Estuaries have high concentrations of particles suspended in the water (see Part VA). These particles reduce the clarity of the water and give it color. Underwater visibility may at times be less than a meter (several feet), indicating high turbidity conditions. Tropical waters with few suspended sediments and clear water are said to have low turbidity. The suspended particles absorb sunlight required for plant photosynthesis, so times of high turbidity tend to be times of low primary productivity.
Particles consist of sediments (primarily clay), detritus, and plankton. Sediments are carried from the surrounding land surfaces into the Bay by rivers. Once in the Bay they either settle onto the bottom or are carried through the Bay into the open ocean. Strong tidal currents and wind waves can resuspend these sediments into the water column. Most trace elements, such as lead, copper, and zinc, are associated with the surfaces of sediment particles, and are transported, deposited, and eventually buried with the Bay sediments. Many trace elements are toxic to marine life in very small quantities. Human activities have accelerated the cycling of trace elements and increased deliveries of these substances to the marine environment.
Studying the concentration of particles in water has implications for the cycling of toxic substances and for the ability of phytoplankton to photosynthsize. The concentration of particles is measured as total suspended solids (TSS). Within the Bay-Delta system, TSS concentrations range from 1-2 milligrams per liter (mg/liter) to maximum concentrations that occasionally exceed 1,000 mg/liter. The USGS measures TSS with a Optical Backscatter Sensor mounted on the submersible instrument package. Light waves emitted from the sensor bounce off of particles in the water; the more light that is bounced back to the instrument, the higher the turbidity (amount of suspended solids).
This plot shows two vertical profiles of suspended solids (yellow lines) obtained at two locations along the USGS sampling transect.
Description of circled numbers:
1. This profile was measured near the Bay Bridge (Central Bay) on March 26, 1996. It shows low suspended solids concentration and uniform distribution from the surface to bottom depths. This condition is observed when tidal currents are weak and winds are calm, and thus the current stresses applied to the seafloor are not strong enough to suspend bottom sediments.
2. This profile was measured in San Pablo Bay on February 6, 1996. It is an example of elevated suspended solids concentrations in the bottom waters. This condition is observed when tidal currents are rapid enough to resuspend sediments off the bottom and move them up into the water column.
1. Summarize the conditions that lead to high turbidity (lots of suspended particles) in Bay waters. Consider conditions that get particles into the Bay originally and conditions that cause them to be resuspended after they have settled to the Bay bottom.
The diagram below shows how TSS varies at the surface of the Bay both spatially, along its length, 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 shows the Delta Outflow Index for 1993-1995. The lower panel shows the changing distribution of total suspended solids (TSS) along the USGS sampling transect. Color is proportional to TSS, with darker (brown) shadings indicating turbid water and lighter (green) shadings indicating clearer water with smaller concentrations of suspended solids. 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, large quantities of suspended sediments are carried into the Bay from the Sacramento and San Joaquin Rivers. The water can become very turbid during these periods of rapid sediment input.
2. During years of low Delta outflow, such as 1994, sediment inputs are reduced and the concentrations of suspended sediments are smaller. So the turbidity, and the color, of Bay waters changes over time because the concentrations of suspended solids change from season to season and from year to year.
3. The concentrations of suspended sediments are smallest in the Central Bay (the region of the Bay-Delta that is farthest from the river sources of sediments). For this reason, Bay waters often appear green near San Francisco but brown in Suisun Bay.
4. The small local streams that flow into the South Bay also carry sediments. During the intense storms of early 1995 these stream inputs greatly increased the concentrations of suspended solids in the South Bay, especially in the region below the San Mateo Bridge.
5. During very wet years, such as 1995, the Bay waters are turbid throughout the year because of sustained riverine inputs of suspended sediments.
2. During which times of the year and in which part(s) of the Bay is the amount of TSS greatest at the Bay's surface?
3. Which process (from 1) causes high amounts of TSS at the Bay's surface? Why might this cause the timing of high TSS to be different in different parts of the Bay? (Hint: see circled number 4 in diagram.)
4. Since the 1980s, Delta waters have become more transparent, that is, less turbid. The reason for this is not well understand, but what are some possibilities? (Keep in mind that plankton as well as sediments are part of the suspended solid load. Also think of sedimentary changes that have occurred.)
The plots below show TSS spatial variation along the length of the Bay and from the surface to the Bay bottom. Temporal variation is shown by data from two different seasons of the year: winter and early fall.
Total suspended solids (TSS; in mg/liter)
January 28, 1997
October 7, 1997
5. Explain the differences in TSS between these two months in the same year.
6. On January 28, 1997, is there any evidence for runoff from local streams (from rain around the Bay) or not?
Because toxic substances often adhere to sediment particles, understanding the fate and transport of sediments also tells us something about the fate and transport of Bay contaminants. The sources below contain useful information about pollutants in the Bay.
7. What kind of contaminants are discussed in the USGS site above and what evidence is there for harmful effects to marine life? Explains the historical trends in toxic concentrations. What are the sampling methods used?
8. How have pollutant levels in the Bay been effected by the implementation of advanced wastewater treatment plants? What evidence is shown in Figures 2 and 3 of the USGS site above?