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Modelling
the relationships between the morphological and
functional characteristics of an atoll
A
modelling project was launched by Michel Claereboudt
using two different approaches based on kriging and
parametric modelling procedures.
The
Kriging approach
Kriging
procedures can be used to predict the internal variables
(IVs) at work on atolls on which no measurement
campaigns have been carried out. They consist of
interpolating the most probable values of
the IVs depending on the most significant forced
variables (FVs) obtained in the data analysis: the
logarithm of the porosity and the logarithm of the area
of the atoll. A kriging algorithm was used in which the
contribution of the points was taken to decrease
exponentially with the distance. Figures 11 à 13 give
as an example the estimates obtained for three IVs on
atolls where the porosity and the surface area were
known.
With
the whole set of IVs modelled using this approach, the
following main patterns emerged:
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Fig.
11
- NO3
+ NO2 concentrations predicted by
interpolating and/or extrapolating from
the concentrations recorded, as a function of
the area of the lagoons and the porosity of
their coral rings. The coordinates of the atolls
on which measurements were carried are marked
with the sign °
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Fig.
12
- PO4
Concentrations predicted by extrapolating
from the concentrations recorded, as a function
of the area of the lagoons and the porosity of
their coral rings. The coordinates of the
atolls on which measurements were carried
out are
marked with the sign °
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Fig.
13
- Predicted particulate organic carbon
concentrations extrapolated from the
concentrations recorded, as a function of the
area of the lagoons and the porosity of their
coral rings. The coordinates of the atolls on
which measurements were carried out are marked
with the sign °
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The
parametric modelling approach
Figure
14 gives a diagram of the typical C and N flow patterns
in an atoll lagoon, showing the main components
resulting from the primary and secondary production
processes and the microbial loop.
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Fig.
14 – Functional
diagram based on the SIMATOLL model. The heavy
arrows give the flows of matter (or energy); and
the fine arrows give the flows of information.
The control parameters (FVs) are given in the
outer ellipses.
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The
growth, consumption and mortality and the flows into and
out of the lagoon were modelled on the basis of data in
the literature and experimental data obtained in studies
other than the Typatoll field studies. The SIMATOLL
model was developed. One of the components of this model
is shown in figure 15.
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Fig.
15
– A detail of the functional scheme of part of
the SIMATOLL model. The double arrows indicate
the flows of matter. The fine arrows indicate
the flows of information. The clouds stand for
inexhaustible stocks. The simulation procedures
make it relatively easy to subdivide the model
into separate sections. The disks give the
values of the parameters and the results of the
flow rate calculations.
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The
subsequent validation stage will consist of comparing
the results of the simulation obtained with the model
and the quantitative data obtained on the stocks and
flows (primary production rates and bacterial production
rates) during the Typatoll field studies, varying the
values of the forced variables associated with the
various atolls. It will then be possible to make some
cautious predictions about the atolls which were not
included in the initial Typatoll field measurement
campaigns.
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: The
development of the model has been at a
standstill since August 1997, since Michel
Claereboudt has been unable to continue working
on this project. We are looking for someone to
take over this work where he left off. The
person appointed for this purpose would have
free access to the Typatoll database and the
SIMATOLL model in its present stage of
development. We would do everything possible to
facilitate contacts with the specialists in
various disciplines who have participated in
collecting and interpreting the data. Candidates
should have a thorough background in the ecology
of aquatic media. If you are interested in
joining this project, please contact dufour@ird.fr.
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References :
Dufour
P., Andréfouët S, Charpy L., Garcia N. (2001) Atolls
morphometry controls nutrient regime in their lagoons. Limnology
and Oceanography 46 (2): 456-461
Dufour
P., Andréfouët S, Charpy L., Garcia N. (2001) Atolls
morphometry controls nutrient regime in their lagoons. Limnology
and Oceanography 46 (2): 456-461
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Atoll_site_