Many forms of life are found in San Francisco Bay Estuary, and they are all part of the interconnected chain of "who eats whom" that ecologists call the food web. As in terrestrial ecosystems, green plants form the base of the web by using the energy of sunlight to create simple organic compounds that form the plant's tissues (a process called photosynthesis). Because these plants form organic compounds from inorganic compounds, they are called primary producers. A plant may then become food for a small animal, which in turn may be eaten by a larger one. Because they eat food that has already been converted to an organic form, these animals are called secondary producers.
If we want to know why certain fisheries are declining, we must first look at the health of the food groups that are eaten by fish, starting with the primary producers. To learn more about primary producers in San Francisco, click here to go to a web site produced by scientists at the U.S. Geological Survey. Answer the questions below
1. What is primary productivity?
2. Who does this type of productivity? Provide names and descriptions of primary producers (note that the photographs here are magnified many times; most of these organisms are barely as big as a pencil dot).
3. Why is primary productivity important? Look at the chart and estimate how many parts of phytoplankton it takes to produce 1 part of fish. (the units on this diagram are grams carbon per square meter per year, but they could be pounds or other units). What does this chart imply about the efficiency of eating at lower or higher parts of the food web?
4. During which times of year and in which parts of the Bay are the rates of primary production highest?
5. How does the rate of primary productivity in San Francisco Bay compare to other estuaries and to the ocean?
6. How much primary productivity occurs in San Francisco Bay (in humpback whale units)?
The animals in San Francisco Bay feed on the primary producers and on each other; they are the secondary producers. Life in the Bay, and in all other marine environments, can be divided into three major types:
Click here to go to a web site on San Francisco Bay Ecology created by the Marine Science Institute, located in Redwood City along the shores of the South Bay.
7. What are some examples of invertebrates, fish, and mammals that live in San Francisco Bay?
The ecosystem in San Francisco Bay no longer resembles the one that existed 150 years ago, before large numbers of people began to settle the region. Many species are now extinct and many more have been introduced from other locations.
Figure 1. Continuous long-term investigation of San Francisco Bay has given the USGS a rare opportunity to measure ecosystem changes following invasion by a highly successful exotic species. The Asiatic clam, Potamocorbula, was introduced to the Bay in 1986, probably by a cargo ship. A cascading sequence of ecological changes followed, similar to changes in the Great Lakes after their invasion by the zebra mussel. These fundamental ecological changes have likely contributed to the population declines of almost every fish species living in the North Bay. Data are from a USGS report on Managing Coastal Resources of the U.S.
8. In Figure 1, what impact has the Asian clam had on phytoplankton populations in the Bay? What would be the expected subsequent impact on other organisms in the Bay?
Click here to read an article from the San Francisco Chronicle about foreign species that now threaten the Bay's health.
9 From the Chronicle article, summarize the situation that now exists in San Francisco Bay: what species are invading and why they are a problem?