The transcontinental railroad was completed on May 10, 1869 at the Golden Spike National Historic Site (Promontory Summit), successfully linking the east coast of America to the gold of the west. Due to the difficulty of undertaking such a venture with the limited resources, technology and money at the time, building through the lake was only considered as a joke. During World War II, the railroad was stripped of iron for seaports in California, and a more direct line that saved 35 miles was set up across the lake. This railroad was initially a wood trestle and had little resistance to the lake's temperament. In 1959, a 30-foot stone railroad causeway replaced the battered wood trestle. As a result, lake circulation reduced drastically. Great Salt Lake now has two distinct sections, the north arm and the south arm.
Sixty-six percent of the fresh water entering the lake is from the Bear, Weber and Jordan Rivers, all of which flow into the south arm. Additional water comes from rain, springs and groundwater. The water in the north arm stays at a relatively constant high salt (or salinity) concentration of approximately 26–28% due to little freshwater input. Thus, the north arm is so saturated with salt that salt literally precipitates out of the water! In contrast, the south arm fluctuates drastically depending on flood or drought conditions. Average salinity in the South Arm is around 13%. Only two types of known bacteria can currently live in the saturated North Arm waters: the halophiles Halobacterium and Halococcus. It is estimated that there are around 1,000,000 to 100,000,000 bacteria per milliliter! A purplish pigment in these halophiles is responsible for the unique reddish color of GSL's northern section.
Saline levels are crucial in determining the conditions of the south arm from year to year. Higher salinities (less fresh water input and higher evaporation) result in lower algal species diversity in the lake. The number of species are known to fluctuate from as few as six to as many as 144, just from saline levels! Brine shrimp and brine fly larvae are far and away the largest animals in the lake no matter what. However, the variety and quantity of algae available can affect the success of shrimp and flies. There exists a delicate balance between salinity, algal species and brine shrimp success. A big snow year can lead to a larger, fresher lake and more algal diversity. However, brine shrimp are not able to digest all the types of algae (like diatoms with their hard silica shells). Brine shrimp prefer Dunelliela viridis, a unicellular algae, which is able to survive higher concentrations of salt than most other algae. In drought years, D. viridis essentially has a monopoly on the lake, and the brine shrimp flourish. Before we get too excited about drought years, the north arm has taught us a very valuable lesson. When the water reaches a certain high salinity point, not even D. viridis can survive, and the brine shrimp populations are very low.
When the conditions are favorable for Dunelliela viridis and hence, for brine shrimp, the birds that use GSL as a stopover are able to migrate south for the winter with fat content bellies. Whether the year is wet or dry, shrimp, larvae, algae, bacteria and birds all recycle available nutrients back into the ecosystem when they excrete waste and when they perish, completing this salty ecosystem cycle.