Watershed Symposium 2019

Using Citizen Science to Locate Nutrient Sources to Utah Lake

Summary:
Harmful algal blooms on Utah Lake have raised the question: where do the nutrients in Utah Lake come from? We used citizen science to sample hundreds of points in the watershed three times a year, allowing us to determine where solutes are coming from and the consistency of their sources.

Full Abstract:
Anthropogenic inputs of N and P have led to eutrophication of water bodies around the world, intensifying water resource scarcity. While removing point sources has been largely effective, persistence of non-point sources has delayed or negated gains in water quality in many instances. Recent studies have shown that spatial variability of stream chemistry may not be as unpredictable as previously thought, suggesting that periodic sampling of many locations in a stream network (i.e. synoptic sampling) could identify non-point nutrient sources and quantify nutrient retention capacity. Here, we conducted a large participatory science project to synoptically sample Utah Lake’s tributaries. Utah Lake is a large, shallow, eutrophic lake that has been listed as impaired for phosphorus and total dissolved solids, but disagreements exist about the source of solutes entering the lake. We collaborated with members of the public to sample ~200 points across the 2,950 km2 Utah Lake watershed three times during 2018 (March, July, and October). We calculated spatial stability, a metric of the consistency of patterns in concentrations, and temporal synchrony for a suite of stream solutes, including phosphate, nitrate, DOC, and sulfate. We used these data to calculate a distributed mass balance of solutes throughout the watershed, quantifying how much leverage each subwatershed exerted on the overall catchment load. Solute concentrations showed extreme variation across the watersheds, attributable to differences in land use and natural catchment characteristics. For example, median phosphate concentration was 0.011 mg P/L but it ranged from 0.0015 to 0.62 mg P/L. Likewise, nitrate concentration spanned 5 orders of magnitude (0.0007 to 28 mg N/L), with a median concentration of 0.14 mg N/L. Nitrate concentration was highest in canals and point sources within Utah Valley. Chloride had high spatial stability (0.8 spearman rank correlation, SR), but soluble reactive phosphorus (the most bioavailable form of phosphorus) had low spatial stability (0.3 SR), suggesting substantial seasonal or event-level change through time. Nitrate had intermediate spatial stability (0.5 SR). The low spatial stability of phosphorus compared with other solutes could be due to two non-exclusive phenomena: 1. non-geologic sources of phosphate that are more spatially variable than weathering could be dominant and 2. biological processes could alternatively increase or decrease phosphate based on nutrient demand across space and time. We conclude that citizen science can generate insight into catchment chemistry and hydrologic processes while also increasing community connection with the watershed.