Environmental Engineering Reference
In-Depth Information
overall simulated area of 20
20 km. This two-dimensional topography increases
linearly from zero to 2 m along the full distance of 20 km in
x
-direction, which
simulates the slow increasing field elevation gradient. The model also takes into
account the seasonal cycle of flooding and drying, which is simulated by a sinusoidal
function, producing variability of the water level over the simulation time. For each
cell, the potential food web structure is the same, although living biota are assumed
present in a cell only when it is flooded, and the fish are present only when water is of
sufficient depth. During the execution of the model, the cells show local heteroge-
neity due to the interaction of different elevation, water stands and biomass levels.
There is seasonal net movement of the fish and crayfish: (a) movement out of
cells that are drying, and (b) movement into cells that are becoming flooded. During
the period of rising water, a fraction of the population in a given cell is allowed to
move up the gradient to an adjacent newly flooded cell, while during falling water
some fraction of a population is able to escape being stranded by moving to cells
that are still flooded. The fish are also allowed to diffuse among flooded cells and
exploit the different habitat resources. To examine how the model components
react to annual changes in the water level, long-term simulations over a period of
10 years are performed. In particular, the model predicts biomasses of fish across
the heterogeneous landscape over time.
Due to page limitations, full equations and parameters of the model are not
listed here, but are documented in Jopp et al. (2010), DeAngelis et al. (2010), and
can be found as supplementary material on the MCED webpage (www.MCED-
ecology.org).
We use this modelling framework to meet the following aims:
l
Describe the resulting temporal pattern of the aquatic food web under the above
mentioned conditions
Investigate how important trophic cascades vary temporally in the model and
what this may imply for the southern Florida fish community.
l
18.3 Model and Simulation Results
We now examine the temporal dynamics of the food web subject to this baseline
hydrological regime, with a water level fluctuation of 0.6 m in amplitude.
Figure
18.5
shows six snapshots in time through the annual hydrologic cycle
along the elevation profile throughout a year for the six higher trophic levels):
invertebrates, fish species 1, 2 and 3, piscivorous fish and crayfish. To achieve
stable patterns, long-term simulations over 10 years were performed, and the last
year, year 10, is displayed and analysed here.
In all sub-figures elevation increases from the left to the right, by about 2 m
over a 20-km distance. The first three panels (Fig.
18.5a-c
) refer to the dry
season, when the water levels are decreasing. The most conspicuous features in
these panels are the pulses of fish and crayfish that are retreating towards the
