Dissolved Oxygen Depletion in the Stockton Deep Water Ship Channel: Physical and Chemical Processes Conceptual Model

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Reach 3: Primary Driver—Photosynthesis

Reach 3 photosynthesis diagram

Photosynthesis in Reach 3 (i.e., the DWSC) is affected by the four secondary drivers shown above. Click on a secondary driver to jump down to the discussion of that driver. See the Basic Concepts page for a general discussion of how the secondary drivers affect the primary driver. Factors affecting sunlight are, for the most part, not reach specific and are therefore not described.

The proportion of DO concentrations that is derived from photosynthesis has not been identified but is expected to increase with higher algae biomass concentrations. The seasonal DO concentrations in Reach 3 at the Rough and Ready Island station, along with Stockton DO data for 2003 are shown in the figure below. Seasonal declines in the DO concentrations in Reach 3 below saturation levels, as well as below the regulatory minimum, indicate that algae photosynthesis is not sufficient to maintain Reach 3 DO concentrations above these levels.

DO in the DWSC in 2003

Secondary Driver—Algal Biomass Concentration

Potential sources of algal biomass in Reach 3 are described in Reach 3: Primary Driver—BOD Concentration: Secondary Driver—Carbonaceous BOD Concentrations. One study, which evaluated net daily oxygen production by comparison of photosynthesis and respiration processes in the water column in the DWSC, found that an average amount of algal biomass produced an additional 2072 kg of chlorophyll a per day, an increase of 1422%. This algal photosynthesis generated a net increase of oxygen in the photic zone. The photic zone was relatively shallow, however, and the phytoplankton in the remaining dark portion of the water column generated a much greater net oxygen demand of 3,8007400 kg per day (Lehman and Ralston 2001).

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Secondary Driver—Turbidity

High concentrations of suspended sediment restrict the photic zone to the upper 2 meters (6.6 feet) of the DWSC (Lehman 2003; Lehman et al. 2001). Suspended inorganic sediment more severely reduces light penetration when compared to limitations caused by algae alone (Lee and Jones-Lee 2003; Lehman et al. 2001).

Suspended particles in the DWSC are subject to both settling and resuspension forces. Measured values of turbidity, TSS, VSS, and phytoplankton pigments show that a large percentage of particles settle out of the water after they enter the DWSC because of the substantially larger channel and slower velocities (Litton 2003). Data taken in 2001 suggest that about 30% of measured TSS settles out between Light 48 and Light 43 in the DWSC (i.e., upstream end of the DWSC) (Litton 2003). Downstream of Light 43, concentrations of TSS are nearly constant throughout most of the water column, indicating that resuspension rates are about equal to settling rates (Litton 2003).

Turbulence has been determined to be sufficient to cause resuspension and generally prevent stratification of the water column in the DWSC, although stratification may occur as a result of warm air temperatures and solar radiation in the afternoon (Litton 2003). Resuspension is primarily influenced by tidal fluxes but may also be enhanced by ship traffic (Litton 2003). Turbidity limits light penetration, thus limiting the depth of the photic zone, which in turn limits the level of DO created by algal production (Lee and Jones-Lee 2003).

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Secondary Driver—Water Temperature

In general, an increase in water temperature produces a corresponding increase in the rate of photosynthesis. The figure below indicates that DWSC temperatures are similar to the San Joaquin River temperatures.

Rough and Ready Island, Mossdale, and Fernalis Temperatures in 2001

The second figure (below) shows the temperature stratification for July 2002. Surface heating produces a surface layer 13 meters deep that allows increased algal growth near the surface of the DWSC in the summer. This heating accounts for the increased afternoon DO measurements at the surface Rough and Ready Island station. The surface layer is subsequently mixed into the DWSC during the night, and the net effect of the algae photosynthesis on DO in relatively small.

Water Temperature and DO at Rough abd Ready Island

The water temperature conditions in Reach 3 are described below.

As shown in the first figure above, water temperatures in Reach 3 (Rough and Ready Island) are generally very similar to temperatures in Reaches 1 (Vernalis) and 2 (Mossdale). Water temperatures in the upstream portions of Reach 3 (i.e., near Channel Point) are slightly higher than temperatures in the downstream portion of the reach (i.e., near Disappointment Slough) (Lehman and Ralston 2001). Near surface water temperatures in the DWSC are slightly higher (0.51°C) than temperatures near the bottom during the day in July and August (Jones & Stokes 2001a). However, the DWSC is generally well mixed at night, and substantial temperature stratification typically does not occur (Jones & Stokes 2002a; Jones & Stokes 2001a).

Seasonally, water temperatures in the DWSC are generally greater than 77°F (25°C) in the summer and less than 50°F (10°C) during the winter (Lehman and Ralston 2001). Surface water temperatures in the Turning Basin are slightly higher than temperatures in the DWSC (Lehman 2003). In addition, Turning Basin water temperatures are generally more stratified (2°C difference between surface and bottom temperatures) than in the DWSC (Lehman 2003).

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Secondary Driver—Nutrients

Nutrients are a necessary component of the photosynthesis process. Nutrients in the DWSC are either discharged directly to the DWSC from the Stockton RWCF discharge or imported from the San Joaquin River.

Many sources note that algal growth is probably not nutrient-limited in the DWSC and that levels of macronutrients, such as inorganic nitrogen and orthophosphate, are an order of magnitude greater than required for algal growth (Lehman et al. 2001; Lehman 2003; Hunt 2002). Growth of algae in the San Joaquin River and DWSC appears to be limited instead by light availability and travel time (Lee and Jones-Lee 2003; Leland et al. 2001).

The results of a mass balance model demonstrated that the dissolved ammonia discharge of the Stockton RWCF has a residence time of up to 25 days (during low river flows) and could make up a large percentage of the total dissolved ammonia in the DWSC ( Lehman 2003). The total upstream nitrogenous load was much greater than that of the RWCF, but its decay rate was slow and most of the available nitrogen was already decomposed ( Lehman 2003). Either source could be the primary driver of oxygen demand on any given day (Lehman 2003).

The concentrations of nitrogen and phosphorus compounds in the DWSC are substantially more than adequate for algal production.

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Uncertainties in Photosynthesis

Uncertainties related to understanding how rates of photosynthesis in Reach 3 may contribute to low DO concentrations in the DWSC include:

  • the relative contribution of local algal photosynthesis and subsequent respiration/decay of the algae biomass (Lehman 2003),
  • the relative importance of live versus decaying algal biomass to oxygen demand ( Lehman and Ralston 2001), and
  • the effects of the temperature stratification on the algal photosynthesis.

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