Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 123

CE-QUAL-W2 58 has five different organic matter state variables. To calculate the
amount of phosphorus incorporated into organic matter, two stoichiometric coefficients
(phosphorus to organic matter and phosphorus to carbonaceous BOD) are used. Thus,
organic phosphorus is not a state variable in this model. Organic matter decay produces
orthophosphate, which is the main phosphorus state variable. Decay of labile and
refractory dissolved organic matter and carbonaceous BOD corresponds to organic
phosphorus 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 model can
simulate an unlimited number of algal and epiphyton groups, each group/member
having different half-saturation constants and uptake rates for phosphorus.
CE-QUAL-R1 57 has phosphate as the main state variable. Similar to CE-QUAL-
W2, organic matter is represented by three state variables, namely, labile dissolved
organic matter, refractory dissolved organic matter, and detritus. The aerobic decom-
position of these three materials contributes to the phosphate pool. Respiration of
three types of phytoplankton, one type of macrophyte, zooplankton, and fish are
also included in phosphate sources.
The CE-QUAL-ICM 56 model has four state variables: total phosphate, dissolved
organic phosphorus, labile particulate organic phosphorus, and refractory particulate
organic phosphorus. Total phosphate comprises dissolved phosphate, particulate
phosphate, and algal phosphorus. Three algae groups are defined: green algae,
diatoms, and blue-greens. In CE-QUAL-ICM, the rate of dissolved organic phos-
phorus mineralization is related to algal biomass. When phosphate is scarce, algae
stimulate production of an enzyme that mineralizes dissolved organic phosphorus
to phosphate. Mineralization is highest when algae are strongly phosphorus limited
and is the lowest when there is no limitation. Similar calculations are also made for
hydrolysis of labile and refractory particulate organic phosphorus.
The HEM-3D 90 model uses kinetic process descriptions mostly from CE-QUAL-
ICM model. Macroalgae are added to HEM-3D as the fourth algal group.
The mass balance model equations for various phosphorus forms are presented
in the following subsections.

4.1.5.2.1 Inorganic Phosphorus

∂ () C

t

C
KC CGaC

( T − 20 )

3

= Da ( 1 − f C k ) − θ

4

− (4.29)

12

P

4443

1 PC

444

OP 4 83 83 8

144

P 1 PC 4

∂ +

12 4444

m PC

4444

4

3

Death Growth

Mineralization

where
C 3

= inorganic phosphorus concentration [mg P l −1 ]

D P 1 = death plus respiration rate constant of phytoplankton [day −1 ]

a PC = phosphorus to carbon ratio [mg P mg −1 C]

1 − f OP = fraction of dead and respired phytoplankton recycled to the phosphate

phosphorus pool [none]

C 4 = phytoplankton carbon concentration [mg C l −1 ]