Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 103

The sediment depth at which oxygen is exhausted and the redox potential goes
to zero has been defined as the redox potential discontinuity (RPD) layer. The RPD
layer is visible due to its change in color and indicates the zone of habitability for
benthic infauna. The closer this color change appears to the sediment surface, the
lesser is the dissolved oxygen that exists in the sediment porewater. 27,61 Due to
seasonal changes in the metabolic rates of benthic organisms, the boundary between
the oxidized and the reduced sediment often occurs at different depths in the sediment
throughout the year. 30

4.1.5 M ODELING OF N UTRIENT C YCLES

Nutrient cycles are presented in water quality models as a variety of mass balance
equations that contain input, output, and reaction terms (kinetic process equa-
tions). In this section, mainly kinetic process equations in the EUTRO5 module
of WASP5 (Water Quality Analysis Simulation Program) are presented. WASP
is a dynamic compartment model that can be used to analyze a variety of water
quality problems in such diverse water bodies as coastal waters, estuaries, ponds,
streams, lakes, rivers, and reservoirs. This model was developed by the U.S.
EPA 78 and is widely used for water quality modeling in the U.S. and in other
countries. 67,79-89
Due to the complexity of nutrient cycles in aquatic ecosystems, various models
consider different processes and use different simplifications and assumptions.
Therefore, the differences between WASP/EUTRO5 and some other water quality
models that can be used for coastal lagoons are also evaluated and the kinetic
parameters used in various models are presented in tables in this section.
EUTRO5 is a module of WASP applicable to modeling eutrophication. It sim-
ulates eight state variables (ammonium, nitrite/nitrate, orthophosphate, phytoplank-
ton biomass, carbonaceous biochemical oxygen demand (CBOD), dissolved oxygen,
nonliving organic nitrogen, and nonliving organic phosphorus) in the water column
and sediment bed. 78
In EUTRO5, phytoplankton kinetics and dissolved oxygen are considered as
systems interacting with the nutrient cycles. The model characterizes the phytoplank-
ton population as a whole by the total biomass of the phytoplankton present. Min-
imum formulation for nutrient limitation is used, and constant stoichiometry for
algal biomass is assumed. The model does not simulate the kinetics of higher trophic
level organisms such as zooplankton and fishes. Settling terms are not included in
the kinetic equations in the EUTRO5 code, but are coded in the transport sections
of WASP. Four types of model segments (surface water, subsurface water, upper
benthic layer, and lower benthic layer) are defined in WASP. Six flow fields can be
used for calculating the exchanges between segments, which are used for solving
the mass balance equations.

4.1.5.1 Modeling Nitrogen Cycle

Four nitrogen variables are modeled in the EUTRO5 module of the WASP model:
phytoplankton nitrogen (living), organic nitrogen (nonliving), ammonium, and
nitrate. Both ammonium and nitrate are available for phytoplankton growth. However,