Gulf of California
Also called the Sea of Cortez, this marine body exhibits some of the greatest biodiversity of the seas of the world, manifesting over 5000 species of macroinvertebrates.
With a depth of over 3000 meters in the northern reach, the Gulf of California has an areal extent of about 177,000 square kilometers.
The Gulf of California was formed over nine million years ago via rifting of Baja California from the balance of Mexico through tectonic forces.
The Gulf of California came into existence approximately 16.5 to 9.2 million years before present, when tectonic forces rifted the Baja Peninsula from the North American Plate. At that time the East Pacific Rise propagated up the center of the Sea of Cortez along the seabed; the East Pacific Rise is a mid-ocean ridge, situated along the Pacific Ocean floor, separating the Pacific Plate from the North American Plate, Rivera Plate, Cocos Plate, Nazca Plate and Antarctic Plate. This element of the East Pacific Rise is sometimes called the Gulf of California Rift Zone.
The Gulf could extend as far as Indio, California, save for the expansive Colorado River delta that blocks the Gulf from flooding Mexicali as well as Imperial Valley.
Volcanic activity has been exhibited in the East Pacific Rise. For example, the felsic volcanism coincided with extensional rotation of the rifting, while in the Sierra Santa Ursala area the volcanic activity preceded significant tectonic rotation. The island of Isla Tortuga is a testament of this ongoing volcanism.
The Gulf of California holds 37 islands, with the largest being Isla Ángel de la Guarda and Tiburón. Most of the islands are found on the Baja side of the Gulf. Moreover, many of the islands of the Gulf of california are the product of volcanism arising about ten million years before present. The islands of Islas Marías, Islas San Francisco, and Isla Partida are considered the result of such volcanism. The volcanic islands were formed as a result of an individual structural occurrence. Several islands, including Isla Coronado, are home to active volcanoes.
See main article: Gulf of California large marine ecosystem
The Gulf of California Large Marine Ecosystem is classified as a Class I, highly productive (greater than 300 grams of carbon per square meter per year) ecosystem. Sardines and anchovies are the principal food for large numbers of squid, fish, marine birds, and marine mammals (e.g. California sea lions, Fin whales, dolphins, orcas, and Northern elephant seals). The ecosystem serves as a nursery ground for the Gray whale, and is a breeding areas for a number of marine birds (e.g. Herring's gull, Larus heermanni and Royal tern, Thalasseus maximus) as well as many mammals.
The northern gulf has many endemic species including endemic populations of the Gulf of California harbor porpoise (a critically endangered marine mammal also known as a Vaquita) and the totoaba (a large endangered fish). Since the Colorado River once discharged into the Gulf of California, but now rarely does, many changes are occurring in the estuarine environment at the far northern end. Sedimentation and changes in the environment brought about by the loss of inflow favor the isolation of species and their consequent speciation.
Human activities are altering water quality and ecosystems in the Gulf of California. The decrease of Colorado River freshwater input has changed the ecological conditions of what used to be an estuarine system, important for fish reproduction. It is now an area of elevated salinity. Water pollution problems include agriculture inputs and runoff from intensive cropping pratices needed to feed a rapidly expanding Mexican population. Pesticides and herbicides are used in the agricultural areas of the Mexicali Valley and of the Sonora and Sinaloa states.
The main issue affecting ecosystem health has been the escalation of fishing vessels and fishing gear types, from small pangas, handlines with multiple hooks and spearguns, to gill-nets, trawls and longlines. Species in danger of economic extinction in the Gulf of California include the Cabrilla, Black and White seabass, Gulf grouper, Yellowtail, Dog snapper and Sierra. Sea turtles, Hammerhead sharks, and Giant manta rays have virtually disappeared. The Gulf of California Large Marine Ecosystem has however shown a resilience that is partly due to its coastal watersheds and to the submarine topography that causes the upwelling of nutrients.
Along the eastern coast of the Gulf of California the Sonoran-Sinaloan transition subtropical dry forest ecoregion comprises a distinct zone of dry forest that forms a north-south transition between the Sonoran Desert to the north and the Sinaloan dry forests to the south. The ecoregion shares elements of climate, as well as soil and topographic attributes, with both neighboring ecoregions. Nonetheless, it exhibits a unique mixture of temperate and tropical biota, many species of which find either their northern or southern limits here. Characteristically tropical species include the magnificent black-throated magpie jay (Calocitta colliei). On the other hand, many more typically northern desert species are also found here, including jumping cholla cactus (Opuntia fulgida) and barrel cactus (Ferocactus wisliznei). The southeastern boundary of these dry forests marks the end of the Nearctic and the beginning of the Neotropical biogeographic realm.
Along the southwestern edge of the Gulf of California lies the Sierra de la Laguna dry forests ecoregion was earlier an isolated island and correspondingly contains a high number of endemic species. After joining the balance of the Baja Peninsula, this ecoregion gave way to speciation, resulting in high species diversity. This subtropical dry forest is threatened by cattle grazing and by hunting of high level predators. Designated as a Protected Natural Area, this unique and significant ecoregion is threatened by ongoing habitat fragmentation.
- S.Alvarez-Borrego. 1983. Gulf of California. In: Ecosystems of the World: Estuaries and Enclosed Seas. B.H. Ketchum, ed. Elsevier Scientific Publishing House. ISBN: 0444419217.
- Richard C. Brusca. 1973. A Handbook to the Common Intertidal Invertebrates of the Gulf of California. Tucson, Arizona: University of Arizona Press. ISBN 0816503567.
- G.Ceballos and A. García. 1995. Conserving neotropical diversity: The role of dry forests in Western Mexico. Conservation Biology 9:6 (1349-1356).
- Hugo Delgado-Granados. 2000. Cenozoic tectonics and volcanism of Mexico (Google eBook) Geological Society of America. 275 pages