By Janice Brahney, Guest Blogger
DEQ invites guest bloggers to share their thoughts on issues that impact our environment. We appreciate their insights and the opportunity to broaden the conversation with others in the community.
Dust is as old as dirt. Wind erosion and dust storms are natural phenomena in the semi-arid regions of the western United States. However, human land use and drought have the potential to increase the susceptibility of soil to erosion by reducing vegetation cover, exposing lakebeds, and by disturbing the landscape through a variety of recreational or industrial activities.
We have long known that this erosion removes the fraction of the soil that people care about – the soil fraction that is rich in nutrients, water holding capacity, and organic matter.
A reasonable question is then, where is that dust going and what impact does it have when it gets there?
As it turns out, elevated dust deposition appears to have significant effects on low-nutrient freshwater systems through the enhanced delivery of phosphorus – one of the most important nutrients for aquatic environments.
Without going into too much detail, an intense sampling campaign over several years indicated that dust from southwestern Wyoming’s Green River Valley was facilitating the transport of phosphorus to high mountain lake ecosystems in the nearby Wind River Range. Dust-affected lakes had higher phosphorus concentrations, greater biological productivity (e.g., increased phytoplankton and zooplankton growth), and altered organismal communities. Furthermore, elevated dust deposition and the associated changes to these lake systems was a new phenomenon that we were able to tie directly to human land-use upwind.
An interesting point is that the land-use was not generating dust storms akin to the dust-bowl era; rather small chronic erosion events were having a large cumulative effect. These results led to many more questions. For example, how widespread are the lakes affected by dust-phosphorus?
I conducted two separate studies in an attempt to address this question. The results strongly implied phosphorus is entering many lakes though atmospheric pathways and that this flow has been increasing in recent decades. Though both published papers from this work are circumstantial, they are provocative and hint at a widespread, previously undocumented pathway for one of the most important ecosystem nutrients.
In the first paper, “Is atmospheric phosphorus pollution altering global lake stoichiometry?” we compare dust chemistry from mountain environments around the world to lake chemistry in the same regions. Mountain lakes are really good indicators of atmospheric nutrient deposition because their catchments tend to be small, steep, and lack vegetation. This means the rainwater that ultimately ends up in the lake has very little time to take up nutrients from the land around the lake. As a result, lake water in these lakes looks like the regional rainwater. If phosphorus emissions were increasing due to human activity, then we would expect to see greater deposition rates of phosphorus — and greater lake water phosphorus concentrations — around regions where this type of human activity occurs. That is exactly what we found. However, correlation is not causation, which is why the title has a question mark.
Collaboration with the EPA produced a second paper: “Continental-scale Increase in Lake and Stream Phosphorus: Are Oligotrophic Systems Disappearing in the United States?” Here, we examined data from thousands of rivers and lakes in the continental U.S. looking for changes in chemistry over three sampling periods in the last decade. Shockingly, we found that the number of lakes and streams that could be classified as low-nutrient had dropped precipitously from approximately 30 percent to less than 7 percent of lakes and 3 percent of streams. In addition, the lakes and rivers that changed the most were the least impacted by human activity in their watershed.
This raises the question: If the phosphorus is not coming from human activity within the watershed, is it coming from outside of the watershed via the airshed?
There is, however, another potential explanation. Even though rainfall has not increased in much of the continental US, rain intensity may have increased, which can lead to greater erosion rates in the catchment, increasing the flux of phosphorus to these systems. At present, a lack of monitoring data prevents us from determining the primary cause of increasing phosphorus concentrations in these remote systems.
Though neither of these latest papers provides a smoking gun for the atmospheric pathway of phosphorus, they indicate that more research and more data are needed. Many government organizations around the world collect rain, but do not measure phosphorus. Many organizations measure atmospheric aerosols, but only the fraction smaller than 10 micrometers (about 1/10th the size of a grain of salt). Further, aerosols collected at any size are rarely analyzed for phosphorus content. There are many reasons for these omissions — dust is hard to measure, samples are often contaminated with bird poop, and soluble reactive phosphorus will start to disappear as soon as it is collected. But there are some manageable solutions to these challenges.
Clearly, given the importance of phosphorus to aquatic ecosystems, this issue should be investigated further. Beyond the transport of nutrients and effects on aquatic systems, many questions remain. For example: What else is being transported and what are the potential effects to human health?
I look forward to answering some of these questions and more with my graduate students at Utah State University. Stay tuned…
I am new faculty in the Department of Watershed Sciences at Utah State University. I have a Bachelor of Science in Environmental Science, and a Master of Science and PhD in the Geological Sciences. I am interested in understanding human and natural controls on freshwater ecosystems and the consequences for the organisms that live within them. I aim to conduct research that both makes progress on important scientific questions and produces knowledge that is immediately useful to land and ecosystem managers who must make the best decisions possible as they balance the needs of our human communities and the natural systems they affect. I live in Logan, Utah, and enjoy almost any activity, so long as it is outside.