Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 105

includes a Monod-type function to represent the effect of ammonium concentration
on the nitrification rate. Similar to EUTRO5, the dissolved oxygen correction term
is also a Monod-type function.
CE-QUAL-W2 58 has five different organic matter state variables. To calculate the
amount of nitrogen incorporated into organic matter, two stoichiometric coefficients
(nitrogen to organic matter and nitrogen to carbonaceous BOD) are used. Thus,
organic nitrogen is not a state variable in this model. Two of the state variables used
for modeling the nitrogen cycle are ammonium and nitrate. Organic matter decay
produces ammonia. Decay of labile and refractory dissolved organic matter plus
carbonaceous BOD form the elements of the dissolved organic nitrogen mineralization
process, while decay of labile and refractory particulate organic matter corresponds
to hydrolysis plus mineralization. Unlike EUTRO5, which simulates phytoplanktons
as a single population biomass, CE-QUAL-W2 can simulate an unlimited number of
algal and epiphyton groups, each group/member having different half-saturation con-
stants and uptake rates for nitrogen. For the nitrification and denitrification, CE-
QUAL-W2 and EUTRO5 both have a temperature correction term; however, their
oxygen correction functions are different. In EUTRO5, the effect of dissolved oxygen
concentration on nitrification and the denitrification rates are expressed as Monod-
type functions. In CE-QUAL-W2, the nitrification and denitrification terms do not
include such functions, but rather include discrete sign functions. These sign functions
shut down nitrification if the dissolved oxygen concentration is less than a prespecified
minimum value. On the contrary, denitrification shuts down when dissolved oxygen
concentrations exceed the same minimum value. In CE-QUAL-W2, nitrogen fixation
by blue-green algae can be modeled by setting the half-saturation concentration for
nitrogen to zero for the corresponding algal group(s).
CE-QUAL-R1 57 also uses ammonium and nitrate as state variables. Similar to
CE-QUAL-W2, the organic matter is represented by state variables, namely, labile
dissolved organic matter, refractory dissolved organic matter, and detritus. Aerobic
decomposition of the organic matter contributes to the ammonium pool. Respiration
of three types of phytoplankton, one type of macrophyte, zooplankton, and fish are
also included as ammonium sources. Nitrification is allowed only in layers with
dissolved oxygen concentrations greater than a prespecified minimum value, which
is also the case in CE-QUAL-W2.
The CE-QUAL-ICM 56 model has five state variables, namely ammonium, nitrate,
dissolved organic nitrogen, labile particulate organic nitrogen, and refractory particu-
late organic nitrogen. Three algae groups are defined: green algae, diatoms, and blue-
green algae. In CE-QUAL-ICM, the rate of dissolved organic nitrogen mineralization
is related to algal biomass. When dissolved inorganic nitrogen (ammonium nitrate)

is scarce, algae stimulate production of an enzyme that mineralizes dissolved organic
nitrogen to ammonium. Mineralization rate is highest when algae are strongly nitrogen
limited; it is lowest when no limitation exists. Similar calculations are also made for
hydrolysis of the labile and refractory particulate organic nitrogen. Similar to the case
in the model developed by Park and Kuo, 69 the nitrification term in CE-QUAL-ICM
includes a Monod-type function representing the effect of ammonium concentration
on nitrification. Similar to EUTRO5, the dissolved oxygen correction term is also a
Monod-type function.

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