Variability Between Bottom Type Categories
Box plots are presented in Figure 2 to compare the variability between and within bottom type categories by
year. The box plot for 1996A in Figure 2 reveals that there is greater variability in reflectance measurements in
bleached coral measurements than the healthy coral and the algae values. The variability in bleached coral
reflectance may be a result of different degrees of bleaching (i.e. different amounts of zooxanthellae present)
such that some samples may have recently been placed under stress while others may have been bleached for
some time and already imperceptibly colonized by macro algae. The differences in median reflectance in the
1996 dataset may be large enough to expect discrimination based on magnitude.
Figure 2. The above box plots compare the SPOT 1 simulated reflectance separately for each substrate group on
a per year basis.
The variability within and between populations of bottom types is low in the 1997 dataset as seen in the box
plot in Figure 2B. The spread is relatively large in the rubble category (n=35) possibly due to the presence of
variable amounts of dead coral debris and rubble resulting in variable rubble reflectance measurements. For the
1997 data set, the median marks are similar between populations, rendering discrimination on the basis of
magnitude of reflectance difficult.
As with the 1997 dataset, the variability within the rubble category (n=33) is considerably larger than the
other bottom type categories in the 1998 box plot (Figure 2C). The median reflectance values for bleached and
healthy corals are nearly identical with very similar spreads as well, which will therefore not enable
discrimination based on SPOT 1 magnitude of reflectance. Likewise, median values for algae-covered surfaces
and rubble are nearly identical, which would also render discrimination difficult.
Finally, the box plot for 1999 (Figure 2D) reveals that the reflectance values of rubble (n=10) are
significantly higher than the other bottom type categories. These measurements suggest that the rubble sampled
in 1999 was not as “contaminated” with debris and macro algae as the measurements in other years and at other
locations. Unfortunately, the medians and spreads of reflectance values of the remaining bottom type categories
are similar and would therefore be difficult to discern on this basis. It is worth noting that the healthy coral
spectra have the “tightest” range indicating low variability within the population.
From this initial qualitative presentation and description of the variability between bottom types, it is
apparent that it is unlikely that discrimination between substrate types could be accomplished simply using
SPOT 1 magnitude of reflectance. There may be other means of discrimination, however, that would allow
confident identification of broad categories of substrate type. The particular SPOT reflectance characteristics
within each bottom type category will therefore be examined next.
Variability Within Bottom Type Categories
Constructing box plots to compare all measurements of a given category of bottom type collected from
1996 to 1999 allows further examination of SPOT 1 reflectance within the broad categories. Figure 3 contains
box plots comparing healthy coral, bleached coral, rubble, algae covered surfaces, and grass separately
including measurements taken from 1996 to 1999.
The median values of healthy coral are similar for all years, although the spread is the greatest in 1999
(n=10) (Figure 3A). Conversely, while the median values of bleached coral reflectance are similar in 1997
(n=17) and 1998 (n=30), the median values are considerably higher and the spread considerably greater in both
1996 (n=9) and 1999 (n=59) (Figure 3B). The median shown in the box plot for rubble shows relatively small
variability in 1997 (n=35) and 1998 (n=33), but is considerably higher in 1999 (n=10) (Figure 3C).
Figure 3. The above box plots compare the SPOT 1 simulated reflectance values for similar substrate types
measured from 1996 to 1999.
The variability in magnitude of SPOT 1 reflectance is relatively small for the algae covered surfaces,
although the spread is larger in 1996 (n=10), 1998 (n=76) and 1999 (n=14) than in 1997 (n=6) (Figure 3D).
Finally, no comparison is possible of sea grass measurements between years/locations, but it is useful to
examine the median and spread of reflectance values in sea grass measurements (n=24) collected in 1999
(Figure 3E). The box plots aid visualization of the variability within the bottom type populations, which appears
to be qualitatively high.
