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xv Wetlands occur where land meets water. Wetlands , therefore, are not evenly spread over the surface of the Earth. Some areas, such as the Amazon and Siberia, have enormous wetlands, each of these being more than one million square kilometers. Most wetlands along the ocean coasts are much smaller, being limited on one side by uplands and on the other by ocean water that is too deep for rooted plants. The water levels in wetlands along coasts are influenced by tides, hence the general name tidal marsh. Tidal marshes can be further divided into two types, the narrow fringing marshes along steeper shorelines , and the somewhat larger areas of wetlands associated with estuaries and deltas. This book is about the latter type, wetlands in a large estuary, San Francisco Bay. The steep and rocky shorelines of California offer few estuaries and deltas. San Francisco Bay is therefore an ecologically important exception . Indeed, it is the biggest estuary on the west coast of the New World (Nawi and Brandt 2008). Here, two rivers that collectively drain nearly 40% of California meet and empty into the Pacific Ocean. These rivers deposit silt, and they dilute ocean salinity. Depending upon how you do the calculations, these rivers have created from one to four thousand square kilometers of estuary and tidal marsh. The rarity of such large wetlands on the West Coast magnifies their importance to wildlife. Millions of waterfowl using the Pacific flyway visit the wetlands and mudflats. The Dungeness crab, California halibut, and Pacific salmon all rely on the Bay as a nursery. Some 750 wildlife species, along with 120 species of fish, use the estuary (Nawi and Brandt 2008). San Francisco Bay may be exceptional on the West Coast, but it should not be treated as an exception. This bay exemplifies all the general principles that drive the formation of estuaries around the world. In all such tidal wetlands, a physical template controls the kinds of wetlands that arise and the particular plant and animal species that live in the wetlands. In general, the physical template is created by the flood regime, the nutrients, and natural agents of disturbance such as fire and grazing (Keddy 2010). In estuaries , salinity is also an important factor. In San Francisco Bay, this template is created and controlled by tides near the coast but also by inputs of freshwater and sediment from the rivers, the Sacramento and San Joaquin. Together, such rivers carry freshwater into the Bay at an average rate of about 24 million acre-feet per year (900 m3 /s). The key ecological factors in the Bay are therefore (1) the area of silt and shoreline upon which wetland plants can potentially grow and (2) the salinity gradient, which ranges from fresh to saline. If you know how much of the Bay area is wet and how much wetland arises under each salinity regime, you can predict, rather well, the kinds of plants and animals that will occupy the Bay. The salinity gradient, of course, varies. Pulses of freshwater from the rivers will reduce salinforeworD : some thoughts on san franCisCo Bay anD its wetlanDs xvi ForEword ity, while periods of drought will lead to higher salinity. The range of variation and the extremes may be more important than the mean conditions . The idea that extreme conditions rather than mean conditions drive ecosystems has been around for a long time, but because extremes are often more difficult to observe and measure, we tend to overlook them. Tidal wetlands are a class of ecosystems where extremes must be considered. San Francisco Bay was formed over a long series of geological events, the most recent being the general rise in sea levels at the end of the ice age some twenty thousand years ago. During each ice age, sea levels fall as water is stored in vast continental glaciers. At the end of each ice age, the water from melting glaciers steadily raises sea levels. During the maximum of the last ice age, sea levels were about 100 m lower, and one large river drained through a canyon that is now called Golden Gate, into the Pacific Ocean. That would have been a sight to see. The current bay, then, the one with which we are familiar, is a flooded river valley. Prior ice ages led to similar alterations in ecological conditions. This book sensibly includes a chapter on these events. Here is another general principle that the Bay illustrates: global temperature and sea level are...

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