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A. Collin et al.
displays a diversity of the three bands obviously superior to this of the outer reef,
further offshore, as well as the pass. This result reflects the variability of the spectral
reflectance in shallow water, which is, in agreement with water depth, gradually
replaced by a dominance of the blue-green, then the blue, thus the diminution and
disappearance of the entropy. However it is possible to detect changes between
the two dates on the barrier reef which appears less diverse in 2010 than in 2006
in the southwestern study area. Moreover the systematic analysis of the transect
tends to establish a manifest differentiation between the two models. Both averages
and variances of the entropy significantly diverge (0.31 and 0.05, p-value < 0.01;
2 × 10
−2 and 4 × 10
−4 , p-value < 0.01, respectively). From 2006 to 2010, the
color diversity has decreased by a factor six, and its variability has been strongly
eroded. It is also noteworthy that the 2010 curve decreases in average and variability
from west to east, whilst this trend is not detectable in 2006.
16.4 Discussion
The coral cover and the structural complexity of Tiahura outer reef have experienced
two major disturbances between 2005 and 2010: the invasion of the predator Acanthaster planci and the wake of the cyclone Oli (February 2010). The frequency of
such recurring disturbances on the Polynesian outer reef is in the order of 10 to 25
years (Adjeroud et al. 2005, 2009) and varies according to the type (high volcanic
island or atoll) and the geographical location of the island or the site.
16.4.1 Coral Response to Disturbances
The synergy of the three methods (in situ and VHR satellite imagery) allows gaining
more insight into the response of coral to both the invasion of A. planci and the cyclone
Oli. From 2005 to 2009, the coral cover of Tiahura site surveyed by photo-quadrats
has experienced an annual decrease of 10 % until reaching a complete absence of
coral cover. This rapid fall may be due to the demographic outbreak of the carnivorous starfish. The impact of the swell generated by the cyclone Oli on the coral
cover was not significant since it was already close to 0 % before this disturbance
happened. The satellite products we derived such as the bathymetry and the entropy
of the bottom albedo confirm the rapid fall of the coral cover by providing some further explanatory mechanisms. From 2006 to 2010, two trends were revealed by the
remote sensing approach: the erosion of the habitat and the mitigation of the colour
diversity. Intuitively, the erosion can be attributed to the wake of the cyclone that
produced a powerful and ample swell likely to break the tri-dimensional structure of
coral colonies which can explain the smoothing observed throughout the evolution
of the mapping products. The benthos discolouration may be related to the mechanical destruction of the colonies by the cyclone but may also be the consequence of
the predation by A. planci. These two phenomena drive the disappearance of the
A. Collin et al.
displays a diversity of the three bands obviously superior to this of the outer reef,
further offshore, as well as the pass. This result reflects the variability of the spectral
reflectance in shallow water, which is, in agreement with water depth, gradually
replaced by a dominance of the blue-green, then the blue, thus the diminution and
disappearance of the entropy. However it is possible to detect changes between
the two dates on the barrier reef which appears less diverse in 2010 than in 2006
in the southwestern study area. Moreover the systematic analysis of the transect
tends to establish a manifest differentiation between the two models. Both averages
and variances of the entropy significantly diverge (0.31 and 0.05, p-value < 0.01;
2 × 10
−2 and 4 × 10
−4 , p-value < 0.01, respectively). From 2006 to 2010, the
color diversity has decreased by a factor six, and its variability has been strongly
eroded. It is also noteworthy that the 2010 curve decreases in average and variability
from west to east, whilst this trend is not detectable in 2006.
16.4 Discussion
The coral cover and the structural complexity of Tiahura outer reef have experienced
two major disturbances between 2005 and 2010: the invasion of the predator Acanthaster planci and the wake of the cyclone Oli (February 2010). The frequency of
such recurring disturbances on the Polynesian outer reef is in the order of 10 to 25
years (Adjeroud et al. 2005, 2009) and varies according to the type (high volcanic
island or atoll) and the geographical location of the island or the site.
16.4.1 Coral Response to Disturbances
The synergy of the three methods (in situ and VHR satellite imagery) allows gaining
more insight into the response of coral to both the invasion of A. planci and the cyclone
Oli. From 2005 to 2009, the coral cover of Tiahura site surveyed by photo-quadrats
has experienced an annual decrease of 10 % until reaching a complete absence of
coral cover. This rapid fall may be due to the demographic outbreak of the carnivorous starfish. The impact of the swell generated by the cyclone Oli on the coral
cover was not significant since it was already close to 0 % before this disturbance
happened. The satellite products we derived such as the bathymetry and the entropy
of the bottom albedo confirm the rapid fall of the coral cover by providing some further explanatory mechanisms. From 2006 to 2010, two trends were revealed by the
remote sensing approach: the erosion of the habitat and the mitigation of the colour
diversity. Intuitively, the erosion can be attributed to the wake of the cyclone that
produced a powerful and ample swell likely to break the tri-dimensional structure of
coral colonies which can explain the smoothing observed throughout the evolution
of the mapping products. The benthos discolouration may be related to the mechanical destruction of the colonies by the cyclone but may also be the consequence of
the predation by A. planci. These two phenomena drive the disappearance of the