The salinity of Bay water measures the relative proportion of fresh water and seawater, which changes drastically both spatially (at different places in the Bay) and temporally (in different seasons and different years). Because saltier water is heavier, it sinks beneath fresh water. The entry of fresh water from the Delta, which floats on the Bay's surface, and salt water from the ocean, which sinks to the bottom, works together with tidal currents to drive the water's circulation (see Part I). Understanding salinity variations therefore provides information about how sediments, toxins, and organisms are transported by water circulating in the Bay. Peak spring Delta flows and their fluctuations have a large influence on maintaining healthy habitat for fish species. Because the Bay's fisheries resources have declined sharply during the last two decades, resource managers are interested in how salinity varies in spring, especially in the area near the Delta that is a nursery for important species. Managers need to know how natural and human-induced changes affect water availability and fisheries especially during times of low-water inflow. Figure 1. This top diagram shows the variation in fresh water inflow through the Delta throughout the year for the period of 1932-1981. The bottom diagram shows the variation in salinity at Fort Point (just inside the Golden Gate near the mouth of the Bay) during the same time period Diagram from Peterson and others, Variations in Spring Delta Discharge to San Francisco Bay, USGS web site. 1. Summarize the relationship between Delta inflow and the salinity of Bay water.
The salinity of Bay water measures the relative proportion of fresh water and seawater, which changes drastically both spatially (at different places in the Bay) and temporally (in different seasons and different years). Because saltier water is heavier, it sinks beneath fresh water. The entry of fresh water from the Delta, which floats on the Bay's surface, and salt water from the ocean, which sinks to the bottom, works together with tidal currents to drive the water's circulation (see Part I). Understanding salinity variations therefore provides information about how sediments, toxins, and organisms are transported by water circulating in the Bay.
Peak spring Delta flows and their fluctuations have a large influence on maintaining healthy habitat for fish species. Because the Bay's fisheries resources have declined sharply during the last two decades, resource managers are interested in how salinity varies in spring, especially in the area near the Delta that is a nursery for important species. Managers need to know how natural and human-induced changes affect water availability and fisheries especially during times of low-water inflow.
Figure 1. This top diagram shows the variation in fresh water inflow through the Delta throughout the year for the period of 1932-1981. The bottom diagram shows the variation in salinity at Fort Point (just inside the Golden Gate near the mouth of the Bay) during the same time period Diagram from Peterson and others, Variations in Spring Delta Discharge to San Francisco Bay, USGS web site.
1. Summarize the relationship between Delta inflow and the salinity of Bay water.
Salinity in Bay and Delta waters ranges from 0 psu (fresh river water) to 34 psu (salty ocean water). Salinity is measured in practical salinity units, psu, which are about equal to parts per thousand, ppt. Thus 25 psu is about 25 ppt, or 2.5 percent (parts per hundred). The USGS submersible instrument package contains a device that measures the amount of conductivity in the water. Conductivity can be converted to a salinity measurement because salts in water are electrically charged ions of such elements as sodium and chloride. The more charged ions there are in the water, the more conductive it will be. In the next sections, we will examine salinity data obtained from the USGS Water Quality of San Francisco Bay web site, to analyze variations in time and space throughout the Bay.
The diagram below shows how salinity varies spatially, along the length of the Bay, and temporally, from season to season and year to year. These data are from the USGS Water Quality program and combine measurements from three years that had high natural variability. For example, 1994 was a dry year, while 1995 was a wet year.
The upper panel displays the Delta Outflow Index for 1993-1995. This is the amount of fresh water that enters into the Bay through the Delta. The lower panel demonstrates the changing distribution of salinity at the Bay's surface along the USGS sampling transect. Color is proportional to salinity, with dark (blue) shadings indicating high salinity and light (yellow) shadings indicating low salinity. The vertical axis represents variability in space from the lower Sacramento River (top of image), to the Central Bay, and then to the lower South Bay (bottom of image). The horizontal axis represents change over time from 1993 through 1995.
Description of circled numbers:
1. The thick black line shows the changing position in the Bay where surface salinity was 2 psu (an indicator of the fresh water-seawater boundary). This location was in Carquinez Strait (between Suisun and San Pablo Bays) during the period of high Delta outflow in 1993.
2. As Delta outflow recedes during the dry season (summer to fall), the fresh water-seawater boundary moves landward. In fall 1993, surface salinity of 2 psu was located in the eastern part of Suisun Bay.
3. During dry years, such as 1994, the fresh water-seawater boundary moves further landward. Surface salinity of 2 psu was located in the lower Sacramento River during fall 1994.
4. During years of exceptionally high river low, such as 1995, the fresh water-seawater boundary moves far seaward. After the large Delta outflows of 1995, surface salinity of 2 psu moved all the way into eastern San Pablo Bay.
5. The color transition here, from yellow to blue, represents the seasonal change of salinity from low values during the wet winter-spring period to progressively higher values during the dry summer-autumn period. When river flows are small, evaporation and mixing with the coastal ocean cause the Bay's salinity to increase.
6. Salinity in the South Bay also changes in response to inputs of fresh water, both from local streams and from Delta outflows. Notice how quickly the salinity dropped in early 1995, soon after a series of storms brought heavy flows into the Bay.
2. How does salinity in the Bay vary spatially; that is, which places along its length tend to be more salty and which tend to be less salty? What factors explain this spatial variability?
3. How does salinity in the Bay vary temporally during the year? During which seasons do Bay waters tend to be saltiest and why? During which seasons do Bay waters tend to be least salty and why?
4. How does salinity in the Bay vary temporally, from year to year? What factor is most important in explaining this temporal variation?
The diagram above illustrates the variation in salinity at the Bay's surface. Bay salinity also varies from the surface to the bottom. When we look at vertical profiles of Bay salinity, we once again see both temporal and spatial variations. Remember from Part 1 that saltier water is more dense (heavier) and that it sinks below fresh water. When the salinity variation between bottom and surface water is large, the Bay is said to be stratified; stratified estuaries may also be called salt wedge or partially-mixed types. When salinity variation is small, the Bay is unstratified and may be called a well-mixed type estuary. Inputs of large amounts of fresh water and weak tidal currents tend to create stratified water, while small amounts of fresh water and stirring by strong tidal currents tend to create well-mixed water.
This diagram shows how the vertical distribution of salinity varies temporally, during different times of the year at the same location (near San Mateo Bridge in the South Bay). This is one of the sites along the USGS sampling transect. It illustrates the data that result from lowering the submersible instrument package from the Bay surface to the bottom at that site.
1. This profile was measured near the San Mateo Bridge on April 11, 1995. Measurements were made after a period of high river flows that diluted the salt content of Bay waters and established a strong vertical gradient (salinity stratification) between low-salinity surface and higher-salinity bottom waters. Salinity gradients act to slow vertical mixing, and they create distinct layers that have their own water quality characteristics. Salinity gradients are broken down by tidal stirring, so the shape of the salinity profiles changes with the tides.
2. This profile was measured near the San Mateo Bridge on September 21, 1995. This profile was measured after months of low river flow when the salt content of Bay waters progressively increased toward ocean salinity. Notice that the salinity was uniform from surface to bottom depths (the salinity gradient disappeared). This condition implies thorough vertical mixing of the water.
5. What are the primary factors that produced stratified conditions in April and well mixed conditions in September of the same year?
6. Imagine that you are a phytoplankton (single-celled algae) that requires sunlight to do photosynthesis and produce food? Would stratified conditions or well-mixed conditions be better for keeping you in sunlit waters and able to produce the organic compounds that support your life and other life in the Bay?
7. How would you expect the vertical distribution of salinity to vary spatially in the Bay and why? That is, which locations would you expect to have a strong salinity gradient between surface and bottom waters and which locations would you expect to have well mixed conditions? (Hint: think about the source of fresh water.)
A particularly good way to view spatial variability is the following plots, which show salinity measurements along the length of the Bay AND from the surface to the bottom. Remember that the Central Bay (near the Bay's connection to the ocean) is near the center of the diagrams. The South Bay is to the left and the North Bay and Delta are to the right. Click here to see a map of the USGS sampling transect. Keep in mind that the vertical scale is highly exaggerated relative to the horizontal scale, which make the Bay's bottom (black) look MUCH more rugged than it really is. In reality, the Bay's bottom is a smooth surface with gradually changing depths. The yellow line is the location of the 2 psu salinity measurement (a measure of the fresh water - saltwater interface).
The temporal variability can be analyzed by comparing diagrams from different times of year and different years. Carefully examine and compare the data from the same month (April) in different years and from the same year (1994 and 1995) in different months.
Salinity Scale in psu (practical salinity units, about parts per thousand)
April 19, 1994
April 4, 1995
September 27, 1994
September 21, 1995
8. Describe the salinity distribution in the Bay at different months of the same year (1994 and 1995). Note differences in salinity and in the amount of stratification or mixing. Which year shows the most change between April and September?
9. Describe the salinity distribution in the Bay during the same months (April and September) of different years. What can you say about the climatic differences between the two years illustrated here?
10. One problem for fisheries in low-flow years is that pumps, that function to divert water from flowing into the Delta, can suck up salt water and organisms instead of fresh water. Looking at the diagrams, can you see any danger of this situation occurring in the years illustrated?
11. Based on these diagrams, which part of the Bay would you say is likely to get less flushed by fresh water and is in more danger of accumulating toxic substances, the northern parts of the Bay or the South Bay?
Where the downstream flow of fresh water at the Bay's surface meets the upstream return flow of saltier water near the bottom, suspended sediment, detritus, plankton and young fish can be concentrated at levels up to several hundred time the upstream or downstream concentrations. Some studies suggest that the summer production of phytoplankton in the northern part of the Bay is greatest when the entrapment zone is located in Suisum Bay, where the water is shallow (mostly less than 6 feet depth). The 2 psu (yellow) line is a good indicator of where fresh water and salt water meet.
12. Which months in the diagrams above are likely to be beneficial for phytoplankton growth in the northern part of the Bay?
13. What conditions do you think make Suisun Bay a good location for these photosynthesizing algae? Why would April 1995 have been a poor month for phytoplankton growth?