Water Quality Management - River Dynamics and Integrated River Management - page 571

Many complex models have been developed to relate growth, death (respiration), and photosynthetic
activity to nutrient and light availability for a single algal species (e.g., Brown and Barnwell, 1987; Ambrose
et al., 1988) and multiple algal species (e.g., DUFLOW, 2000). The details of these models are beyond
the level of detail for the simple presentation of basic water quality problems and management solutions
presented in this chapter. Instead this chapter takes what Thomann (1972, p. 105) refers to as the “naive”
view that the inclusion of two terms, P and R in the mathematical model for the DO balance, can roughly
represent the complexity of the photosynthesis-respiration process of aquatic plants. In this view,
photosynthesis is modeled as a distributed source of oxygen, P , and respiration is modeled as a distributed
sink of oxygen, R . The effect of the combined action of photosynthesis and respiration on the DO deficit,
D PR t , may then be computed as

ª RP º

D (1 e

Kt

) (9.27)

« » PR t

K K

¬ ¼

a a

Table 9.4 lists some average values of gross photosynthetic ( P - R ) production of DO as compiled by
Thomann (1972). Thomann (1972, p. 107) noted that the values in Table 9.4 have many uncertainties
including sunlight conditions, depths of samples, and method of determination. Nevertheless the values
listed in Table 9.4 provide information on the order of magnitude from gross photosynthetic production
to aid in starting the DO analysis. Finally, the values in Table 9.4 have the same units as distributed sources
or sinks of DO like SOD, thus, like SOD the values in Table 9.4 must be divided by the flow depth before
they can be used in Eq. (9.27).

Table 9.4 Some average values of gross photosynthetic (photosynthesis minus respiration) production of dissolved
o xygen (after Thomann, 1972)

Gross production
(g/m 2 /day)

Water type and/or location

Truckee River, Nevada, U.S., - bed attached algae 9

Tidal Creek, U.S. - diatom bloom 6

Delaware River estuary, U.S. - Summer 3-7

Duwamish River estuary, Seattle, Washington, U.S. 0.5-2.0

Neuse River System, North Carolina, U.S. 0.3-2.4

The presence of algae and other aquatic plants varies throughout the year because of the complex
combinations of the four previously listed factors. In particular, the total incoming light energy is a function
of time of day and time of year. With intermittent or heavy cloud cover, random variations are imposed on
the regular periodicity of incoming daily light energy. The combination of random hour to hour and day
to day fluctuations and periodic fluctuations in light energy indicates that care must be taken in assigning
photosynthetic input of DO as a steady-state source (Thomann, 1972, pp. 106-107). Thus, Fair et al. (1971,
p. 649) note that as important as photosynthesis may be in the total oxygen economy of natural waters, it
cannot be included in engineering calculations of the oxygen balance for waste-load allocation. Only
oxygen absorbed from the atmosphere can be relied on to input oxygen at all times except when ice cover
eliminates the air contact.
Normally, most water-quality managers think of DO problems in light of DO-BOD interactions, however,
in some cases photosynthesis and respiration can be the primary cause of DO problems. For example,
Manache and Melching (2004) found that for the Dender River in Belgium model parameters related to the
simulation of algal growth had a dominating effect on the uncertainty in simulated DO concentrations.
During the study period only 12% of the waste flow entering the Dender received treatment, thus, water