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Exploring watershed effects on nutrient concentrations in shallow lakes through stable isotope analysis
Thomas A Langer1, Kyle D Zimmer1, Brian R Herwig2
1Department of Biology, University of St. Thomas, St. Paul, MN, 55105, USA.
The Science of the Total Environment
|February 12, 2022
Summary
Lake managers should reduce agricultural land use in watersheds to improve shallow lake water quality. Minimizing agriculture helps maintain clear water, reducing total nitrogen (TN) and total phosphorus (TP) levels.
Area of Science:
- Environmental Science
- Limnology
- Biogeochemistry
Background:
- Shallow lake biogeochemistry is shaped by in-lake conditions (ecosystem state) and watershed factors (land use).
- The relative importance of these factors is unclear, hindering effective lake management strategies.
- Understanding these influences is crucial for managers aiming to maintain clear-water states in lakes.
Purpose of the Study:
- To determine the relative influence of lake size, watershed land use, and ecosystem state on nutrient concentrations and fish stable isotopes in shallow lakes.
- To assess how different land use types (natural, row crop, all agriculture) impact lake biogeochemistry.
- To inform management decisions regarding watershed versus in-lake interventions for lake stabilization.
Main Methods:
- Studied 48 shallow lakes in Minnesota, USA.
- Analyzed water column total nitrogen (TN) and total phosphorus (TP).
- Measured stable isotopes (δ15N and δ13C) in three fish species.
- Employed model selection to identify key influencing factors.
Main Results:
- Fish δ13C was primarily influenced by lake size.
- Fish δ15N showed a strong correlation with watershed agriculture intensity.
- Water column TN and TP were significantly lower in clear-water lakes compared to turbid lakes.
- Land use effects on TN and TP were masked by the dominant effect of ecosystem state.
Conclusions:
- Watershed agriculture directly influences nutrient processing, as indicated by fish δ15N signatures.
- Ecosystem state (clear vs. turbid) is a primary driver of water column TN and TP concentrations.
- Reducing agricultural intensity in watersheds is a key strategy for promoting and maintaining the clear-water state in shallow lakes.
