حوضچه های رنگارنگ نمک در خلیج سانفرانسیسکو

Multicolored Salt Ponds at San Francisco Bay

If you ever fly over San Francisco Bay, be sure to peer out of the window to catch a glimpse of one of the world's most incredibly coloured landscapes - the salt evaporation ponds operated by Cargill, Inc.

Salt evaporation ponds are shallow artificial ponds designed to produce salts from sea water or other brines. The seawater or brine is fed into large ponds and water is drawn out through natural evaporation which allows the salt to be subsequently harvested. During the five years it takes for the bay water to mature into salt brine, it is moved from one evaporation pond to another. In the final stages, when the brine is fully saturated, it is pumped to the crystalizer where a bed of salt 5 to 8 inches thick is ready for harvest.

Salt ponds range from blue green to deep magenta – colored naturally by the microorganisms that thrive as salinity levels increase. The color indicates the salinity of the ponds and the type of microorganisms that’s breeding on it. Three microorganisms in particular, Synechococcus, Halobacteria, and Dunaliella, influence the color of salt ponds.

In the low-salinity ponds, both color and microbiology match the blue green waters of San Francisco Bay. As the brines concentrate, several algae, including Dunaliella, impart a green cast to the brines. With increasing salinity, Dunaliella out-competes other microorganisms and hues vary from pale green to bright chartreuse. About midway through the pond system, the increased salinity promotes huge populations of tiny brine shrimp, which clarify the brine and darken it. The saltiest brine, or pickle, appears deep red, because Halobacteria take over and the hypersaline brine triggers a red pigment to form in the Dunaliella’s protoplasm.

The palette of salt pond colors reflects an unusual micro-biota. Yet it is more than just a curiosity of nature. The algae and other microorganisms create the basis for a rich ecosystem, supporting more than a million shorebirds, waterfowl and other wildlife. At the same time, these tiny creatures regulate water quality -- which promotes development of a higher quality salt.