The life cycle of the brine shrimp is depicted in the drawing. The shrimp hatch from overwintering cysts in January through May and the first adults appear in the lake around the middle of May. These early adults may bare live young which will mature rapidly in the warm layer of the lake. This life cycle is depicted by the inner loop in the drawing.
For this exercise, we will concentrate on the larger shrimp population which is born from the cysts. (Their life cycle is depicted by the outer loop). The females switch from live reproduction to oviparous reproduction. by June of each year. The diapause eggs lie dormant on the bottom of the lake until the following winter. The brine shrimp may be found each summer in the oxygenated upper waters, but they begin to decline in number by September and are almost absent from the plankton by December. The shrimp feed on (filter) planktonic algae, and the algae concentration varies dramatically from month to month within the year. It also varies from year to year depending on the salinity of the lake.
The Birds
The brine shrimp provide a main source of food for the Eared Grebe, the Wilson's Phalarope and the California Gull. (Click here to see the foodweb drawing.) The Eared Grebes use Mono lake primarily as a stopover site during fall migrations, and their departure each year is precipitated by the seasonal collapse of the lake's population of adult brine shrimp. The Wilson's Phalarope is also a migratory bird; it relies on Mono Lake as a stopover and staging site before commencing what may be a nonstop migration to South America (NRC 1987, p. 101). The California Gulls nest on several of the islands and islets in Mono Lake They arrive in the spring and lay their eggs during a period when the brine shrimp are relatively scarce. Chick hatching occurs mainly in June, and chick growth usually coincides with the period of peak abundance of the brine shrimp.
Life Cycle of the Mono Lake Brine Shrimp
(drawing from Hart (1996), courtesy of Joyce Jonte)
