Pesticides have harmful effects on freshwater organisms

ENVIRONMENT

By GELINDE NAREKINE
EVERY now and then, we hear and read about big industrial waste disposals into aquatic environments (freshwater bodies and the ocean), and the subsequent degree of pollution, contamination, and toxicity caused.
However, very little attention is accorded to harmful impacts of pesticides that are, directly or indirectly, discharged into aquatic environments.
It is probably due to the fact that their toxic effects may not be obvious, and therefore, any physical evidence of damage caused is so often attributed to climate change and other human activities.
Research shows that key agents of biodiversity decline in aquatic ecosystems are water pollution, overexploitation and harvesting, habitat degradation and destruction, introduced invasive species, and climate change.
Effects of dams and water management along with pollution from urban, agricultural, and forestry sources are the main threats in freshwater ecosystems, whereas, a broad range of factors have impacts on marine invertebrates.
Invertebrates inhabiting freshwater (and marine) ecosystems are important contributors to global diversity.
They provide significant services that have cascading effects across ecosystems.
The ecosystem functions provided by aquatic invertebrates include water puriﬁcation and ﬁltering, processing of organic matter, recycling of nutrients, and mediating transfer of carbon through food webs.
And similar can be said of fish and other animals.
Their contribution to the economy and sustenance of freshwater environments is of utterly significance importance.
These roles and functions played by both the invertebrates and vertebrates are fundamentally important in maintaining the health, hygiene, and integrity of freshwater environments.
Due to the ever-increasing human populations across the globe, and our dependency on available natural resources, it is obvious that there is need for technological advancements to cope with the need of societies. This has led to use of different synthesized chemicals in the environment for pest control and therefore, to increase the quality and quantity of products.
Pesticides have become a necessary part of the production cycle for killing pests and eliminating diseases that can drastically reduce harvestable products. This exercise has not only been beneficial, but has also introduced pollution and toxicity, specifically into freshwater bodies.
Pesticides are used to control pests, such as aquatic weeds, insects, aquatic snails, and plant diseases.
They are extensively used in forestry, agriculture, veterinary practices, and even in urban environments. They can be categorized according to their use into three major types, namely, insecticides, herbicides, and fungicides.
Thus, water contamination by pesticides is known to induce harmful impacts on the production, reproduction, and survival ability of living freshwater organisms, such as algae, aquatic plants, aquatic invertebrates (include insects, crabs, shrimps, and prawns), and ﬁsh.
Since they are not easily broken down into environmentally safe substances, pesticide residues can be sustained for long periods in the ﬁelds after application.
This could lead to sustained negative influences on the health and wellbeing of aquatic organisms. Upon absorption, pesticides are transformed especially in liver and blood cells, leading to undesirable effects on the survival and growth rate of ﬁsh.
Synthetic pyrethroid insecticides such as permethrin, deltamethrin, tetramethrin, cyhalothrin, and cypermethrin can cause serious toxic impacts on the exposed aquatic organisms. Most aquatic invertebrates and fish are highly vulnerable to these synthetic forms of pyrethroid. Serious hazardous effects have been reported in several ﬁsh species leading to high mortalities and economic losses of the exposed ﬁsh.
When in the organisms, pyrethroids suppress release of certain important chemicals in specific regions of the brain, leading to inhibition of other vital functional enzymes. This can further disrupt hormone-related functions in the affected organism. Pyrethroids have also been shown to inhibit cell cycle progress, cause cell stress, and have immunosuppressive effects.
Additional long-term effects may be caused by damage to respiratory surfaces, and interference with functions of the kidney. All these have serious negative effects on exposed freshwater organisms. Thus, the occurrence of pyrethroids in freshwater environments has become an important ecological health concern.
Toxic effects can have severe consequences for the fitness, reproductive success, and survival of aquatic organisms, ultimately leading to population-level effects. Biological responses to such effects include, altered behavior, reduced growth, immune system effects, reproductive effects, histopathological effects, as well as biochemical responses. Some of these stress responses divert an organism’s energy away from normal metabolic functions, resulting in “higher-level” effects such as growth inhibition and reduced reproductive success.
Pyrethroids are among those contaminants identified to cause immunosuppressive effects and therefore, reduced disease resistance in fish. The immune responses of fish and invertebrates play key roles in the control of aquatic diseases, fitness and reproductive success. When the very essence of their survival is affected, these organisms become highly susceptible to infections and diseases. Most probably, this is a major contributing factor in the extinction of some species of freshwater organisms.
Thus, abnormal behaviors produced by water contaminants include changes in preference or avoidance activities, feeding, performance, learning, predation, competition, reproduction and species-specific social interaction. Such changes can have significant consequences for fitness, survival and reproductive success of an individual. For example, many neurotoxic compounds cause abnormal swimming behavior in fish and other aquatic animals. Such changes can directly translate into increased vulnerability to predation or decreased food acquisition and intake.
Water temperature is an important factor affecting biochemical and physiological processes of individual organisms. It affects rates of contaminant transformation and excretion in organisms. Thus, temperature is inversely related to pyrethroid toxicity. This negative temperature dependence of pyrethroid action has in the past been ascribed to the slow metabolism of pyrethroids at low temperature. Recent studies have further shown that this effect is mostly due to the increased activity of an important biochemical called sodium that increases sensitivity of nerve cell membranes at low temperature.
In natural aquatic systems, surface water temperature is often lower than standard laboratory toxicity testing temperatures. For example, the standard temperature for aquatic toxicity testing of sediment-dwelling invertebrates is 23oC. This is well above temperatures in creeks that can serve as habitat for many freshwater water fish species, for which preferred creek average temperatures are commonly below 20oC.
Pre-exposure or simultaneous exposure to other contaminants, disease or stressful environmental conditions such as salinity and temperature may considerably alter the physiological condition and therefore susceptibility of the organism, as well as modify the toxicity of a given contaminant. Organisms in the environment often experience many stressors simultaneously, including those of a physical, biological, and chemical nature.
Studies have revealed that pyrethroids are more lethal out in the environment than laboratory toxicity assays predict. Even the lowest concentrations can lead to dramatic reductions and even extinctions in a number of animal species. With addition of other chemicals such as piperonyl butoxide to pyrethroid formulations, this enhances the effects of the active ingredient. Piperonyl butoxide is known to inhibit a group of enzymes which are involved in pyrethroid detoxification. In so doing, it enhances the killing effect of pyrethroids by 10-150 times. Such as this is an added danger for exposed freshwater organisms.
Pollution is the critical universal off-putting factor, which is worsened by the hasty growth of human populations and rapid industrialization. The polluted aquatic environment is a hazardous worldwide problem, and the drainage of agricultural, industrial, and commercial chemicals into the aquatic environment has induced several harmful effects on living freshwater organisms. Moreover, these contaminants could directly accumulate in ﬁsh and other organisms, leading to contamination of the food chain, with consequent effects on human consumers.
Within the past decade, the use of pyrethroids have drastically increased. Despite the risks of these highly toxic chemicals, the full suite of direct and indirect effects of pyrethroid insecticides have not been a concern for serious discussions at the local and global levels. And as the use of pyrethroid insecticides continue to increase, understanding their toxic effects will become increasingly important, so to develop strategies to address their overall impact on aquatic ecosystems and therefore, on human life and survival as a whole.

Gelinde Narekine, Disciple of Medical Laboratory Science, Division of Health Sciences, School of Medicine & Health Sciences, University of Papua New Guinea