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A. Collin et al.
climate change, rising sea temperatures and acidification, disease, pollution, direct
destruction, sedimentation, unsustainable fishing practices (Hoegh-Guldberg 1999;
Lesser 2004; Wilkinson 2008).
In response to disturbances, the external areas of coral reefs suffer from large
temporal variations (Hughes 1989; Connell et al. 1997; Ninio et al. 2000) both in
terms of quantity (coral cover) and qualitatively (coral species richness). This high
variability is confirmed by observations made on the monitoring network Polynesia
mana. For almost two decades, the network has collected and analyzed the health
status of the outer reefs in 15 islands of Eastern and Western Polynesia in the South
Pacific. Variations are generally correlated with natural disturbances both physical
(cyclones or strong waves) and biological (outbreak of predator Acanthaster planci).
The direct influence of warmer waters is still a minor threat, in this context, but
bleaching events inducing moderate to low coral mortality, however, were reported
in French Polynesia in 1992, 2002 and 2003 (Salvat 1992; Adjeroud et al. 2009).
The method used to detect changes in health status in the context of this monitoring is the Point Intercept Quadrat conventionally used in phytosociology, and
then adapted to benthic studies (Loya 1978). Like other similar methods (e.g. linear
transect), it provides, with appropriate sampling strategies, accurate results usable
for relevant spatio-temporal reef comparisons. This type of method is dedicated to
sampling units at the meter or decameter scale and requires discrete investigations
such as snorkelling or scuba diving. This method therefore constrains both the size of
the entities studied and the geographical representativeness of coastal observations,
not exceeding the linear kilometer. However, quantitative and qualitative benthic
variables characterizing a linear outer reef exhibit spatial homogeneity that does not
exceed the scale of tens of kilometers. In addition, within the same coastline, the
inter-site variability of coral substrates may become significant (e.g. Murdoch and
Aronson 1999). Of special importance, these methods do not take into account the
three-dimensional aspects of sampled entities and therefore do not reveal the landscape changes of structural species, living or dead, which shape most of the reef
habitats (Connell 1978). Understanding the variability between sites and mapping
the biological morphology of outer reefs over large areas is thereby hardly achievable by manual fieldwork, by definition punctual and disparate. On the other hand,
hydrographic and oceanographic campaigns by ship may be dangerous or impossible over shallow reefs. Recent advances in remote sensing can partially address
these challenges. Satellite sensors endowed with very high spatial resolution (VHR)
now provide spectral information at a sub-meter accuracy, continuously over several
square kilometers.
The present work aims to demonstrate, using Tiahura case study (Moorea, French
Polynesia), how the VHR spaceborne remote sensing (QuickBird-2 and WorldView2) can be used to supplement the observations obtained with conventional in situ
methods implemented in the monitoring network Polynesia mana, so that the
temporal variations of parameters characterizing the coral communities and their
architecture can be assessed. The long term ambitions of this work are to provide a
tool for finely evaluating the health of Polynesian outer reefs on a very large scale,
compounded with conventional methods of benthic surveys.
A. Collin et al.
climate change, rising sea temperatures and acidification, disease, pollution, direct
destruction, sedimentation, unsustainable fishing practices (Hoegh-Guldberg 1999;
Lesser 2004; Wilkinson 2008).
In response to disturbances, the external areas of coral reefs suffer from large
temporal variations (Hughes 1989; Connell et al. 1997; Ninio et al. 2000) both in
terms of quantity (coral cover) and qualitatively (coral species richness). This high
variability is confirmed by observations made on the monitoring network Polynesia
mana. For almost two decades, the network has collected and analyzed the health
status of the outer reefs in 15 islands of Eastern and Western Polynesia in the South
Pacific. Variations are generally correlated with natural disturbances both physical
(cyclones or strong waves) and biological (outbreak of predator Acanthaster planci).
The direct influence of warmer waters is still a minor threat, in this context, but
bleaching events inducing moderate to low coral mortality, however, were reported
in French Polynesia in 1992, 2002 and 2003 (Salvat 1992; Adjeroud et al. 2009).
The method used to detect changes in health status in the context of this monitoring is the Point Intercept Quadrat conventionally used in phytosociology, and
then adapted to benthic studies (Loya 1978). Like other similar methods (e.g. linear
transect), it provides, with appropriate sampling strategies, accurate results usable
for relevant spatio-temporal reef comparisons. This type of method is dedicated to
sampling units at the meter or decameter scale and requires discrete investigations
such as snorkelling or scuba diving. This method therefore constrains both the size of
the entities studied and the geographical representativeness of coastal observations,
not exceeding the linear kilometer. However, quantitative and qualitative benthic
variables characterizing a linear outer reef exhibit spatial homogeneity that does not
exceed the scale of tens of kilometers. In addition, within the same coastline, the
inter-site variability of coral substrates may become significant (e.g. Murdoch and
Aronson 1999). Of special importance, these methods do not take into account the
three-dimensional aspects of sampled entities and therefore do not reveal the landscape changes of structural species, living or dead, which shape most of the reef
habitats (Connell 1978). Understanding the variability between sites and mapping
the biological morphology of outer reefs over large areas is thereby hardly achievable by manual fieldwork, by definition punctual and disparate. On the other hand,
hydrographic and oceanographic campaigns by ship may be dangerous or impossible over shallow reefs. Recent advances in remote sensing can partially address
these challenges. Satellite sensors endowed with very high spatial resolution (VHR)
now provide spectral information at a sub-meter accuracy, continuously over several
square kilometers.
The present work aims to demonstrate, using Tiahura case study (Moorea, French
Polynesia), how the VHR spaceborne remote sensing (QuickBird-2 and WorldView2) can be used to supplement the observations obtained with conventional in situ
methods implemented in the monitoring network Polynesia mana, so that the
temporal variations of parameters characterizing the coral communities and their
architecture can be assessed. The long term ambitions of this work are to provide a
tool for finely evaluating the health of Polynesian outer reefs on a very large scale,
compounded with conventional methods of benthic surveys.