Started from the Bottom: Predicting Risk of Toxin Formation in Wetland Mud

Article: Pollman, C. D., Swain, E. B., Bael, D., Myrbo, A., Monson, P., & Shore, M. D. (2017). The Evolution of Sulfide in Shallow Aquatic Ecosystem Sediments: An Analysis of the Roles of Sulfate, Organic Carbon, and Iron and Feedback Constraints Using Structural Equation Modeling. Journal of Geophysical Research: Biogeosciences122(11), 2719-2735.

They say that you only live once, but for wild rice plants in the Great Lakes Region, whether or not they live depends on what tiny microbes living deep within the mud are doing. Although small, these microbes can poison the rice plants and have some big impacts, especially for everyone that depends on the food these plants provide.

Rice for What: Importance of Wetland Environments

Freshwater wetlands are dynamic environments that provide important habitat for a variety of organisms ranging from larger plants and animals, to tiny microbes within the mud that we cannot see with our naked eye. Although small, these microbes can have a big impact on the wetland because they can transform chemical compounds. One compound that is of particular concern is sulfide. Microbes can form sulfide from sulfate, which is present in the water of the wetland. High levels of sulfate in wetlands can result from mining waste and wastewater that ends up in the water. Scientists are concerned about the formation of sulfide in wetland mud because it can be toxic to plants that grow in the wetlands.

The Prairie Wetlands in Fergus Falls, Minnesota. Photo by Courtney Celley/USFWS

While the potential for a wetland to become too toxic for plants is concerning from an ecological standpoint, it can also be an issue for people. One example, and the focus of the study from Pollman and colleagues, is the wild rice plant, which can be particularly sensitive to sulfide. Wild rice typically grows in wetlands across the Great Lakes Region of the United States where it is an important food source for people and animals. It is also farmed and harvested, making it economically important. It is also culturally important to some indigenous peoples in the region.

Wild rice growing in a Minnesota wetland. Photo by Eli Sagor

Best (Model) I Ever Had: Using Statistics to Find the Best Predictors

Recognizing the importance of wild rice and the threat of sulfide formation, Pollman and colleagues set out to understand which factors lead to sulfide formation and help improve management strategies.  While sulfide is made from sulfate, whether or not this transformation occurs depends on other conditions as well. For instance, scientists have observed some wetlands where there is a lot of sulfate in the water, but little to no sulfide being formed in the sediment.

Some other compounds can affect sulfide. If sulfide forms and there is iron present, the two can combine to form iron sulfide, and it is no longer toxic to plants. Organic carbon, which are the carbon compounds that from decaying plants and other living things, is also involved in the reaction that transforms sulfate to sulfide. However, all of these interactions can be complex. Thus, this study sought to figure out what combination of conditions put a wetland at risk of forming sulfide in the mud, making it an unsuitable habitat for wild rice.

Some examples of how different conditions can lead to a wetland either being a toxic or healthy environment for wild rice plants. In the real world, these conditions can occur in different combinations, which is why it can be a difficult problem to tackle!

In order to accomplish their objectives, the researchers took samples of mud and water at a number of wetlands across Minnesota. At each site, they took measurements including sulfate concentration in the water, sulfide in the mud, and iron content in the mud. They then used this data to test a model that they had created to predict the formation of sulfide based on the physical and chemical conditions in the wetland. The scientists used statistical analyses to figure out which of these conditions were most important in predicting sulfide formation, so they could arrive at a model that could best predict which wetlands were in trouble.

Location of sites (red dots) sampled for the study. Figure by Pollman et al., 2017

The Motto: It’s not just sulfate that’s important!

After testing how including different conditions in the model affected the ability of the model to predict sulfide formation, the scientists found that there were three different environmental conditions that were most important in determining whether a wetland was at risk. These conditions include the concentration of sulfate in the water, the amount of iron in the mud, and the amount of organic carbon in the mud. High concentrations of organic carbon and sulfate put a wetland at higher risk of forming sulfide. Iron, which converts sulfide to a non-toxic form, has the opposite effect: high concentrations of iron put a wetland at a lower risk level. The researchers found that each of these factors was equally important. In other words, none of these had absolute control over whether or not sulfide formed, rather it was the balance between these sulfate, iron, and organic material.

Take Care: Managing Wetlands to Prevent Sulfide Formation

While the model developed by Pollman et al. cannot determine exactly how much sulfide will form, it can help to improve management practices to maintain wetlands as healthy habitats for wild rice. It would be difficult to control how much iron or organic carbon ends up wetland sediments since these are a result of natural processes, but it is possible to control how much sulfate ends up in water from waste products and other human activities. As the findings suggest, the risk of forming sulfide depends on more than just how much sulfate there is in the overlying water. Therefore, just setting an overall limit for sulfate likely would not work, since sulfide will start to form at different levels of sulfate depending on the other conditions mentioned.

To be more effective, sulfate limits should be site-specific and this model can help to identify which wetlands should have higher limits and which should have lower. For instance, a wetland with a lot of organic carbon and only a small amount will start to form sulfide at a lower amount of sulfate and should therefore have a lower limit relative to a wetland with low levels of organic carbon and high levels of iron. Through this combination of scientific research and environmental policy, it is possible to ensure a sustainable future with healthy wetlands and plenty of wild rice!

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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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