Nutrient Uptake and Water Quality in Great Salt Lake Wetland Impoundments

The Great Salt Lake (GSL) is the largest inland body of water on the Pacific flyway, a major pathway for migratory birds in the Americas that extends from Alaska to Patagonia. The lake is surrounded by approximately 360,000 acres of wetlands, providing critical food, shelter, cover, nesting areas, and protection to between 4–6 million birds that visit each year. Impounded wetlands were created as part of the GSL ecosystem to support waterfowl habitat. These large, shallow, submergent wetlands are diked to control water levels to sustain aquatic plants which are an important food source. Besides providing critical habitat, these impoundments also perform critical functions, including water quality improvement. Wetlands improve water quality by removing sediments, nutrients (nitrogen (N) and phosphorus (P)), and other contaminants from water. This function is especially important because excessive N and P in waterbodies leads to algal blooms which can be directly toxic or cause oxygen depletion and fish kills. However, these wetlands are threatened by climate change and human impacts like water diversions, excessive nutrient inputs, and invasive species. There is a critical need to understand how these wetlands will be affected by environmental stressors, specifically their capacity to assimilate nutrients and improve water quality. First, we tested a method to measure nutrient assimilative capacity using mesocosms, intermediately-sized enclosures where nutrient concentrations can be manipulated within a confined space. We performed experimental perturbations within the mesocosms, to measure the change in nutrient assimilative capacity. Our treatments included aquatic plant removal, increasing salt content, and altering the baseline phosphorus concentration. We found that nutrient uptake was not different among the treatments, suggesting that wetlands are resistant to change in this function with regard to the treatments measured. The next summer we performed extensive sampling, including a nutrient assimilative capacity measurement, at 18 wetland impoundments along the eastern side of the Great Salt Lake. We classified the health of these wetlands using multimetric indices (MMIs) that take into account important biological features of the impoundments, including the amount of aquatic vegetation, diversity of macroinvertebrates, and amount of algal mat cover. Contrary to our hypothesis, we found that wetland quality (according to the MMI) was not related to the amount of nutrient uptake measured within the mesocosms. We found that abiotic variables including nutrient concentrations within the water column and soil, dissolved oxygen, water depth, and soil texture were correlated with nutrient assimilation, suggesting that they are important characteristics to measure for future research. Finally, we wanted to assess nutrient limitation within the impoundments to control phytoplankton growth. We performed a container study at two locations where we added increasing amounts of N or P and compared them to the growth rate of phytoplankton. We found that the locations we assessed are N limited, meaning that decreasing the amount of N to the wetlands will decrease the amount of phytoplankton or algal growth. Bacterial evenness or the relative abundance of the different species at both sites decreased once the N concentration was higher than our calculated threshold. These results support management efforts to minimize N and P discharge to these sensitive wetlands