Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 151

may be dominated by phytoplankton. In shallower waters, plants may be dominated by aquatic
macrophytes. Aquatic macrophytes may be emergent plants such as cattails, rooted plants with
loating leaves such as lily pads, or submerged vegetation such as the invasive Eurasian milfoil. In
shallower rivers and streams, plants that are attached to rocks or other substrates, the periphyton,
may dominate. The periphyton can include algae growing on the surface of rocks and stones (epil-
ithic forms), on submerged plants (epiphytic forms), or on the bottom sediments (epipelic forms, or
the benthos) of rivers.
Primary productivity is the process by which chemical energy is stored as biomass, while pro-
ducing oxygen as a by-product. The total rate is called gross primary production (GPP). Since some
of the energy is used in respiration, the difference between the gross productivity and that remain-
ing after respiration is called net primary production (NPP).
Various methods have been developed to estimate GPP and NPP, primarily by measuring varia-
tions in oxygen or carbon. One commonly used instream method is the total community oxygen
metabolism. This method, originally described by Odum and Hoskins (1958), has been widely used
in productivity studies. Generally, the method involves the monitoring of DO concentrations over
a diel period (diurnally) and analyzing the variations graphically. The method involves estimating
productivity, respiration, diffusion, and any accrual from other sources (a source of error). USEPA
SESD-EAB (2007) described a variation in the method whereby diffusion is measured rather than
estimated using methods originally described by Odum and Hoskins (1958). Grace and Imberger
(2006) describe a variety of open water methods for measuring stream metabolism.
A second commonly used method to estimate water column productivity and respiration utilizes
light and dark bottles. Essentially, bottles are suspended in a water column, where in the light bottle
(clear bottles) productivity and respiration occur allowing an estimation of the NPP, while in the
dark bottle (a covered bottle) only respiration occurs, allowing an estimation of the GPP. The light
and dark bottle method is described in detail in USEPA SESD-EAB (2007) and Standard Methods
for the Examination of Water and Wastewater (APHA 2005).
5.5.5 S edIMent d eMandS
Sediment oxygen demand (SOD) is a process that has long been known to impact the water qual-
ity of surface waters. SOD, due to the mineralization (diagenesis) of organic materials in bottom
sediments, can contribute to oxygen declines in waterbodies. While impacted by the lux, or
deposition, of organic materials to sediments, the relationship is nonlinear (Hatcher 1986; Chapra
1997). This nonlinear relationship results from the complex physical, biological, and chemical
cycling that occurs in anaerobic sediments and the interface between the sediments and the water
column.
Methods are available to estimate the SOD based on the downward lux of carbon, nitrogen, and
phosphorus from the water column. In a landmark paper, Di Toro et al. (1990) developed a model
of the SOD that mechanistically arrives at the observed nonlinear relationship (Chapra 1997). Di
Toro, in his topic Sediment Flux Modeling (2001), provided a further description of the techniques.
Martin (2004) described the implementation of the SOD algorithms into the WASP.
SOD is also commonly measured using in situ chambers or cores, or based on diel variations in
oxygen concentrations. Figure 5.28 illustrates a SOD chamber developed by the U.S. EPA (Murphy
and Hicks 1986), and the standard operating procedure for its deployment is described in USEPA
SESD-EAB (2007). A typical procedure is to place and seal a chamber on the bottom sediments and
measure the DO concentrations using a probe. The rate of DO utilization provides a measure of the
SOD. Grace and Imberger (2006) describe similar SOD chambers and their applications.
An alternative to measuring the SOD is community substrate oxygen demand (CSOD), which
is an assessment of the respiration of all substrates. Components of CSOD include the diel curve
method (Odum and Hoskins 1958), the light and dark bottle method (water column productivity
and respiration), and estimates of reaeration/diffusion. A community oxygen metabolism analysis