RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 107

where Δ is the energy derived from sunlight, and C(H 2 o)
is a general representation of organic carbon; for
example, glucose is C 6 H 12 o 6 or 6·C(H 2 o). The process
of respiration can be represented by the (reverse)
reaction

is the chlorophyll a concentration in µ g/L. As an approx-
imation, the ratio of algal biomass (dry weight) to algal
chlorophyll a is 100:1. Aside from using the empirical
Equations (4.108) and (4.109) to estimate the photosyn-
thetic production and respiration rates from chlorophyll
a concentrations, production and respiration rates can
be measured directly using the light- and dark-bottle
method (APHA et al., 2005) or the delta method (Di
Toro, 1975).

C H O O ( ) + → CO H O + + ∆ (4.107)

2 2 2 2

Microbial ecologists refer to the respiration described
by Equation (4.107) as aerobic respiration because
oxygen is utilized as the electron acceptor. When oxygen
is absent, anaerobic respiration takes place, utilizing a
variety of other compounds as electron acceptors. Since
the amount of photosynthetic and respiration activity
depends on the amount and intensity of sunlight, there
is a diurnal (daily) and seasonal variation in dissolved
oxygen amounts contributed by photosynthesis and res-
piration. In fact, the diurnal variation can sometimes be
so extreme that the stream is supersaturated with
oxygen during the afternoon and severely depleted of
oxygen just before dawn. oxygen produced by photo-
synthesis above saturation during bright sunshine is
released to the atmosphere and is lost from the water.
Photosynthesis and respiration are a major source and
sink of oxygen (respectively), particularly in slow-
moving streams and lakes, and can be expected to be
significant for algal concentrations in excess of 10 g/m 3
(dry mass).
Quantification of oxygen fluxes associated with pho-
tosynthesis and respiration is difficult, and is generally
dependent on such variables as temperature, nutrient
concentration, sunlight, turbidity, and whether the plants
are floating (phytoplankton) or on the bottom (macro-
phytes, periphyton). Reported photosynthetic oxygen
production rates, S * (averaged over 24 hours) are in the
range of 0.3-3 g/m 2 ·d for moderately productive surface
waters up to 10 g/m 2 ·d for surface waters that have a
significant biomass of aquatic plants. Reported respira-
tion rates, S r * , have approximately the same range as
photosynthetic oxygen production rates. Empirical
equations that have been proposed for estimating pho-
tosynthetic oxygen production and respiration rates are
(Di Toro, 1975)

Benthic Oxygen Demand. Benthic oxygen demand or
sediment oxygen demand (SoD) results primarily from
the deposition of suspended organics and native benthic
organisms in the vicinity of wastewater discharges and
can be a major sink of Do in heavily polluted streams.
Most benthic sludge undergoes anaerobic decomposi-
tion, which is a relatively slow process; however, aerobic
decomposition can occur at the interface between the
sludge and the flowing water. The products of anaerobic
decomposition are Co 2 , H 2 S, and CH 4 , and if gas pro-
duction is especially high, the floating of bottom sludge
may result, leading to an aesthetic problem, as well as
depletion of Do. Benthic oxygen demand, S * (ML −2 T −1 ),
is typically taken as a constant in most applications, and
the benthic flux of oxygen, S b (ML −3 T −1 ), used in the
ADE is derived from S * using the relation

S A S
d

* *

b s b

(4.110)

S = =

b

∀

where A s is the surface area of the bottom of the stream
(L 2 ), ∀ is the volume (L 3 ) of the st r eam section contain-
ing a benthic surface area A s , and d is the average depth
of flow (L). In many water-quality modeling studies, S *
values are obtained through calibration. Typical values
of S * at 20°C are given in Table 4.7, and calculated
values of S b at 20°C can be converted to other tempera-
tures using the relation

TABLE 4.7. Typical Benthic Oxygen Demand Rates, S * ,
at 20°C

Range
(g/m 2 ·d)

Average value
(g/m 2 ·d) Bottom Type

S p = 0.25 Chla (4.108)

Filamentous bacteria
(10 g/m 2 )

- −7

Municipal sewage
sludge near outfall

−2 to −10 −4

and

Municipal sewage
sludge downstream
of outfall (aged)

−1 to −2 −1.5

S r = 0.025 Chla (4.109)

Estuarine mud −1 to −2 −1.5

Sandy bottom −0.2 to −1.0 −0.5

where S p and S r are the average daily oxygen production
and respiration rates, respectively, in mg/L·d, and Chla

Mineral soils −0.05 to −0.1 −0.07