Date of Award:

5-2023

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Janice Brahney

Committee

Janice Brahney

Committee

Andre Geraldo de Lima Moraes

Committee

John Stark

Abstract

Mountain watersheds provide a critical supply of clean water to millions of people around the world. In recent years, evidence of a concerning trend has emerged in these watersheds: rising phosphorus (P) concentrations. P often acts as the limiting nutrient in mountain lake ecosystems, so increasing P availability in mountain lakes and streams has the potential to drastically increase algal growth in these systems. In extreme cases, increasing mountain lake P concentrations may even cause harmful algal blooms that degrade downstream water quality. While the implications of rising P concentrations in mountain lakes are serious, the cause driving this widespread change remains unclear. In this study, we tested whether higher soil temperatures could be increasing P export from mountain soils and into lakes and streams by increasing the rate of soil organic matter decomposition and mineral weathering. We conducted lab soil incubations and field soil transplant experiments along elevation/temperature gradients to test the effects of warming on soil P export using soils from five different western United States mountain ranges. Our results show that large increases in temperature can increase both the amount of rain-soluble P and the amount of P leaching through mountain soils, suggesting that higher soil temperatures may explain some of the observed increases in mountain lake P concentrations. However, our results also show that soil moisture plays a critical role in mediating the effects of temperature on soil P production, particularly when changes in temperature are small. In addition, we found that soils with certain properties have much higher rain-soluble P concentrations, which may result in much larger warming-induced increases in P export from those soils. In total, these results suggest that rising temperatures may increase P export from mountain soils into mountain waterbodies the most in regions with higher-P soils that experience warming concurrent with relatively high soil moisture. These findings may be helpful for land managers in predicting which watersheds may experience the largest influxes of P due to climate change and, as a result, would make the best candidates for implementing management strategies to mitigate other sources of nutrient pollution.

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