Development and Nutrient Enrichment in Southwest Colorado

Three years ago, Science of the Fly's first volunteer sampler and now executive director—Allie Cunningham—began collecting samples around Telluride, CO. Sampling in this region quickly grew, and new volunteers helped expand onto the Animas and La Plata rivers. Today, there are more than 30 sampling sites across Colorado alone and over 300 sites across the globe.

While each sample is significant, without consistent and prolonged data collection we cannot discern how dissolved nutrients change over time. Without a robust set of data, it is next to impossible to conclusively understand what the work of SOTF volunteers tells us. The consistent collection of samples on the San Miguel, Animas, La Plata, and connected tributaries provides an opportunity to study and understand patterns in water quality and their effects on fisheries in this region.

Understanding water quality and its impact on fisheries is undoubtedly important, but the science behind these patterns can often feel daunting. Why focus on dissolved nutrients like phosphate and nitrate? What can the concentrations of these nutrients tell us about the health of our rivers? And, what anthropogenic and environmental conditions cause nutrient concentrations to change? This projects sets out to answer these questions and illustrate the importance of consistent, long-term river monitoring, with one of the first analyses of SOTF's growing database of water quality.

Nitrate and phosphate are naturally occurring compounds made up of nitrogen and oxygen or phosphorus and oxygen respectively. These compounds readily dissolve in water (i.e. dissolved nutrients), which allows them to be easily transported through waterbodies like rivers or lakes. In plants, phosphate and nitrate serve as an essential nutrient in plant metabolism, growth, development, and adaptation to various environments (Zhang et al., 2018). While vital to all life on Earth, in excess, dissolved nutrients pose serious risks to ecosystems worldwide.

Nutrient enrichment is the leading cause of water contamination in the country and can cause eutrophication, harmful algal blooms, or nutrient limitation, which can impact biological communities across all trophic levels (Figure 3). The most common effect of nutrient enrichment is eutrophication. Eutrophication is a processes where increases in dissolved nutrients (often from urban and agricultural runoff) induce excessive algal growth, which causes oxygen levels to quickly decrease as the algae dies and is decomposed by microbes (Figure 4). Dissolved nutrients are vitally important to aquatic ecosystems. However, in excess, they can induce drastic changes to an ecosystem, from a disruption to the aquatic insect community to hypoxia (low oxygen) induced fish kills.

Rivers such as the San Miguel, La Plata, and Animas play an important ecological and economic role in Colorado, by contributing to the $1.9b fishing industry, providing a recreational resource to thousands, and serving as the lifeline to biological communities in the area. For this reason, it is necessary to understand how changes in dissolved nutrients may impact these fisheries and the preemptive steps that can be taken to mitigate the cascading effects of nutrient enrichment.

Globally, acute increases in anthropogenic emissions of nitrogen and phosphorus have led to increases in aquatic and terrestrial nutrient deposition (Gao et al., 2014). Simultaneously, urban and agricultural areas are the leading causes of human caused nutrient enrichment in river ecosystems, both of which have expanded significantly in recent decades (Allen et al., 2004; Evans et al., 2014; Lintern et al., 2018). In agricultural areas, excess use of nitrogen and phosphorus based fertilizers can lead to the leaching of nutrients into waterways during heavy rains, floods, or erosion events. In urban areas, nitrogen and phosphorus accumulate primarily from municipal wastewater and non-point source pollution in stormwater (Walsh et al., 2014). Due to the abundance of impervious surfaces in urban areas, these nutrients are transported directly back to aquatic ecosystems in high concentrations, rather than absorbed and retained overtime by healthy soils and plants.

Understanding that urban areas are one of the leading causes of aquatic nutrient enrichment, this project investigates if small, isolated urban areas (i.e. Telluride or Durango) significantly impacted phosphate and nitrate concentrations in this region. In order to accomplish this goal, unique watersheds were created directly upstream of each sampling location using  USGS StreamStats  (Figure 5). This allowed for the inclusion of any land cover upstream that may be impacting water quality at each site, instead of generalizing conditions across all sites within the same shed or on the same river. Land cover data for Colorado was sourced from the  Multi-Resolution Land Consortium . Using an open source mapping software, qGIS, I quantified the proportion of each land cover type within each watershed.

Eastern Colorado and the greater Denver area is dominated by urban, agricultural, and grassland areas (Figure 6). In contrast, the study area, and western CO as a whole, is largely comprised of high-elevation, woody vegetation and grasslands (Figure 7). Woody vegetation includes all deciduous, coniferous, mixed forests, and woody shrubs. All in all, developed area—which is defined here as the sum of non-impervious, rural, and urban areas—made up merely 0.169-7.104% across all sampling sites (Figure 8).

Despite making up a small percentage of the total land area, there was a positive relationship between percent developed area and total dissolved nitrate (p=0.2963) and phosphate (p<0.05) concentrations. In other words, as developed area increases so do nutrient concentrations. However, only the positive relationship with phosphate was statistically significant, indicating the need for more data and further research to understand the patterns between nutrients and developed area in this region. While most of the sampling sights were relatively similar, one outlier—"San Miguel River Above Confluence with South Fork"—demonstrated much higher levels of both dissolved nutrients. Conversely, there was a significant inverse relationship between woody vegetation cover and average nitrate concentrations (p<0.0005), indicating that forested areas could be a valuable tool to mitigate excess nutrient runoff into these rivers.

Rivers naturally contain low concentrations of both phosphate and nitrate, which play a vital role in primary production and river health. Generally speaking, nitrate concentrations below 250 µg/L and phosphate concentrations below 25µg/L are indications of a healthy river (SOTF, 2019). Overall, the sampling sites in this region are well below standards for both nitrate and phosphate concentrations. This could be due to the regions abundance of forested area and relative lack of both agricultural and developed areas.

Although the conditions of these rivers is generally something to be proud of, the strong relationship between nutrient enrichment and developed areas persists. This pattern is particularly important understanding that Colorado's population is expected to increase by more than 1 million people by 2050 (DOLA, 2022). The majority of this growth will occur on the Northern Slope; however, mountain towns like Telluride and Durango can expect increases in populations and expansions of developed areas.

The relationship between developed areas and nutrient enrichment is particularly evident on the San Miguel River. This watershed contains the highest concentration of developed area and sampling sites with above average levels of nutrients.

These locations are a mere miles from the watershed's headwaters, and yet clear signs of human caused nutrient deposition persist. As this river begins, it flows directly through Telluride, by golf courses, highways, and rural communities. This leaves the San Miguel particularly susceptible to nutrient runoff from fertilizers and wastewater. One site just miles downstream from Telluride and a large rural community averaged 978.28µg/L and 72.69µg/L of nitrate and phosphate respectively. This sampling site site is just downstream of the Telluride Waste Water Treatment Plant, which could explain the elevated nutrients recorded at this site. Notably, this was one of only two locations to exceed 250µg/L nitrate and the only site above 25µg/L phosphate.

Urbanization is a leading cause of anthropogenic fish decline across the U.S., due to its impact on water quality via nutrient enrichment and habitat degradation (Brain and Prosser, 2022). Equally, climate change is expected to disrupt and induce stress on a range of aquatic organisms. The compounding effects of increased development and climate change will likely induce a reconfiguration of headwater streams, including a loss of ecosystem structure and services, which will be more costly than climate change alone (Nelson et al., 2009).

While locals can not prevent climate change, preventative action to mitigate nutrient contamination and maintain resilient river systems can help uphold the health of these ecosystems. For example, the limiting effect of woody vegetation on nitrate concentrations (Figure 10) could inform future development projects by minimizing losses of forested areas in order to mitigate nutrient runoff.

Perhaps most importantly, volunteers can continue to diligently and consistently collect samples in this region. While these rivers are generally healthy, the only reason we know this is because of volunteer sample collection. This project, however informative, is merely a start to the work that can be done using Science on the the Fly's data. Many of the sites used in this project consisted of only a handful of samples, and more consistent collection is necessary to grow our understanding of these waterways. After roughly three years, we are just now starting to see the potential of SOTF's work. As more samples get collected, the data base grows and so does our understanding of these rivers.

The current story of the rivers of Southwest Colorado is one to be proud of. It is a story of healthy waterways and renowned trout fisheries. A story of isolated developed areas that have far reaching effect on these waterways. Effects that can be mitigated with proper preemptive action. In order to uphold this story—to maintain the health of these invaluable resources—sampling at these sites and beyond must continue, so we can grow our knowledge of these rivers and be prepared to take action to protect them when the time comes.

I would like to acknowledge Middlebury College's Assistant Professor of Biology Eric Moody, Derrick Burt of the Geography Department, the Woodwell Climate Research Center, Andie Norton of Woodwell, Allie Cunningham of Science on the Fly for their invaluable guidance in completing this project, and the volunteer samplers that make this work possible.