A Lake Divided by Rail
The Great Salt Lake is bisected by a 20-mile rock causeway built for the Lucin Cutoff railway line. Construction of this solid embankment, which replaced an earlier wooden trestle, finished in 1959. It effectively partitioned the lake into a northern arm (Gunnison Bay) and a southern arm (Gilbert Bay). This division created two distinct aquatic environments. The southern arm receives nearly all of the freshwater inflow from the Bear, Weber, and Jordan rivers, which keeps its salinity relatively low. The causeway severely restricts this freshwater from reaching the northern arm. Intense evaporation in the isolated north arm concentrates its salts, making it significantly more saline than the south.
This sharp salinity gradient dictates which microorganisms can survive in each section, directly influencing the water's color. The difference is so pronounced that it is visible to astronauts in orbit. Salinity in the southern arm typically fluctuates between 8% and 17%. The northern arm, by contrast, is often near saturation at approximately 27% salinity.
The Biology of Color
The southern arm's blue-green color comes from cyanobacteria and various species of green algae that tolerate moderate salinity. This ecosystem also supports populations of brine shrimp (Artemia franciscana) and brine flies, which feed on the algae.
In the hypersaline north arm, the conditions are too extreme for most of this life. The water's characteristic pink-to-red hue is produced by two main types of salt-loving microbes. One is the alga Dunaliella salina, which, under the stress of high salinity and intense sunlight, produces large amounts of a protective red-orange pigment called beta-carotene. The other important organisms are extreme halophilic archaea, such as those from the genus Halobacterium. These microbes contain a purple-pigmented protein in their cell membranes called bacteriorhodopsin, which captures light energy. The combined effect of these pigments gives the north arm its reddish color.