Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 114

dissolved oxygen concentration is less than a prespecified minimum value at which
anaerobic processes begin. The minimum dissolved oxygen default value is 0.1 mg l −1 .
CE-QUAL-R1 57 models macrophytes. Thus, ammonium and nitrate uptake by
macrophytes from sediment is included. Anaerobic ammonium release, aerobic decom-
position of organic matter, and settling of adsorbed ammonium are also considered in
the sediment compartment model balance equations.
Both the CE-QUAL-ICM 56 and HEM-3D 90 models use the same predictive
sediment submodel developed by Cerco and Cole 56 for the Chesapeake Bay eutroph-
ication model study. This submodel consists of three basic processes: deposition of
particulate organic matter to the sediment, organic matter diagenesis (decay), and
release of nutrients (phosphate, ammonium, and nitrate) to the overlying water
column or their burial into deeper sediment.
Benthic equations for organic, ammonium, and nitrate nitrogen are given in the
following subsections.

4.1.5.1.5 Organic Nitrogen (Benthic):

∂
∂

C
t

7 ( T − 20) ( T − 20)

= k θ a f C − k θ C (4.26)

12

PZD

444

PZD

444

nc on

3

4

12

OND

443

OND

44

7

Algal decomposition Mineralization

where

C 7 = organic nitrogen concentration [mg N l −1 ]

k PZD = anaerobic algal decomposition rate constant at 20 ° C [per day −1 ]

θ PZD = anaerobic algal decomposition temperature coefficient [none]

T = temperature [ ° C]

a nc = nitrogen to carbon ratio [mg N mg −1 C]

f on = fraction of dead and respired phytoplankton recycled to the organic

nitrogen pool [none]

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

k OND = organic nitrogen decomposition rate constant at 20 ° C [day −1 ]

θ OND = organic nitrogen decomposition temperature coefficient [none]

4.1.5.1.6 Ammonia Nitrogen (Benthic)

∂
∂

C
t

1 ( T − 20 ) ( T − 20 )

= k ( )

12

θ a 1 − f C + k θ C (4.27)

4444 4444 3 12 443 44

PZD PZD nc on 4 OND OND 7

Algal decomposition Mineralization

where

C 1 = ammonium nitrogen concentration [mg N l −1 ]

k PZD = anaerobic algal decomposition rate constant at 20 ° C [day −1 ]

θ PZD = anaerobic algal decomposition temperature coefficient [none]

T = temperature [ ° C]

a nc = nitrogen to carbon ratio [mg N mg −1 C]

1 − f on = fraction of dead and respired phytoplankton recycled to the ammonia

nitrogen pool [none]