The Gulf of California: Biodiversity and Conservation

2. Physical, Chemical, and Biological Oceanography of the Gulf of California

CHAPTER 2 Physical, Chemical, and Biological Oceanography of the Gulf of California saúl alvarez-borrego Summary The Gulf of California is a dynamic marginal sea of the eastern Paci fic Ocean. Today, nutrient input to the Gulf from rivers is very small and has only local coastal effects. The Gulf has three main natural fertilization mechanisms: wind-induced upwelling, tidal mixing, and water exchange between the Gulf and the Pacific Ocean. These natural fertilization mechanisms have made the Gulf of California more resistant to anthropogenic effects (e.g., those due to construction of dams) than are other ecosystems, such as the eastern Mediterranean Sea. Upwelling occurs off the mainland (eastern Gulf ) coast with northwesterly winds (“winter” conditions from December through May) and off the Baja California peninsula coast with southeasterly winds (“summer” conditions from July through October), with June and November as transition periods. Upwelling off the mainland coast is strong and has a marked effect on phytoplankton communities (chlorophyll concentration values can exceed 10 mg m−3 ). When combined with eddy circulation, upwelling increases the phytoplankton biomass across the Gulf. However, because of strong stratification during summer upwelling off the Baja coast, the southeasterly winds have a weak effect on phytoplankton biomass, causing an increase in chlorophyll a only to values around 0.5 mg m−3 in spite of wind magnitudes similar to those of winter. Tides in the Gulf of California are produced by co-oscillation with the adjacent Pacific. In the Gulf, the tidal wave is progressive: the time of high or low water is progressively later traveling northward in the Gulf. The time difference between the entrance and the vicinity of the Colorado River is approximately 5.5 hr for high water and 6 hr for low water. The result is that low water at one end of the Gulf occurs at about the same time as high Physical, Chemical, and Biological Oceanography / 25 water at the other end. The northern Gulf exhibits spectacular tidal phenomena , with a range of >7 m during spring tides in the uppermost Gulf and >4 m in the Midriff Islands region. Tidal mixing in the latter region produces a vigorous stirring of the water column down to >500 m depth, with the net effect of carrying colder, nutrient-rich water to the surface and creating an ecological situation similar to constant upwelling. This also has the effect of making the areas around the islands of the Gulf a source of CO2 to the atmosphere. Satellite ocean color data show that most of the Gulf has low photosynthetic pigment concentrations during the summer, but they remain high in the waters around the Midriff Islands because of tidal mixing. The lowest offshore surface temperatures and the highest surface nutrient and total dissolved inorganic carbon concentrations in the entire Gulf are persistently found in the Midriff Islands region. As a result, primary productivity is high year-round, and this area supports large numbers of sea birds and marine mammals. South of the Ángel de la Guarda and Tiburón Islands, the Gulf has basically the same thermohaline structure as that of the Eastern Tropical Pacific, with modifications at the surface due to excess evaporation. At intermediate depths, the concentration of oxygen in some places is undetectable by the Winkler method. This oxygen minimum plays an important role in the ecology and geology of the Gulf, as it influences the distribution of pelagic and benthic organisms and sedimentation patterns. Laminated diatomaceous sediments are formed where the basin slopes intersect the oxygen minimum in the water column. These laminated sediments are paleo-ecological records of seasonal, interannual, and decadal changes of hydrographic conditions. Satellite-derived data revealed a dramatic reduction in chlorophyll at the entrance to the Gulf during El Niño 1982–1983, with values down to ∼20 percent of those for non–El Niño years, but there was a relatively small impact in the Midriff Islands and northern regions. Introduction The first physical oceanographic data for the Gulf were collected in 1859–1861, when John Xantus measured tides at Cabo San Lucas for the U.S. Coastal Survey. The first physical oceanographic work off the west 26 / saúl alvarez-borrego coast of mainland Mexico was during the C. & I.S.S. Hassler cruise with Alexander Agassiz on board. The Hassler expedition worked at Cabo Corrientes in 1872 (Schwartzlose and Alvarez-Borrego 2002). The first...