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Outsized nutrient contributions from small tributaries to a Great Lake
Robert J Mooney1, Emily H Stanley2, William C Rosenthal2,3
1Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706; rjmooney@wisc.edu.
Summary
Excessive nitrogen and phosphorus from small streams significantly impact large lakes like Lake Michigan. Managing nutrients in smaller watersheds is crucial for protecting valuable nearshore aquatic ecosystems.
Area of Science:
- Environmental Science
- Limnology
- Ecosystem Ecology
Background:
- Excessive nitrogen (N) and phosphorus (P) loading causes widespread eutrophication in aquatic ecosystems.
- Nonpoint nutrient sources from tributaries are major drivers of eutrophication in U.S. lakes.
- Small tributaries of large water bodies are often overlooked despite their potential local importance.
Purpose of the Study:
- To investigate how land cover and dams affect nutrient inputs from tributaries of varying watershed sizes into Lake Michigan.
- To assess the nutrient loads, concentrations, stoichiometry, and bioavailability across Lake Michigan's tributaries.
- To understand the role of small streams in coastal eutrophication of large freshwater lakes.
Main Methods:
- Conducted a snapshot assessment of nutrient inputs from nearly all tributaries of Lake Michigan.
- Analyzed nutrient loads, concentrations, N:P stoichiometry, and dissolved inorganic nutrient percentages.
- Compared nutrient delivery across different watershed sizes and land cover types.
Main Results:
- Nutrient loads, concentrations, stoichiometry, and bioavailability varied significantly among tributaries.
- The six largest tributaries contributed approximately 70% of the total daily N and P load to Lake Michigan.
- Small tributaries showed disproportionately high nutrient loads and a greater proportion of bioavailable dissolved inorganic nutrients.
Conclusions:
- Small streams play an underappreciated role in driving coastal eutrophication in large water bodies.
- Nutrients from small watersheds, though modest in total load, are highly bioavailable and can fuel nearshore algal blooms.
- Expanding nutrient management to smaller watersheds is essential for mitigating ecological impacts on valuable nearshore ecosystems.
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