Tracking hot spots and hot moments in an urban freshwater estuary

Gaston Small, University of St. Thomas (MN)

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This module explores the hydrology and biogeochemistry of the St. Louis River Estuary (Duluth, Minnesota). The overarching question of the module is: when, and where, is the estuary acting as a source vs. a sink for nutrients? Students analyze seasonal trends in discharge and solute concentrations, apply a mixing model to estimate contributions of water from different sources, use these results to make inferences about spatial patterns in biogeochemical processes, and test these inferences against measured rates of microbial denitrification.

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Learning Goals

After completing this module, students will be able to:

  • Explain seasonal patterns in nutrient concentrations based on physical characteristics (river discharge, temperature).
  • Estimate residence time of water in an estuary based on river discharge (from a USGS hydrograph), area (calculated from Google Earth), and mean depth.
  • Use a mixing model, based on conservative solutes, to partition water contributions from different chemically-distinct sources.
  • Use differences (residuals) between observed and expected values (based on conservative mixing) to infer net biogeochemical processing rates.

Context for Use

This module is designed for an upper-level undergraduate course in limnology, hydrology, or environmental science. These computer-based exercises take approximately 3-5 hours. Students need access to Google Earth (online version) and Excel (desktop app is needed for full functionality of interactive spreadsheet model).

Prior to beginning the module, students will benefit from having a basic understanding of eutrophication (causes and consequences of nutrient loading in aquatic ecosystems) and nitrogen transformations (nitrification and denitrification)

How Instructors Have Used This Module

Using Project EDDIE modules in Environmental Science Senior Seminar
Gaston Small, University of St. Thomas (MN)
This module explores seasonal dynamics in hydrology and biogeochemistry in a freshwater estuary. Students use a dynamic mixing model to tease apart the contribution of different sources of water at different locations throughout the estuary, and then use these results to infer hotspots of nutrient dynamics.

Description and Teaching Materials

Why this Matters:

Estuaries are complex physical and biological systems, with water from various sources mixing and moving in three dimensions, along different environmental gradients. The standard approaches used to conceptualize and quantify nutrient cycling rates in streams or in lakes do not strictly hold true. This module introduces students to a hydrologic mixing model. Rather than simply analyzing patterns in data, this approach synthesizes this information, combining empirical observations with assumptions to generate inferences (Activity B), that can then be validated against independent observations (Activity C).

Quick overview of the activities in this module

  • Activity A: Students analyze seasonal patterns in river discharge from a USGS hydrograph, estimate the area of the estuary in Google Earth, compare water residence time and temperature in spring vs. summer, and generate a hypothesis about seasonal nutrient processing rates in the estuary.  
  • Activity B: Students compare the chemical signatures of five distinct water sources in the estuary and use an interactive mixing model to estimate the relative contribution of these sources to water in the estuary at 9 different longitudinal locations, in spring and summer. These estimates are then used to generate predictions of nutrient concentrations under a conservative mixing scenario. Students calculate deviations from this conservative prediction as an indicator of biological processing rates.
  • Activity C: Students build on findings from Activity A and B, formulating and answering a follow-up question using a dataset of sediment characteristics and microbial denitrification rates.

Workflow of this module:

  1. Prior to class, students watch this 18-minute YouTube video that shows how the investigators collected the data.
  2. The instructor gives a brief PowerPoint presentation with background material on eutrophication, hydrology, or other connections to course materials, as appropriate.
  3. Students can then work through the module activities.

Teaching Materials

Teaching Notes and Tips

We suggest having students work in small groups, with the instructor circulating to answer questions and stopping periodically to share answers from different groups and to facilitate discussion. Several of the questions are well-suited for class discussion: B.1, B.12, and C.3.

In Activity B, we suggest assigning different teams of students the early June (SLRE 11) and late July (SLRE 13) datasets for analysis, and then pairing teams to compare findings.

Technology notes: the .kml file showing sampling site locations should be able to be opened with either the Google Earth app or the online version. However, if students have trouble opening the file, they can still view the site locations from the image in the handout, and they can still complete Activity A.2.a, measuring the area of the estuary, without the station location data.

The interactive mixing model file should be opened in the Excel desktop app for full functionality. If opened in Google Sheets, the graph will not update.


Student teams complete guided questions, including an inquiry-based data analysis and synthesis question in Activity C. Possible responses are given in the instructor handout.

At the end of each module, provide an opportunity for class discussion and debrief.  What did students find surprising, difficult, or confusing during each module? Students can share preliminary findings for each module through a Google Jamboard or similar software to provide real-time feedback.

References and Resources

This 18-minute video explains the project and shows how the investigators collected the data.

Original dataset:

Lake Superior National Estuarine Research Reserve: