Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 104

ammonium is the preferred form because of physiological reasons. The ammonium
preference factor used in the model is given in Equation (4.25). During phytoplankton
respiration and death, a fraction of the phytoplankton biomass is recycled to the
nonliving organic nitrogen pool, while the remaining fraction contributes to the
ammonium pool. Although released ammonium is readily available for algal
growth/nitrification, released organic nitrogen must undergo mineralization or bac-
terial decomposition into ammonium before utilization. EUTRO5 uses a saturating
recycle rate, which is directly proportional to the phytoplankton biomass present (see
Equation (4.22) and Equation (4.23)). Saturating recycle permits second-order depen-
dency at low phytoplankton concentrations, and permits first-order recycle when the
phytoplankton concentration greatly exceeds the half-saturation constant, K mPC . Basi-
cally, this mechanism slows the recycle rate if the phytoplankton population is small
but does not permit the rate to increase continuously as the phytoplankton concen-
tration increases.
Equations describing the nitrification of ammonium in EUTRO5 module contain
temperature and low dissolved oxygen correction terms. The latter is a Monod-type
function, which represents the decline of nitrification as the dissolved oxygen con-
centration approaches zero. The denitrification equations also have a key term for
dissolved oxygen. However, this term is designed to reduce the denitrification rate
as a function of decreasing dissolved oxygen concentration above zero. However,
in the benthic layer, where anaerobic conditions always exist, denitrification is
always assumed to occur.
In AQUATOX, 73 the nitrogen cycle is modeled with two state variables:
ammonium and nitrate. The atmospheric deposition of nitrate and biota excretion
of ammonia are two nitrogen sources not modeled in WASP. Also in AQUATOX
during the decomposition of detritus only inorganic nitrogen is released, while
in EUTRO5 both inorganic and organic nitrogen are formed. Furthermore, three
types of algae (blue-green, green, and diatoms) plus macrophytes are modeled
in AQUATOX, and each of them has a different nitrogen uptake rate. For green
algae and diatoms, the Redfield ratio may be used. Blue-green algae can supply
nitrogen by fixation, and this is accounted for by smaller nitrogen uptake ratios
in the model. Algae can be either phytoplankton or periphyton. Phytoplanktons
are subject to sinking and washout, while periphytons are subject to substrate
limitation and scour by currents because they are fixed in space. Macrophytes
supply their nitrogen from sediment; therefore, nutrient limitation of macrophytes
is not considered in the model. Macrophyte ammonium excretion to the water
column is modeled.
Nitrification and denitrification processes described in AQUATOX have temper-
ature and dissolved oxygen correction terms similar to those in EUTRO5; however,
a pH correction term is also included in AQUATOX.
The model developed by Park and Kuo 69 uses three state variables—organic
nitrogen, ammonium, and nitrite-nitrate-for modeling the nitrogen cycle. Similar
to the EUTRO5, phytoplankton is considered as a single population biomass. The
mineralization rate is modeled as a function of the organic nitrogen concentration.
In EUTRO5 mineralization rate is modeled as a function of both the organic nitrogen
and phytoplankton concentrations. In this model, the nitrification process description