Changes of coral communities over 35 years: Integrating in situ and remote-sensing data on Saint-Leu Reef (la Réunion, Indian Ocean)
Introduction
Around the world, many coral reefs have become degraded (Pandolfi et al., 2003, Souter and Lindén, 2005, Bruno and Selig, 2007, Obura et al., 2008) as anthropogenic stresses and natural disturbances (cyclones and bleaching) have increased in strength and frequency (Hughes et al., 2003). Coral communities respond with different levels of sensitivity to these stressors (Lirman and Fong, 1997). Current and future management programmes also need to be cognisant of normal successional changes on reefs (Pichon, 1974, Kleypas et al., 2001, Perry et al., 2008) driven by natural catastrophes such as cyclones and flood plumes, whose influence on the state of corals at a particular place reflects both the normal environment and the site-specific history of disturbance (Done, 1995, Connell et al., 1997). Monitoring programmes attempt to capture such changes in the spatial distribution and composition of coral communities, and to understand the dynamics associated with degradation and recovery at different spatial and temporal scales, from the global (e.g. Wilkinson, 2008) to the local (e.g. Done et al., 2007). However, most studies have focused on spatially limited quadrats or transects within a set of particular reefs, and were often established in response to a single event. Consequently, they do not include description of a historical baseline of undisturbed communities and rarely provide synoptic (i.e. reef-scale, regional-scale km2 – 1000 km2), spatially continuous observations, helpful for the development of spatially explicit conservation and management plans (Purkis and Riegl, 2005).
Remotely sensed images and maps derived from them can provide synoptic and long-term observations of reef-top habitats. Medium and high spatial resolution (30 m to 1 m), multi-spectral digital satellite images are now routinely available at moderate cost, on demand, or from a regular collection schedule (Andréfouët and Riegl, 2004, Elvidge et al., 2004). For many places, black and white or color analog aerial photographs provide synoptic views of reefs before the satellite era, often extending back to the 1930s, well before coral reef monitoring programmes expanded in the 1990s. Aerial photographs may be the only source of observations of the state of a coral reef before many major disturbance events (Lewis, 2002). The combination of satellite and airborne images provides long-term and synoptic observations on map reef-top features and detects changes therein (Yamano et al., 2000, Cuevas-Jiménez et al., 2002, Lewis, 2002, Palandro et al., 2003, Purkis and Riegl, 2005).
Manual delineation in photo-interpretation has proven to be a successful mapping technique for characterising patch dynamics in the seagrass environment (McKenzie et al., 2001, Hernandez-Cruz et al., 2006). In the coral reef environment, this technique applied at a fine geomorphological scale allowed detection of structural reef loss in response to a cyclone in the West Indies (Lewis, 2002).
Some changes in reef-top features detected from satellite-image-based maps can be used to infer dynamic characteristics, both ecological and geomorphic. For example, Purkis and Riegl (2005) detected episodes of coral community development in the Arabian Gulf after recurrent coral mass mortality due to bleaching. Palandro et al. (2008) also documented habitat changes in the Florida Keys using remotely sensed imagery and in situ surveys at two different scales. These examples suggest that the right combination of historical imagery and in situ observations at suitable spatial scales and temporal sampling intervals enables mapping of the dynamic reef-top communities and substrata over periods of years to decades. In particular, observations at suitable regular intervals, yearly or less, over a longer term (>10 years), may capture cycles of reef degradation and recovery, or long-term changes of state (McClanahan et al., 2002). This provides essential support information for reef science, monitoring and management.
The goal of this study was to combine available imagery (satellite and airborne) and in situ data to map coral community and other reef-top changes over 35 years on Saint-Leu Reef, La Réunion. More specifically, we aimed to use the imagery to scale up localised in situ observations to characterise the dynamics of an entire shallow reef. The challenges were both technical (processing of disparate imagery across time with limited coincident field-surveys) and thematic (identifying a relevant coral community typology trackable in imagery and compatible with in situ monitoring data).
Section snippets
Study site and disturbance events
La Réunion is a mountainous volcanic island of the Mascarene Archipelago in the South West Indian Ocean (Fig. 1A). Fringing reefs occur along 12 km of the protected coast (Montaggioni and Faure, 1980). These are high-energy reefs, with strong currents running across and along the reef flats, especially on Saint-Leu Reef (Naim, 1989), the focus of this study. Saint-Leu Reef is 5.7 km long and 300 m wide, bordered by a growing residential area, with a fringing reef typical of the Indian Ocean (Naim,
Typology of coral communities
The composition of the 21 habitats ‘a’ to ‘u’ directly derived from the 90 in situ reef surveys are presented in Fig. 3A. The simplified community typology of fifteen coral community categories (including three categories dominated by debris) is presented in Fig. 3A.
Fig. 3B depicts a scatter-plot of the mean and variance of the Red and Blue band digital values of color and texture in the initial 21 habitat classes. The distribution of mean values and spread of the variance in the classes shows
Discussion
The goal of this study was to combine available imagery (satellite and airborne) and in situ data to map coral community and other reef-top changes over 35 years on Saint-Leu Reef, La Réunion Island. More specifically, we aimed to use the imagery to scale up localised in situ observations to characterise the dynamics of the entire shallow reef. Challenges in the mapping exercise were both technical and thematic.
Conclusions
In this study, remotely sensed images and in situ data sets were combined to produce a spatial and temporal picture of the dynamics of Saint-Leu Reef coral communities and other features. The changes in distribution and composition of coral communities were tracked across four decades and contextualised in the light of pulse and press disturbances. The study allowed us to draw a baseline, as far back in time as it was technically possible, providing an improved basis for judging the current
Acknowledgements
We thank G. Pennober, from Université de la Réunion, who provided aerial photographs for 1973, 1978, 1989, 1997, and Quickbird images for 2002, 2006; E. Tessier and B. Cauvin of Réserve Marine de la Réunion who provided the aerial photograph for 2003 and bleaching reports; and L. Bigot from Université de la Réunion who assisted in the field. The manuscript was greatly improved while under review by M. Schleyer's comments. The structure of the manuscript benefited from comments by I. Leiper.
References (51)
- et al.
Medium scale approach (MSA) for improved assessment of coral reef fish habitat
Journal of Experimental Marine Biology and Ecology
(2006) - et al.
Recovery in rubble fields: long-term impacts of blast fishing
Marine Pollution Bulletin
(2003) - et al.
Quantification of two decades of shallow-water coral reef habitat decline in the Florida Keys National Marine Sanctuary using Landsat data (1984–2002)
Remote Sensing of Environment
(2008) - et al.
Modelling coral reef habitat trajectories: evaluation of an integrated timed automata and remote sensing approach
Ecological Modelling
(2007) - et al.
Remote sensing: a key tool for interdisciplinary assessment of coral reef processes
Coral Reefs
(2004) - ARVAM, 2002. Evaluation des impact du cyclone Dina (21–22 Janvier 2002) sur le milieu marin cotier de la Réunion,...
- et al.
Regional decline of coral cover in the Indo-Pacific: timing, extent, and subregional comparisons
Public Library of Science One
(2007) - et al.
Twenty-five years of change in coral coverage on a hurricane impacted reef in Hawai‘i: the importance of recruitment
Coral Reefs
(2007) - et al.
A 30-year study of coral abundance, recruitment, and disturbance at several scales in space and time
Ecological Monographs
(1997) - et al.
Mapping of shallow coral reefs by colour aerial photography
International Journal of Remote Sensing
(2002)
Sources de l'enrichissement en sels nutritifs de l'écosystème récifal à la Réunion: impact des eaux souterraines
Ecological criteria for evaluating coral reefs and their implications for managers and researchers
Coral Reefs
Regional and local variability in recovery of shallow coral communities: Moorea, French Polynesia and central Great Barrier Reef
Coral Reefs
Decadal changes in turbid-water coral communities at Pandora Reef: loss of resilience or too soon to tell?
Coral Reefs
Satellite observation of Keppel Islands (Great Barrier Reef) 2002 coral bleaching using IKONOS data
Coral Reefs
Hurricanes and Caribbean coral reefs: impacts, recovery patterns, and role in long-term decline
Ecology
Documenting decadal spatial changes in seagrass and Acropora palmata cover by aerial photography analysis in Vieques, Puerto Rico: 1937–2000
Bulletin of Marine Science
Recruitment failure, life histories, and long-term decline of Caribbean corals
Ecology
Climate change, human impacts, and the resilience of coral reefs
Science
Rapid phase-shift reversal on a Jamaican coral reef
Coral Reefs
Thematic map accuracy assessment
Defining “coral reef” for the age of global change
International Journal of Earth Sciences
Who are the players on coral reefs and does it matter? The importance of coral taxonomy for coral reef management
Bulletin of Marine Science
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