Mono Lake
Mono Lake is a saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in an endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake which make its water alkaline.
The desert lake has an unusually productive ecosystem based on brine shrimp, which thrive in its waters, and provides critical habitat for two million annual migratory birds that feed on the shrimp and alkali flies. Historically, the native Kutzadika'a people ate the alkali flies' pupae, which live in the shallow waters around the edge of the lake.
When the city of Los Angeles diverted water from the freshwater streams flowing into the lake, the lake level dropped, imperiling the migratory birds. The Mono Lake Committee formed in response and won a legal battle that forced Los Angeles to partially replenish the lake level.
Geology
Mono Lake lies within the Mono Basin, an endorheic basin with no outlet to the ocean. Dissolved salts in the runoff thus remain in the lake, raising the water's pH and salt concentration. The tributaries of Mono Lake include Lee Vining Creek, Rush Creek and Mill Creek which flows through Lundy Canyon.Geological forces formed the basin over the last five million years: basin and range crustal stretching and associated volcanism and faulting at the base of the Sierra Nevada.
From 4.5 to 2.6 million years ago, large volumes of basalt were extruded around what is now Cowtrack Mountain ; eventually covering and reaching a maximum thickness of. Later volcanism in the area occurred 3.8 million to 250,000 years ago. This activity was northwest of Mono Basin and included the formation of Aurora Crater, Beauty Peak, Cedar Hill, and Mount Hicks.
Lake Russell was the prehistoric predecessor to Mono Lake, during the Pleistocene. Its shoreline reached the modern-day elevation of, about higher than the present-day lake. As of 1.6 million years ago, Lake Russell discharged to the northeast, into the Walker River drainage. After the Long Valley Caldera eruption 760,000 years ago, Lake Russell discharged into Adobe Lake to the southeast, then into the Owens River, and eventually into Lake Manly in Death Valley. Prominent shore lines of Lake Russell, called strandlines by geologists, can be seen west of Mono Lake.
The area around Mono Lake is currently geologically active. Volcanic activity is related to the Mono–Inyo Craters: the most recent eruption occurred 350 years ago, resulting in the formation of Paoha Island. Panum Crater is an example of a combined rhyolite dome and cinder cone.
Tufa towers
Many columns of limestone rise above the surface of Mono Lake. These limestone towers consist primarily of calcium carbonate minerals such as calcite. This type of limestone rock is called tufa, a term for limestone that forms at low to moderate temperatures.Tufa tower formation
Mono Lake is a highly alkaline lake, or soda lake. Alkalinity is a measure of how many bases are in a solution, and how well the solution can neutralize acids. Carbonate and bicarbonate are both bases. Hence, Mono Lake has a very high content of dissolved inorganic carbon. Through supply of calcium ions, the water will precipitate carbonate-minerals such as calcite. Subsurface waters enter the bottom of Mono Lake through small springs. High concentrations of dissolved calcium ions in these subsurface waters cause huge amounts of calcite to precipitate around the spring orifices.The tufa originally formed at the bottom of the lake. It took many decades, or even centuries, to form the well-known tufa towers. When lake levels fell, the tufa towers rose above the water surface and stand as the pillars seen today.
Tufa morphology
Description of the Mono Lake tufa dates back to the 1880s, when Edward S. Dana and Israel C. Russell made the first systematic descriptions of the Mono Lake tufa. The tufa occurs as "modern" tufa towers. There are tufa sections from old shorelines, when the lake levels were higher. These pioneering works on tufa morphology are cited by researchers and were confirmed by James R. Dunn in 1953. The tufa types can roughly be divided into three main categories based on morphology:- Lithoid tufa - massive and porous with a rock-like appearance
- Dendritic tufa - branching structures that look similar to small shrubs
- Thinolitic tufa - large, well-formed crystals of several centimeters
Tufa chemistry
Russell studied the chemical composition of the different tufa types in Lake Lahontan, a large Pleistocene system of multiple lakes in California, Nevada, and Oregon. Not surprisingly, it was found that the tufas consisted primarily of CaO and CO2. However, they also contain minor constituents of MgO, Fe/Al-oxides, and PO5.Climate
Limnology
The limnology of the lake shows that it contains approximately 280 million tons of dissolved salts, with salinity varying with the amount of water in the lake at any given time. Before 1941, average salinity was approximately 50 grams per liter . In January 1982, when the lake reached its lowest level of, the salinity had nearly doubled to 99 g/L. In 2002, it was measured at 78 g/L and is expected to stabilize at an average of 69 g/L as the lake replenishes over the next 20 years.An unintended consequence of ending the water diversions was the onset of a period of "meromixis" in Mono Lake. In the time before this, Mono Lake was typically "monomictic"; which means that at least once each year the deeper waters and the shallower waters of the lake mixed thoroughly, thus bringing oxygen and other nutrients to the deep waters. In meromictic lakes, the deeper waters do not undergo this mixing; the deeper layers are more saline than the water near the surface, and are typically nearly devoid of oxygen. As a result, becoming meromictic greatly changes a lake's ecology.
Mono Lake has experienced meromictic periods in the past; the most recent episode of meromixis, brought on by the end of water diversions, commenced in 1994 and ended by 2004.
Lake-level history
An essential characteristic of Mono Lake is that it is a closed lake, meaning it has no outflow. Water can only escape the lake if it evaporates or is lost to groundwater. This may cause closed lakes to become very saline. The reconstruction of historical Mono Lake levels using carbon and oxygen isotopes has also revealed a correlation with well-documented changes in climate.In the recent past, Earth experienced periods of increased glaciation known as ice ages. This geological period of ice ages is known as the Pleistocene, which lasted until ~11 ka. Lake levels in Mono Lake can reveal how the climate fluctuated. For example, during the Pleistocene, when the climate was colder, the lake level was higher because there was less evaporation and more precipitation. Following the Pleistocene, the lake level was generally lower due to increased evaporation and decreased precipitation associated with a warmer climate.
The lake level has fluctuated during the Holocene, since the end of the ice ages. The Holocene high point is at elevation, reached in approximately 1820 BCE. The low point before modern diversions is at elevation, reached in 143 CE. The lowest modern level due to diversions is at, reached in 1980.
Ecology
Aquatic life
The hypersalinity and high alkalinity of the lake mean that no fish are native to the lake. An attempt by the California Department of Fish and Game to stock the lake failed.The whole food chain of the lake is based on the high population of single-celled planktonic algae present in the photic zone of the lake. These algae reproduce rapidly during winter and early spring after winter runoff brings nutrients to the surface layer of water. By March, the lake is "as green as pea soup" with photosynthesizing algae.
The lake is famous for the Mono Lake brine shrimp, Artemia monica, a tiny species of brine shrimp, no bigger than a thumbnail, that is endemic to the lake. During the warmer summer months, an estimated 4–6 trillion brine shrimp inhabit the lake. Brine shrimp have no nutritional value for humans but are a staple for birds in the region. The brine shrimp feed on microscopic algae.
Alkali flies, Ephydra hians, live along the lake's shores and swim underwater, encased in small air bubbles, to graze and lay eggs. These flies are an important food source for migratory and nesting birds.
Eight nematode species were found living in the littoral sediment:
- Auanema spec., which is outstanding for its extreme arsenic resistance, having three sexes, and being viviparous.
- Pellioditis spec.
- Mononchoides americanus
- Diplogaster rivalis
- species of the family Mermithidae
- Prismatolaimus dolichurus
- 2 species of the order Monhysterida
Birds
Nearly 2,000,000 waterbirds, including 35 species of shorebirds, use Mono Lake to rest and eat for at least part of the year. Some shorebirds that depend on the resources of Mono Lake include American avocets, killdeer, and sandpipers. One to two million eared grebes and phalaropes use Mono Lake during their long migrations.
Late every summer, tens of thousands of Wilson's phalaropes and red-necked phalaropes arrive from their nesting grounds, and feed until they continue their migration to South America or the tropical oceans, respectively.
In addition to migratory birds, a few species spend several months nesting at Mono Lake. Mono Lake has the second largest nesting population of California gulls, Larus californicus, second only to the Great Salt Lake in Utah. Since abandoning the landbridged Negit Island in the late 1970s, California gulls have moved to some nearby islets and have established new, if less protected, nesting sites. Cornell University and Point Blue Conservation Science have continued the study of nesting populations in Mono Lake that began 35 years ago. Snowy plovers also arrive at Mono Lake each spring to nest along the northern and eastern shores.