Chlorophyll is a measure of the biomass (abundance) of phytoplankton, the suspended microscopic algae that are the largest living component in San Francisco Bay. Phytoplankton produce organic food from inorganic substances (primary production) using the chemical energy found in sunlight (see Part IV). Chlorophyll is a pigment in phytoplankton that captures sunlight for the process of phytosynthesis. In the production process, plants take up nutrients, such as nitrogen and phosphorus, and trace metals that are contaminants in the marine environment, such as cadmium, zinc, and nickel.
This diagram shows a vertical profile of chlorophyll concentrations collected at the same location along the USGS sampling transect at two different times of the year. The USGS measures chlorophyll concentration with a fluorometer, mounted on the submersible instrument package. The fluorometer measures the amount of red light that suspended algae fluoresces when stimulated by the blue-light beam of the meter.
Description of circled numbers:
1. This profile was measured near the San Mateo Bridge on September 21, 1995. It is an example of low phytoplankton biomass and uniform distribution from top to bottom in the water column.
2. This profile was measured near the San Mateo Bridge on April 11, 1995. It illustrates a condition in which phytoplankton biomass was high, especially in the surface layer. This profile was made during the spring bloom, a period of rapid phytoplankton population growth. Spring blooms occur when the water becomes stratified by salinity gradients that slow the rate of vertical mixing.
1. During which times of year are phytoplankton likely to experience a stratified water column?
A stratified water column is important in estuaries because phytoplankton need to remain near the surface where they can obtain sunlight to do photosynthesis. The high amount of suspended particles in estuaries prevents sunlight from penetrating deeply into the water. Because a stratified has low density water at the surface, phytoplankton remain floating there and are not mixed into lower depths.
This plot contains two
vertical profiles of light penetration (yellow lines) that were obtained using
a sensor mounted on the submersible instrument package. Light Penetration refers
to the amount of sunlight that penetrates the water column and reaches various
depths. In the water column, sunlight is absorbed and scattered by suspended
particles, dissolved substances, and the water itself. The USGS monitors light
penetration to determine if it is sufficient to support phytoplankton photosynthesis.
Light penetration is presented as the percentage of the light measured at 1
meter depth. The data were collected at two different locations along the USGS
sampling transect during the same sampling cruise.
Description of circled numbers:
1. This profile was measured in San Pablo Bay on February 6, 1996. The sunlight was being absorbed rapidly by suspended particles and was completely absent at about 2.5 meters depth. This condition is typical of northern San Francisco Bay, with high concentrations of suspended solids. Only the shallow upper layers of the water column have sufficient sunlight for photosynthesis.
2. This light profile was measured in Central Bay on February 6, 1996. In this case sunlight was present down to 5 meters. Usually the Central Bay has relatively clear water, with maximum light penetration reaching 8 meters.
2. In addition to a stratified water column that keeps phytoplankton
floating in the sunlit surface zone, plants require light and nutrients.
Might there be times when these factors provide limits on the amount of
phytoplankton production? Explain.
CHLOROPHYLL AT THE BAY'S SURFACE
The upper panel displays the Delta Outflow Index for 1993-1995. The
lower panel shows the changing distribution of chlorophyll along the USGS
sampling transect. Color is proportional to chlorophyll concentration, with
darker (green) shadings indicating high phytoplankton abundance and lighter
(yellow) shadings indicating low abundance. The vertical axis represents variability
in space from the 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.
Descriptions of Circled Numbers:
1. Bay waters usually have small populations of phytoplankton, with chlorophyll concentrations typically less than 5 mg/m3.
2. In the South Bay, the phytoplankton population has a period of explosive growth (a bloom) in March or April. During these blooms, the phytoplankton abundance (measured as chlorophyll concentration) increases more than ten times.
3. Prior to 1987, the North Bay had abundant phytoplankton in summer, with chlorophyll concentrations reaching 30 to 60 mg/m3. But these episodes of elevated summer biomass disappeared in 1987 after the Asian clam Potamocorbula invaded the estuary and began to consume phytoplankton cells as fast as they could reproduce.
4. The patterns of the spring phytoplankton blooms change from year to year, with large and prolonged blooms during years of exceptional river flow such as in 1995.
3. Why do periods of high river flow produce large phytoplankton blooms in the spring (hint: remember the discussion above about stratified estuaries and what conditions produce stratification)?
4. Because other life depends on phytoplankton, it is important to
understand the factors that influence their growth. If you wanted to design
an intensive sampling program to study phytoplankton dynamics, in which
part of the Bay and during which part of the year would you focus your study?
Life requires oxygen to sustain its metabolic processes. It is supplied to the Bay water by photosynthesis (which takes up carbon dioxide and releases oxygen) and from the atmosphere (whose primary gases are nitrogen and oxygen). Oxygen is depleted during organism respiration and by decomposition of organic matter.
The USGS measures dissolved oxygen concentration as an indicator of water quality and the activity level of the plants and animals living in the Bay. When the oxygen content of water is undersaturated (less than that at equilibrium with atmospheric oxygen), it indicates that organic matter is consumed by organisms faster than it is produced by the plants. Conversely, when the oxygen concentration is greater than saturation, oxygen is being produced by plant photosynthesis (mostly phytoplankton) faster than it is consumed by all the other organisms. Oxygen concentration is an index of the balance between processes of food production and food consumption. This balance is a key descriptor of the changing status of the ecosystem. When the balance is disrupted, the oxygen concentration can fall to low levels.
Regions of San Francisco Bay experienced episodes of severe oxygen depletion (with fish kills) during the 1950's and early 1960's, before the era of advanced sewage treatment. The oxygen content of Bay waters is now always high enough to supply the oxygen demands of animals, reflecting a positive water quality response to improved sewage treatment techniques. Although nutrient concentrations (e.g., phosphates and nitrates) are very high in San Francisco Bay, the Bay does not have the noxious or toxic blooms of algae that are observed in many other estuaries that receive large inputs of nutrients from waste and land runoff.
This plot displays two examples
of vertical dissolved oxygen profiles (yellow lines) collected at two different
locations along the USGS
sampling transect during the same sampling cruise. An oxygen electrode on
the submersible instrument package measures the amount of dissolved oxygen in
Bay water.
Description of circled numbers:
1. This profile was measured near the Bay Bridge (Central Bay) on March 26, 1996. This example shows that the oxygen content of Bay water is not always uniform from surface to bottom. The surface waters had oxygen concentrations of about 9 milligrams per liter, while bottom waters had oxygen concentrations less than 6 milligrams per liter. These kinds of vertical variations often occur as a result of salinity stratification, which slows the rate of vertical mixing of the water.
2. This profile was measured near the San Mateo Bridge on March 26, 1996. This example shows the typical condition of nearly-uniform oxygen concentrations from surface to bottom. Oxygen is added to the surface layers by atmospheric exchange and photosynthesis. Oxygen is mixed to the bottom waters by tidal and wind-driven stirring. This mixing is rapid in the absence of salinity stratification.
5. Would you expect to see a correlation between dissolved oxygen and chlorophyll concentrations? Explain.
6. What conditions other than high phytoplankton production might be indicated by high dissolved oxygen contents?
The upper panel displays the Delta Outflow Index for 1993-1995. The
lower panel shows the changing distribution of dissolved oxygen at the Bay surface
along the USGS
sampling transect. Color is proportional to oxygen content, with lighter
(yellow) shadings indicating oxygen supersaturation (high values) and darker
(brown) shadings indicating under saturation (low values). The vertical axis
represents variability in space from the 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. During periods of high Delta outflow, such as in early 1993, the oxygen content of the water becomes reduced because bacteria, plants, and animals consume oxygen faster than it can be replenished by phytoplankton photosynthesis or exchanges with the atmosphere.
2. The oxygen content of Bay waters is an indicator of the metabolic activity of all the organisms (bacteria, algae, invertebrate animals, fish) that consume oxygen. We measure oxygen content relative to the saturation concentration at equilibrium with atmospheric oxgyen. Usually, the oxygen concentration is less than 100% of saturation, indicating that organisms are consuming oxygen faster than it is being produced by photosynthesis.
3. During phytoplankton blooms, the algae rapidly produce oxygen by photosynthesis and the oxygen content of Bay waters becomes greater than the saturation concentration. This example illustrates how biological processes can influence water chemistry and water quality of the Bay.
7. During which times of year and in which part of the Bay is dissolved oxygen content greatest? Why?
8. What conditions produce relatively low dissolved oxygen contents?
The data presented in Parts VIID and VIIE have shown a dependent relationship between the amount of suspended particles (and light penetration), chlorphyll, and dissolved oxygen in Bay water. Carefully examine the diagrams below of chlorophyll, dissolved oxygen, and suspended sediments. You have seen them before, but they are shown together here for easier comparison.
9. Explain any correlations you see among the three water measurements.
For example, do you see a relationship between the growth of phytoplankton
(chlorophyll) and the amount of dissolved oxygen and suspended sediment
in the water? Which factors cause which effects?