Environmental Engineering Reference
In-Depth Information
The model was parameterized with data for
Porites astreoides
and
Siderastrea
siderea
for brooders and spawners respectively. Both maturity and reproductive
output were expressed as a function of colony-size and the efflux of larvae from
reefs was quantified from the size-frequency distribution of coral colonies. Coral
recruitment was parametrized with data from an offshore reef in Belize which had
high adult coral cover and high biomass of grazing fish. Coral recruitment in the
model was set to occur at an initial colony size of 1 cm in diameter with the
settlement success being determined by the components of their cell (i.e. rugosity
and algal characteristics). A linear stock-recruitment relationship was created based
on the assumption of high adult coral cover and optimal larval supply. The massive
growth forms of coral colonies were expressed as hemispheres and growth rates
were modelled by linear extension rates of the hemispheres (Table
17.1
).
Mortality rates of corals were also colony-size dependent where whole colony
mortality was generally lower for mature (large) colonies than for smaller ones.
Large colonies were able to overgrow smaller colonies in basic interactions once
colonies had reached the maximum implied size of a cell. Macroalgae were able to
overgrow coral recruits and to cause extensive partial mortality of larger colonies.
Data on hurricane mediated mortality was derived from the impact of Hurricane
Mitch on mature colonies of
M. annularis
in Belize where at least 90% of the
colonies experienced partial-colony mortality. The frequency of hurricanes could
be varied according to geographical area. Since the simulation area of 625 m
2
is
relatively small, the chances that a reef would be either completely destroyed or
missed entirely by a hurricane were high. For this reason the model used the mean
percentage of destroyed reef area for the whole simulation area rather than sub-
dividing it into patches of heavy and light destruction.
Algae were distinguished as either cropped algae (cropped substrata) or macro-
algae. Cropped algae included encrusting coralline red algae, fine filamentous algae
and algal turfs, which were contained within one category because coral recruit-
ment, i.e. coral settlement and post-settlement mortality is associated with all of
these types. If cropped algae were not grazed, spores of macroalgae (here in the
model
Dictyota
spp. and
Lobophora variegata
) developed into a fleshy canopy that
prevented coral settlement. Macroalgal growth progressed by either of the follow-
ing pathways: Cropped algae which were not grazed over the period of 1 year
turned into macroalgae. Once established, macroalgae were able to overgrow
cropped algae in neighbouring cells depending on their relative cover and that of
corals within a von-Neumann-Neighbourhood (see also Table
17.1
). If coral cover
was low, macroalgae could overgrow an area of cropped algae similar in size to the
area they occupied, whereby high coral cover reduced this area by 25%.
Simulations and Results
An a priori sensitivity analyses revealed that initial coral cover, grazing and
hurricane frequency were all important factors influencing coral cover over a period
