What the Watershed Means to Me
Big Sioux River at Akron, Iowa
This article was written by a Jefferson High School student in Melissa Hittner’s 2023-2024 AP Environmental Science class and edited by Friends of the Big Sioux River staff.
Hello, My name is Tehankasandata. That is the name given to me by the Lakota people. Most people know me as the Big Sioux River. I live on the borders of Iowa, South Dakota, and Minnesota in the Big Sioux River watershed. Many people use my waters for fishing, boating, swimming, and irrigation, but I am polluted. Within recent years, the South Dakota Department of Agriculture and Natural Resources, South Dakota Game, Fish and Parks, and the South Dakota Department of Health have begun to test more fish in waterbodies across the state for mercury in fish tissue. These changes in data assessment have resulted in more lakes and streams not meeting their beneficial use standards, including mine.
Mercury is a heavy metal and non-point source pollutant found in waterbodies across the United States. The mercury in my watershed basin is deposited into the soil and aquatic ecosystems from the atmosphere by precipitation. However, how the mercury entered the ecosystem has many explanations. Some factories release mercury as a by-product of fossil fuel combustion. In other instances, mercury and other dangerous chemicals are expelled into the atmosphere through improper waste disposal, particularly that of burning. Mercury is also expelled naturally through volcanic eruptions. Within the last 150 years, the amount of mercury in the atmosphere has more than doubled due to human activities.
When mercury emissions enter the atmosphere, they often travel long distances before being deposited on the earth's surface via precipitation. Once entering my waters, this inorganic mercury is often converted to organic methylmercury through a process known as methylation. Methylmercury has much higher toxicity and bioaccumulates easily in aquatic food chains, leading to unsafe concentrations in the flesh of fish and invertebrates. Almost all mercury found in fish tissue is methylmercury.
So, what is bioaccumulation, and why should the general public know about it? In general terms, bioaccumulation is the build-up of absorbed chemicals or substances inside an organism over time. This build-up occurs because the organism cannot excrete the substances as quickly as they are absorbed, or the substances are too complex for the organism's digestive system to filter out. As a result, biomagnification occurs within the food chain, with larger organisms accumulating mercury or other substances at a faster rate. The more contaminated zooplankton a small fish eats, the more contaminants it will have in its body. The more contaminated small fish a larger fish eats, the more contaminants it will have in its body, and so on. Humans who eat fish from mercury-contaminated waters are at risk for mercury levels building up in their bodies. However, this occurs at a much slower pace than that of a small fish.
Biomagnification puts the entire food chain at risk, but mercury does not have to be in food to be dangerous. Here in the Big Sioux River watershed, many of my waters are used for irrigation. Waters contaminated with mercury can also cause damage to soil and plants, leading to reduced plant growth and even plant death. This potential contamination is particularly alarming for me as a river because I am a tributary of the Missouri River, which connects to many other rivers and streams. The water in my basin could flow to other water biomes and contaminate them with mercury as far down as the Gulf of Mexico, compromising crops across the country.
This article may leave you wondering about the safety of fishing and consuming fish from my waters. Fish provide essential nutrients recommended as part of a healthy diet by the Dietary Guidelines for Americans, and many fish species are both nutritious and low in mercury. Unlike fish bodies, which cannot get rid of mercury, human bodies can slowly get rid of mercury over time. However, if you regularly consume fish with high levels of methylmercury, it can accumulate in your bloodstream faster than your body can remove it. Some local fish, such as bass, northern pike, and walleye, contain higher levels of mercury than others due to their size and age. Smaller and younger fish are less likely to have high mercury levels and can be consumed regularly. There is no method of cooking or cleaning fish that will reduce the amount of mercury in the tissue.
Due to potential effects on developing brains, it is important to limit the amount of mercury consumed by children and those who are pregnant or breastfeeding. The following chart from the Environmental Protection Agency outlines the "best choices" for fish consumption.
You can also check your local state's website for fish advisory consumption information. You can learn more about the fish consumption advisories in the three states that are part of my watershed in the links below.
Mercury is quite challenging to remove from a water body, so it is unlikely that mercury levels in my waters will decrease any time soon. Additionally, the majority of mercury in my waters comes from atmospheric deposition from coal-fired power plants, mining, and other industrial wastes, so until measures to reduce mercury are implemented at a global scale, we will not see a widespread change in the concentration of mercury in waters and soils. However, this should not stop you from fishing, recreating, or consuming fish from my lakes and streams. Awareness is key for your safety and mine because when all is said and done, I am a river who relies on you for my protection.