Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 134

energy. In cyanobacteria (blue-green algae), the phycobiliproteins are the predominant accessory
pigment, giving the group their characteristic blue-green or red color.
Since the absorption of light in photosynthesis is dependent on the wavelength, only a portion of
the electromagnetic spectrum is usable by plants and is referred to as photosynthetically available
radiation (PAR). PAR is usually measured in the visible light spectrum, with wavelengths of about
400-700 nm.
The absorption of light can also cause the degradation of some compounds, such as some toxic
organic chemicals, in a process called photodegradation or photolysis. Photolysis is a function of
the quantity and wavelength distribution of incident light, the light adsorption characteristics of the
compound, and the eficiency at which absorbed light produces a chemical reaction. Photolysis is
classiied into two types that are deined by the mechanism of energy absorption. Direct photolysis
is the result of the direct absorption of photons by the chemical molecule. Indirect or sensitized pho-
tolysis is the result of energy transfer to the chemical from some other molecule that has absorbed
the radiation (Wool et al. 2003). The sorptive properties are speciic to the organic chemicals and
are often available in chemical handbooks.
Processes such as heat transfer, photosynthesis, and photodegradation (photolysis) are functions
of both the wavelengths and the intensity of light. For photosynthesis, productivity usually increases
with light and reaches a maximum at some saturation light intensity, or the point where growth is at
maximum (Figure 5.5). Beyond that point in some aquatic plants, photoinhibition can occur where
growth (photosynthesis) decreases with increasing light intensity. The saturation light intensity var-
ies with the aquatic plant. Since light varies over the course of a day, photoinhibition can vary with
the season and over the course of a day. For example, during the summer months, optimal light may
occur in the morning or afternoon and intensities may be great enough during the middle parts of
the day to be photoinhibiting. Also, some plants are more shade or sun tolerant than others, so the
saturation intensities vary. Diurnal cycles in production also affect variations in DO, pH, and nutri-
ent cycling.
For studies of temperature, productivity, and photolytic chemicals in streams or rivers, estimates
of the light intensity over speciic wavelengths are required. This intensity can be estimated (pre-
dicted) or measured. Methods for predicting shortwave and longwave radiation for heat transfer
in models of water temperature have been widely used and are described in detail by Martin and
McCutcheon (1999). Often, the PAR at the water surface is assumed to be a ixed fraction (such
0.47) of the solar radiation (Szeicz 1974; Baker and Frouin 1987, as cited by Chapra 2007). For mod-
els of toxic chemicals, such as the Water Analysis Simulation Program (WASP) (Wool et al. 2003)
and the Exposure Analysis Modeling System (EXAMS) (Burns and Cline 1985), the light intensity
at each of a series of wavelengths (46 wavelengths) may be computed as a function of time, location,
and atmospheric conditions.
Radiation is also commonly measured using one of a variety of available sensors. Depending
on the needs of a particular study, sensors can be used to measure total radiation (pyranometers),

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0.5

0

0 200 400

Light (langley day -1 )

600 800

FIGURE 5.5 Light limitations to algal growth.