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Mechanical Harvesting vs. Spraying Herbicides

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Mechanical harvesting of aquatic weeds

Mechanical harvesting is one of the type of commonly used harvesting methods to control unwanted growth of aquatic weeds in ponds, lakes, streams, rivers and wetlands. Mechanical harvesting involves physical harvesting, which is generally done by vegetation cutter or harvester, which removes weeds from aquatic ecosystems (Thayer and Ramey 1986). This type of harvesting method is well suited to clear a large area of dense vegetation in a short period of time. Mechanical harvesting can be applied in different species of weeds, however they are most effective for species forming dense growth e.g. water chestnut or water hyacinth. Mechanical harvesters are large machines which cut and collect aquatic plants. They remove the upper portion of the plant and can cut five to ten feet below the water surface. The weeds collected by the harvester are then transferred to an upland disposal site (Greenfield et al. 2004).

One or two harvest in a season is enough to control the growth of commonly targeted dense growing species (Kimbel and Carpenter 1981). Few cycle of removal program can effectively reduce the reoccurring growth of weeds (Louda and Stiling 2004), as seeds are regularly removed from the growing cycle, which results in removal of biomass and finally leads to long-term nutrient reduction.

Harvesting and removing cut plant material is important, because it decreases the excess nutrient loading, which may result in eutrophication (Carpenter and Adams 1978, Mahujchariyawong and Ikeda S 2001). It is predicted that half of the flux of dissolved total phosphorus (DTP) and dissolved organic material (DOM) from the littoral zone to the pelagic zone is due to the decay of aquatic plants (Carpenter 1980). The prediction, based on a eutrophic hard water lake in Wisconsin, suggests that aquatic plants can compose a substantial pool of nutrients that mechanical cutting would free up for algal growth. For instance, harvesting of macrophytes removed 37.4% of the annual phosphorous inputs and 16.4% of nitrogen inputs to a eutrophic lake in Wisconsin during a late August harvest (Carpenter and Adams 1977).

Mechanical harvesting is also effective in removal of invasive aquatic plants (Wilkie and Evans 2010). A wide variety of harvester and cutting machines designed to remove or destroy invasive aquatic plants have been used for several decades effectively (Madsen 2000, Greenfield et al. 2004).

Some of the advantages of mechanical harvesting are summarized below:

  • Control of dense growth of weeds in short term.
  • Removal of biomass and thereby reducing the nutrient load from the water body.
  • Provides effective area selective and long-term control of certain species.
  • Offers eco-friendly alternative to herbicides, which does not introduce unnecessary chemicals into the food web. 

Disadvantages of herbicides

Herbicides are used widely in reduction of aquatic weeds throughout the world. Although herbicides are effective in reduction of weeds due to their easy application and faster affect, they pose a danger to the health of the aquatic ecosystems and in turn affects humans as well. Once the herbicide enters the environment, they may cause unexpected side effects on other organisms and may also interfere with the biological and chemical process of the ecosystem. Some of these compounds are not biodegradable and can persist in the environment for a long time and can enters the food web and affect various organisms. Herbicides may affect organisms in the same ecosystem or in other ecosystems where they are transmitted mainly by wind or by rain. Once applied, the non-biodegradable herbicides dissolve in water or get deposited in sediments, which are then taken up by microorganisms. Other organisms higher in the trophic levels then feed on these microbes and thus the herbicides are also transferred to other organisms (Kortekamp 2011).

Most herbicides pose health risk to humans and also domestic animals. Symptoms vary according to the substance, but generally they are skin irritations and gastrointestinal discomfort. Beside the disadvantages of herbicides due to the eco-toxicological aspect, the emergence of herbicide resistant weeds is an increasing problem worldwide.

Some of the most commonly used herbicides to control weeds in aquatic ecosystems are Copper Sulphate and Glyphosate. Both these compounds have toxic effects on the environment and should be regulated. 

Effect of Copper Sulphate:

Aquatic animals are exposed to different concentration of metals, which are present in the aquatic environments. These metals tend to accumulate in their bodies, which are harmful for their life (Giulio and Hinton 2008). One among these metals is copper, which is an essential element for biological functions in living organisms. For example, it forms part of the plastocyanin protein involved in photosynthetic electron transport and is a cofactor of the enzymes Cu/Zn-superoxide dismutase, cytochrome c oxidase, ascorbate oxidase, amino oxidase and polyphenol oxidase (Leal et al. 2018). However, at higher concentrations copper is highly toxic to aquatic organisms (Hedayati and Safahieh 2012). Copper is introduced in the aquatic ecosystems from industrial wastes or as Copper Sulphate, which is used as herbicide and therapeutic agent (Krishnaja et al. 1987, Nor 1987, Correa et al. 1999). Copper salts (copper hydroxide, copper carbonate and copper sulphate) are widely used in agriculture as fungicide, algaecide and nutritional supplement in fertilizers (Erfanifar et al. 2018).

Copper is also toxic to nitrifying bacteria. At 0.3 mg/L copper sulfate inhibits ammonia and nitrite oxidation and thus increases ammonia or nitrite levels in the ecosystem. By contrast, bacteria that can cause disease in fish are much more resistant to copper (Cardeilhac and Whitaker 1988). Copper promotes oxidative damage by increasing the cellular concentration of reactive oxygen species (ROS) such as the superoxide anion (O2 −), hydrogen peroxide (H2O2) and the hydroxyl radical (OH−), and by disrupting the photosynthetic electron chain and reducing the cellular antioxidant capacity in macroalgae (Collén et al. 2003, Pinto et al. 2003). At high concentrations, ROS are toxic to all organisms, oxidizing proteins, lipids and nucleic acids that often lead to structural aberrations, mutagenesis, and cell death (Pinto et al. 2003). Copper sulfate is also toxic to several fish species (Thayer and Ramey 1986).

Effect of Glyphosate:

Glyphosate is one of the most common non-selective herbicide used extensively (Bradberry et al. 2004). Glyphosate is highly effective in weed control, however due to its high toxicity, it has several negative consequences to environment and living organisms. Glyphosate can actually kill any plant; therefore random application of glyphosate would kill every plant exposed. Glyphosate kills plant specifically by disrupting the production of specific aromatic amino acids, which are the building blocks of protein, which all the organisms require to live. Several studies have shown that even minute exposure of glyphosate can cause some adverse effects in humans. For instance, farmers reported irritation in their eyes if touched after using glyphosate (Goldstein et al. 2002). Ingestion of glyphosate can cause corrosive effects in gastrointestinal tract, mouth, throat and epigastric pain and dysphagia (Bradberry et al. 2004). Exposure of glyphosate to skin can cause eczema in the exposed area. Negative consequences of glyphosate have also been detected on animals that have been tested. Tests on rats showed that inhalation of glyphosate caused reduced respiratory ability and weight loss (Bradberry et al. 2004). Long-term effects also showed development of tumors in liver, thyroid and the pancreas. 

Glyphosate also has several environmental effects, specifically on both land and aquatic environments and it can affect the entire ecosystems. Herbicides leaked into the water bodies can cause lethal effects to many aquatic organisms such as invertebrates and fish. In addition, when applied, glyphosate kills plants in that area and plants also in the surrounding areas (Romano et al. 2010). The death of these living organisms, both on land and in water can have prodigious effects on a broader scale by casing interference in the food chain and leading to the destruction of entire ecosystems.

Moreover, prolonged use of glyphosate can also induce weeds that are resistant to glyphosate (Giesy et al. 2000). The glyphosate resistant weeds can transfer the resistant gene to other plants/weeds. This may turn glyphosate useless and new herbicide development will be needed with other chemicals, which may cause more adverse effects on the environment. 

Therefore, in conclusion although herbicides are beneficial in weed control, given their toxicological and environmental effects other modes of weed control should be preferred, which are less harmful to the environment.


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Hedayati A and Safahieh A. Serum hormone and biochemical activity as biomarkers of mercury toxicity in the yellowfin seabream Acanthopagrus latus. Toxicol Ind Health. 28 (2012): 306-319. 

Kimbel, J.C. and S.R. Carpenter. Effects of mechanical harvesting on Myriophyllum spicatum L. regrowth and carbohydrate allocation to roots and shoots. Aquatic Botany. 11 (1981): 121-127.

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Madsen, John D. Advantages and disadvantages of aquatic plant management techniques. No. ERDC/EL MP-00-1. Engineer research and development center Vicksburg MS environmental lab, 2000.

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