Great Salt Lake

A CASE STUDY OF CARRY CAPACITY: BRINE SHRIMPS IN THE GREAT SALT LAKE

This model simulates the salinity dynamics in the south arm of the Great Salt Lake (GSL) caused by the surface inflows from three local rivers, the water exchanges with the north arm of GSL, and evaporation. The salinity changes, in turn, affect the brine shrimp population in the lake. Therefore, this model illustrates the carrying capacity of the shrimp population influenced by environmental factors.

Grades: 6-12;
NGSS Standards: MS-LS2-1, HS-LS2-1
Related lessons： Colors of Great Salt Lake

20160717_122324.mp4

Video taken by Lin Xiang in GLS, Utah

BACKGROUND

"Great Salt Lake (GSL) is the largest saline lake in the Western Hemisphere, and fourth-largest in the world. The only outflow of water is via evaporation. Over time, this has led to high salinity in GSL, and is responsible for the relatively simple but highly productive saline ecosystem. The lake is a critical link in the Pacific flyway, supporting millions of migratory and resident birds, which feed on invertebrates inhabiting the lake. The lake is also used commercially for mineral extraction and brine shrimp harvest. GSL is vital to the local and regional economy, contributing an estimated $1.3 billion, and supporting nearly 8,000 jobs.

In 1959, Union Pacific Railroad (UPR) constructed a rock-filled causeway across GSL, bisecting the lake from Promontory Point on the east bank, to the West Desert on the west bank (Figure 1). This caused the lake to be separated into north and south bays, Gunnison and Gilbert Bay, respectively. Upon completion, flow between the bays was restricted to the semi-porous fill material and two 4.5 m wide culverts installed during causeway construction. In 1984, an 88 m wide and 4 m deep breach was added to the causeway to increase inter bay flow and alleviate flooding (it was later deepened to 6.4 m). Ninety-five percent of streamflow enters the south bay, causing an elevation gradient to form between the two bays (the south bay is roughly 0.3 meters higher than the north bay). Because of the hydrologic isolation and discrepancy of inflows between the bays, the causeway has become a key driver of salinity in GSL. The north bay is often at or near saturation levels, averaging 317 g/L since 1966, while the south is considerably less saline, averaging 142 g/L since 1966 (White et al., 2014, p. 25) ."

"Salinity differences between the bays have significant impacts on ecology, mineralogy, and commercial and recreational uses The hypersaline north bay is largely inhospitable for macroinvertebrates such as brine shrimp and brine fly. Instead, it is characterized by large populations of archaea (microbes) and red-algae. This causes a discoloration that is easily visible to the naked eye and even satellite images (Figure 2). In contrast, the relatively moderate salinities of the south bay provide habitat for large populations of Artemia and Ephydra, which are vital food sources for birds. However, during periods of high precipitation, salinity drops, allowing freshwater predators such as corixids (water boatmen), usually intolerant to GSL conditions, to prey on Artemia and decimate populations (Wurtsbaugh and Berry 1998) (White et al., 2014, p. 26)."

PROBLEM

"The causeway was built on soft lake sediments and has slowly subsided through time. The two 4.5 m wide culverts became unstable and in 2012 through 2013 they were closed. Lake managers are concerned that if culverts remain closed and new openings are not installed, the causeway will result in water that is too salty in the north bay and too fresh in the south bay to maintain Artemia populations. To replace the flow provided by the culverts, UPR has proposed to build a 55 m trapezoidal bridge. However, changes to lake level and salinity from the bridge had previously not been quantified (White et al., 2014, p. 25)."

Figures and contents above cited from: White, J., Null, S. E., & Tarboton, D. G. (2014). More than meets the eye-managing salinity in Great Salt Lake, Utah. Lakeline Magazine, 34(3), 25.

DESIGN NOTES

1. Only three factors are included in the model to determine the salinity changes in the south arm of GSL: stream inflow, north arm water inflow, and evaporation.

2. Massive precipitation is unusual in the GSL area. When it happens, the stream inflow increases

3. The salinity of the North arm is always high due to fewer freshwater inputs compared to the south arm.

4. To simply the model, only two river entrances, Bear River and Jordan-weber River, are simulated.

5. The highest salinity is 28% (saturation point) with color 90.5 and lowest is 0% (freshwater) with color 99.5. (pcolor= (-0.32) * salinity + 99.5) When salinity is higher than 28%, crystallization starts.

6. The tolerable salinity for brine shrimps ranges from 8% (approximate color of 97) to 20% (approximate color of 93). 100 % of mortality rate is set to shrimps moving to the higher or lower saline area.

7. The brine shrimp population size constantly fluctuates due to salinity diffusion. The curves of shrimp populations have been smoothened to improve visualization.

Reference:

Great Salt Lake Ecosystem Program (. Brine Shrimp. https://wildlife.utah.gov/gsl/brineshrimp/salinity.php

White, J., Null, S. E., & Tarboton, D. G. (2014). More than meets the eye-managing salinity in Great Salt Lake, Utah. Lakeline Magazine, 34(3), 25.

Wurtsbaugh, W., Miller, C., Null, S., Wilcock, P., Hahnenberger, M., & Howe, F. (2016). Impacts of water development on Great Salt Lake and the Wasatch Front. https://qcnr.usu.edu/pdfs/publications/Great%20Salt%20Lake%20Water%20Level_Feb%2024%202016.pdf

HOW TO USE IT

First press on "set up/reset simulation" and then click on "run/pause simulation."
Pull the bars on the sliders to adjust parameters while running the simulation.
Turn on/off the switches to show or hide shrimps.
Observe the system changes over time.

THINGS TO NOTICE

When maximizing the surface inflows from all rivers, the mean salinity drops to around 6%, representing the flooding scenario in the 1980s.
When minimizing the surface inflows from all rivers, the mean salinity increases to about 20.5%. The temperature increase will further boost the mean.

THINGS TO TRY

1. Invite students to consider how to set up simulation to represent droughts, massive precipitation, and opening culverts.

2. Change the inflows from the rivers and north arm and observe how it affects the 1) average salinity and salinity gradient of the lake and 2) population size and distribution of brine shrimps.

3. Change the evaporation rate and observe how it affects the 1) average salinity and salinity gradient of the lake and 2) population size and distribution of brine shrimps.

CREDITS AND REFERENCES

Dr. Lin Xiang creates this module at the University of Kentucky. If you mention this model in a publication, we ask that you include the citations below.

Xiang, L. (2021). Great Salt Lake. Department of STEM Education, University of Kentucky, Lexington, KY.

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