Herbicides and their effects on aquatic ecosystems


Herbicides are an integral part of modern agriculture and will be used in ever-increasing quantities due to the increase in demand for food, the development of biofuel production and the emergence of new pests associated with climate change. Unfortunately, through surface runoff, they easily end up in waterways, where they have a negative impact on the organisms living there. Herbicides and their impact on aquatic ecosystems is an issue that is becoming increasingly important in the context of environmental protection. Sustainable use of chemicals in agriculture is a necessity, but what happens when these chemicals enter waterways? Are they a threat to ecosystems and the organisms that live there?

What are herbicides?

Herbicides, in addition to, among others. insecticides and fungicides, are included in the group of chemical plant protection products, commonly known as pesticides. The word “herbicides” has its origin in the combination of the Latin words “herba” (plant) and “caedere” (to kill). They are defined by the “Weed Science Society of America” as chemicals and cultural organisms used to control or inhibit plant growth, including pathogens used to control weeds.

The common literature uses a simplification of this definition and assumes that these are just chemical compounds that eliminate weeds. So what are weeds? These are plants that compete with crops for sunlight, water and nutrients, so farmers are looking for ways to control them. Crop protection products can be applied before or after crop emergence and can be selective or non-selective (e.g., glyphosate). They are also classified according to the groups of plants on which they are used, and chemical structure. The mechanism of action of herbicides is to interfere with processes such as photosynthesis or enzyme activity. The International Committee for the Study of the World. Weed Resistance (HRAC) assigns letters of the alphabet to the weeds to denote specific mechanisms of action.

Global pesticide production increased significantly after World War II, from about 500,000. t per year in the 1950s. In the 1970s. to more than 3 million tons per year by the beginning of the 21st century. In 2021. The European Union recorded pesticide sales of 355,000. t. Eurostat data confirms moderate growth of 2.7 percent. compared to the previous year. Herbicides dominate the crop protection products market, accounting for 44 percent. share (sales revenue of $27.4 billion). At the top of this ranking, compiled by the Ag News portal, is glyphosate, which – despite numerous controversies – still remains number one in the world, accounting for about 21 percent of the world’s of the total herbicide market.

Herbicides negatively affect the aquatic environment

Aquatic ecosystems are complex biotic systems that include different groups of organisms. Herbicides affect these ecosystems both directly, as a result of their physiological effects on organisms, and indirectly, as a result of ecological interactions between species [1].

The direct effect of herbicides depends mainly on the concentration of chemicals, but also on the developmental stage of the organisms they affect (young individuals are more susceptible than mature ones), the duration of exposure and the presence of additional stressors, albeit other pesticides. For example, the previously cited glyphosate and herbicides based on it are toxic to aquatic microorganisms such as bacteria, algae and protozoa. But also for higher-order organisms such as insects, crustaceans, mollusks, amphibians and fish, a number of negative direct effects caused by these compounds have been observed.

These include impaired reproduction and development, DNA damage, effects on the immune system, oxidative stress, reduced ability to cope with stress, changes in diet and mating behavior. These changes could threaten their survival. In addition, herbicides are subject to bioaccumulation, that is, they tend to accumulate in the tissues of plants and animals, and this in turn leads to a significant increase in their concentration in organisms higher up the food chain, including humans [1,2,3].

Herbicides also have indirect effects on various species, modifying the relationship between them and with the abiotic environment. First order producers (algae and higher plants) are the most negatively affected. The decline in autotroph populations leads to a reduction in the food base of herbivores, which affects the decline in their numbers. In a study conducted by Professor deNoyelles and colleagues [4] on the effects of the herbicide atrazine on freshwater ecosystems (using ponds as an example), it was observed that a reduction in phytoplankton biomass resulted in the inhibition of growth and reproduction of zooplankton species: Simocephalus serrulatus and Daphnia pulex. These effects spread to herbivore-dependent predators (e.g. decline in the bream Lepomis macrochirus population), as demonstrated in subsequent studies [1, 4].

On the other hand, herbicides can induce competition between first-line producer species and promote the growth of those more resistant to their effects. This can lead to algal blooms after the more herbicide-sensitive macrophytes have subsided. This is confirmed, for example, by the research of Professor van der Brick’s team from the Netherlands, where after the application of linuron, a significant decrease in the abundance of the delicate soak Elodea nuttallii and algal blooms of Chlamydomonas sp. were observed. [1, 5].

In addition, herbicides also indirectly contribute to a significant reduction in dissolved oxygen concentration in water and a change in its pH, i.e. an overall deterioration in water quality, which has a negative impact on all aquatic organisms. There are several factors that catalyze the occurrence of this phenomenon, such as a reduction in the number of oxygen-producing organisms and a significant increase in the concentration of chemical compounds produced by the decomposition processes of plants and algae.

Let’s counteract!

Ensuring the safe use of herbicides is a priority to minimize their negative impact on aquatic ecosystems. Choosing the right product, proper dosage and avoiding weather conditions unsuitable for their application are key.

In the context of thefield-to-table initiative”, the European Union aims to halve the use of chemical pesticides by 2030, including those harmful to health and the environment. Member states can include tools and financial incentives in their Common Agricultural Policy Strategic Plans for 2023-2027 to support alternative methods of weed control. Acting responsibly in agriculture is key to achieving production goals with minimal impact of herbicides on aquatic ecosystems. Let’s remember our role in caring for the health of the Earth.

MA. Daria Pęziak-Kowalska – a graduate of the Medical University of Poznan and the Department of Chemical Technology at the Poznan University of Technology. She gained research experience as a manager of an NCN project in the area of “Hybrid processes for removal of bioactive compounds from aqueous systems.” Currently, he serves as a technologist and deals with product quality assurance. She is also an expert for the European Commission and an active popularizer of science. As a hobby, he explores the possibilities of using information technology in the life sciences.

In the article, I used, among others. From the works:

[1] Sánchez-Bayo F., van den Brink P.J., Mann R.M., 2011. Ecological Impacts of Toxic Chemicals, Bentham Science Publishers, https://doi.org/10.2174/97816080512121110101

[2] Schafer R.B., van den Brink P.J., Liess M., 2011. Impacts of Pesticides on Freshwater Ecosystems. [w:] Sánchez-Bayo F., van den Brink P.J., Mann R.M. (eds.) Ecological Impacts of Toxic Chemicals, pp: 111-137 (27). https://www.eurekaselect.com/chapter/690

[3] Rumschlag S.L., Mahon M.B., Hoverman J.T., Raffel T.R., Carrick H.J., Hudson P.J., Rohr J.R., 2020. Consistent effects of pesticides on community structure and ecosystem function in freshwater systems. Nat Commun 11, 6333. https://doi.org/10.1038/s41467-020-20192-2

[4] DeNoyelles F., Kettle W.D., Sinn D.E., 1982. The Responses of Plankton Communities in Experimental Ponds to Atrazine, the Most Heavily Used Pesticide in the United States. Ecology, 63, 5: 1285-1293.

[5] van den Brink P.J., Blake N., Brock T.C.M., Maltby L., 2006. Predictive value of species sensitivity distributions for effects of herbicides in freshwater ecosystems. Hum Ecol Risk Assess; 12: 645-674.