DEVELOPMENT OF HYDRODYNAMIC, WATER QUALITY AND EUTROPHICATION MODELS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 158

Model development and assumptions

The model is developed using the phytoplankton module D3D-ECO:BLOOM module which is a
component under the Ecological module of D3D. In respect of the level of detail of the ecological
modules, the phytoplankton module (BLOOM) is the most extensive as it includes several functional
groups and types. BLOOM is a multi-species algae model, based on an optimisation technique that
distributes the available resources in terms of nutrients and light among the algae species (WL, 1991
and 1992; Los and Brinkman, 1988). BLOOM optimises the species composition to obtain the overall
maximum growth rate under the given conditions. A large number of groups and/or species of algae
and even different phenotypes within one species can be considered. BLOOM distinguishes between
three phenotypes: under nitrogen limiting conditions, under phosphorus limiting conditions and under
light limiting conditions. In general, the availability and accuracy of the data needed to determine the
values of the model coefficients as well as the data for model validation limit the reliability of
modeling results. Naturally, this is true for algae modelling as well.

The main objective of the model is to have a better understanding of the nutrients condition in the lake
which is reflected by the CHL-a prediction results, The CHL-a is a main output from the model that is
used as an indicator of eutrophication. Modeling of CHL-a implies the modeling of the main nutrients
group (NH4, NO3, PO4) Total suspended matter (TSM) and the existing algae species in the lake.

The general mass balance for phytoplankton (in the water column) is given in the following equation
for BLOOM module:

Phytoplank ton

Loads Transport Settling Gross primary production

t

(6-7)

Respiratio n Mortality Grazing

Model Input Parameters

An important parameter to set up the BLOOM module is the definition of the dominant algal species
in the water body. The modeled algae species in the case of Lake Edko are diatoms and green algae as
these are the dominant species in the lake water according to (Fathi et al , 2001). The model was
developed based on the work done by LOS et al, 2007. The following assumptions were used in
developing the BLOOM model for Lake Edko. It is assumed that 1 g CHL-aorophyll-a corresponds to
7.5 g N and 0.75 g P in phytoplankton. This corresponds to a g C/CHL-a ratio of 50 and N/C and P/C
ratios of 0.15 and 0.015, respectively. According to Los et al, 2007, since the primary production is
strongly influenced by light availability, and can even become limited if there is too little light, the
calculation of light conditions in water is an important process in the model. The light limitation
function can be based on daily average and depth average conditions. This function is associated with
the critical ambient extinction coefficient which is species specific. The coefficient is derived from
imposed tables that relate production efficiency to ambient light intensity (irradiation). Growth
inhibition may be included in these tables if the radiation is larger than the optimal radiation. These
tables are part of the BLOOM data base. The main stoichiometric coefficients values used in the
model are shown in Table (7-6) . The model inputs are divided into the following sets:

Substances file including all modeled, processes and results parameters

Boundary conditions

Initial conditions

Processes parameters

Stoichiometric coefficients