Millions of years ago, dinosaurs walked the Earth and the Salton Trough was at the bottom of the ocean. The Salton Trough was part of the Gulf of California until four million years ago when sediment from the Colorado River built up and closed the gap.1 Today, there are 150 miles between the Salton Sea and the Gulf of California, but evidence of their former connection can be found in the soil.
The land around the Salton Sea is composed of minerals of marine origin; many of these minerals are salts. A salt is a compound made up of positively (cation) and negatively (anion) charged ions. Sodium chloride, known as table salt, is the primary salt in the ocean and it dissolves readily in water to form a sodium cation and a chlorine anion. Other chemical components of seawater include magnesium, calcium, and potassium cations and sulfate anions. The Salton Sea has many of the ions of seawater in addition to phosphate and nitrate nutrients from fertilizers that are flushed into the Sea.2 Together, the total dissolved salt content of a body of water is called salinity.
Salinity is the mass of salt per unit volume; it is difficult to measure the mass of dissolved salt in saltwater directly so other properties of water, such as conductivity, refractivity, and microwave emissions, are measured. Conductivity, the ability of a solution to carry an electrical current, is the most common way to measure salinity. In conductivity measurements, a voltage is applied to an electrode pair submerged in a water sample. The drop in voltage between the electrodes is used to measure the resistance of water; which is converted to conductivity. The ability to conduct electricity is directly related to the concentration of ions in water; more ions (high salinity) cause higher conductivity. Refraction is the physical phenomenon that explains the change of velocity of light as it travels from one medium to another. Light travels faster through air than through water. For example, a diving ring at the bottom of the swimming pool looks further away when viewed from above the water than it does when the viewer is submerged in the water. Salt water has a higher refractive index than freshwater, which means light travels more slowly: the higher the refractive index, the higher the salinity. A recent advancement by NASA utilizes microwave emissions from the surface of the ocean to determine salinity.3 Their recent Aquarius mission measured the ocean surface salinity from space for the entire globe (Fig.1).
Just as too much or too little salt in our diet affects our health, too high or too low ocean salinity affects the Earth’s climate. Consistent salinity levels are important for ocean circulation, which buffers the Earth’s temperature from harmful fluctuations.4 In a local ecosystem, salinity levels impact biodiversity. The average salinity of the Salton Sea is 55 g L-1 and this level is continually increasing. The organisms that live in the Sea have different tolerances for salinity, and as the salinity approaches the limit for each species, the food chain shifts. This shift in biodiversity has important implications for the future ecological profile of the Salton Sea.5 The current ecosystem that centers on a detrital food chain (detritus > invertebrates > fish > birds) has gradually been shifting to a system, much like the Great Salt Lake, that has brine shrimp and brine flies at the center. As the salinity increases, many species will die-off or go elsewhere (such as the Colorado Delta), but many saline-tolerant invertebrates and invertebrate-eating birds will thrive at the Salton Sea. The Salton Sea is unlikely to return to its former ecological state or be preserved in its current one, and although the inhabitants may change, as long as agricultural run-off drains into the Salton Sea it will remain an important aquatic habitat.
Written by Melissa Morgan
(1) Alles, D. Geology of the Salton Sea; Western Washington University, 2011.
(2) Holdren, G. C. Chemical and Physical Characteristics of the Salton Sea, California. Hydrobiologia 2002, 473 (1-3), 1–21.
(3) Perez, M. “Aquarius”. <www.nasa.gov>. (accessed Jul 6, 2015).
(4) “Salinity”. NASA Science; Earth. <science1.nasa.gov>. (accessed Jul 6, 2015).
(5) Cohen et al. Haven or Hazard: The Ecology and Future of the Salton Sea; Pacific Institute, 1999.