vermont. This study sampled two distinct bodies of water in the small northern towns of Shelburne and the wooded area near Rutland. Neither Kent Pond nor Shelburne Pond had previously undergone any attempts to control the colonization of M. spicatum. The ponds examined were of significantly different sizes, ranging from roughly 100 acres in size to almost 1800 acres, respectively. The first detections of Eurasian watermilfoil in Kent Pond occurred in 1995 and in 1992 in Shelburne pond. The general proximity of the two bodies of water made examinations fairly convenient.

In order to determine the extent of damage that Oconectus immunis might affect on Eurasian watermilfoil, it was necessary to design an experiment that examined the impact of a variety of crayfish densities. Large cylindrical crates, approximately 1.5 m2 in diameter were employed as a means to contain the mesocosm of Eurasian watermilfoil and O. immunis. These crates were placed approximately 20 ft. from the shore, where the densities of Myriophyllum spicatum were the highest. Additionally, zones were selected that, during the course of prior observation, seemed to be seldom disturbed by boating or fishing activities, or any other external stimuli. The study employed 6 different cages with e distinct densities of papershell crayfish: two cages contained 1 crayfish, two cages were populated with 3 crayfish, and the last two cages were occupied each by 7 crayfish. These densities were termed low, medium, and high, respectively. All of the cages were placed directly atop extremely dense areas of Eurasian watermilfoil and studied periodically over the course of 10 weeks. The cages used in the experiment were randomly placed 2-3 meters apart from one another in no specific direction—the goal in spacing the cages apart was simply to achieve as randomized a placement as possible while ensuring that the locations also coincided with dense colonizations necessary for successful experimentation. The cages used were constructed of galvanized material so as to avoid the potential rusting of the cages. The cages were built with 1.3mm steel sheets and steel mesh approximately 8mm across. Cages were approximately 3 ft. high and protruded from the surface of the water between 2-4 inches, and it was necessary to trench down into the sediment 4-6 inches to prevent the crayfish from burrowing beneath the cages. Additionally, crayfish could neither escape by climbing up the walls of the cage as there was simply nothing there to latch onto but bare steel.

The Oconectus immunis samples were procured from locals who regularly bait the lake for the indigenous crayfish. It was only necessary to inquire at one of the local restaurants concerning whether anyone in the area was involved in such trades.

Using quadrats to determine the percent growth in degradation of current Eurasian milfoil populations, each of the six crates were tested three times during the 10 week experiment, once at three weeks, again at seven weeks, and finally at ten weeks upon completion. Biomass of the milfoil densities in each crate was tested at the end of the experiment. The specimens were first washed and dried, and then allowed to sit for a period of not more than 12 hours. This was done for two reasons: to ensure that constant weights were achieved before measurement, and also so as not to compromise the structure of the specimen by overexposure to the elements.

Results

The results of the experiment were by and large to be expected. While there were minor complications in maintaining the exact crayfish densities originally designated, such variances were neither significant nor material to the outcome of the experiment. However, it should be noted that upon examination in week seven, one of the high-density crates containing O. immunis specimens was found to only contain six live specimens. As such, it is difficult to determine how much time had elapsed since the specimen had deceased, and as such equally difficult to determine the deceased specimen’s role in consumption, or lack thereof, concerning the Eurasian milfoil population of that particular crate.

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