Persistent Nitrate in Alpine Waters with Changing Atmospheric Deposition and Warming Trends.
Sydney C Clark1,2, Rebecca T Barnes3, Isabella A Oleksy4
1Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island 02912, United States.
Environmental Science & Technology
|October 12, 2021
Summary
Elevated nitrate in alpine lakes persists due to atmospheric deposition and catchment nitrification. Microbially produced nitrate is increasingly significant, linked to rising ammonium deposition in these sensitive ecosystems.
Area of Science:
- Environmental Chemistry
- Isotope Hydrology
- Alpine Ecology
Background:
- High-elevation lakes in the Colorado Front Range show persistent elevated nitrate levels.
- Atmospheric nitrate deposition has declined since 2000, making the nitrate source elusive.
- Shifts in nitrogen sources from glacial melt and atmospheric ammonium complicate nitrate origin.
Purpose of the Study:
- To constrain the provenance of elevated nitrate in alpine lakes.
- To investigate nitrate sources during the summer open-water season.
- To understand the role of different nitrogen inputs in alpine aquatic systems.
Main Methods:
- Analysis of complete nitrate isotopic composition (δ15N, δ18O, Δ17O).
- Sampling of nitrate-bearing source waters from two alpine ecosystems (2017-2018).
- Comparison with historical surface water samples from the mid-1990s to present.
Main Results:
- A consistent contribution (13-23%) of uncycled atmospheric nitrate was observed throughout the summer.
- Nitrification within the catchment, utilizing soil organic matter and ammonium deposition, likely contributes up to 87% of nitrate.
- Historical data reveal an increasing importance of microbially produced nitrate over time.
Conclusions:
- Both atmospheric deposition and catchment nitrification are significant nitrate sources in alpine lakes.
- Increasing ammonium deposition is linked to a greater role of microbially produced nitrate.
- Understanding these nitrogen sources is crucial for managing alpine aquatic ecosystems.
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