Digital analysis of multispectral airborne imagery of coral reefs

 The digital airborne sensor, CASI (Compact Airborne Spectrographic Imager) has considerable potential for mapping marine habitats. Here we present an account of one of the first coral reef applications. The CASI was flown over reefs of the Turks and Caicos Islands (British West Indies) and set to view 1 m pixels in 8 spectral bands. In addition, reef habitats were sampled in situ by visual assessment of percent cover in 1 m quadrats. Seagrass standing crop was assessed using a calibrated visual scale. Benthic habitats were classified using hierarchical cluster and similarity percentage analyses of the field survey data. Two levels of habitat discrimination were assessed: a coarse level (corals, algae, sand, seagrass) and a fine level which included nine reef habitats. Overall accuracies of CASI-derived habitat maps were 89% and 81% for coarse and fine levels of habitat discrimination, respectively. Accuracies were greatest once CASI data had been processed to compensate for variations in depth and edited to take account of generic patterns of reef distribution. These overall accuracies were significantly (P<0.001) better than those obtained from satellite imagery of the same site (Landsat MSS, Landsat TM, SPOT XS, SPOT Pan, merged Landsat TM/SPOT Pan). Results from CASI were also significantly better than those from interpretation of 1:10 000 colour aerial photographs of reefs in Anguilla (Sheppard et al. 1995). However, the studies may not have been entirely comparable due to a disparity in the areas mapped. Accepted: 26 May 1997     

Is CITES an effective tool for monitoring trade in corals?

 Despite a legal obligation to do so there is a widespread failure to record coral species on CITES trade permits. The complexity of coral taxonomy and the volume of coral specimens in trade mean that in practice this obligation will never be met, and so it may be sensible to ensure that the identification of genera is as accurate as possible. The accuracy of non-specialists in identifying coral genera using four existing guides was tested experimentally. Only three of ten genera were identified to more than 67% accuracy and none of the guides was ideally suited for coral specimens in trade. A guide designed specifically for this purpose and a change in emphasis under CITES to genera rather than species would therefore improve the effectiveness of CITES in monitoring the international coral trade. Accepted: 11 August 1999     

Spectral discrimination of coral reef benthic communities

Effective identification and mapping of coral reef benthic communities using high-spatial and -spectral resolution digital imaging spectrometry requires that the different communities are distinguishable by their spectral reflectance characteristics. In Kaneohe Bay, Oahu, Hawaii, USA, we collected in situ a total of 247 spectral reflectances of three coral species (Montipora capitata, Porites compressa, Porites lobata), five algal species (Dictyosphaeria cavernosa, Gracilaria salicornia, Halimeda sp., Porolithon sp., Sargassum echinocarpum) and three sand benthic communities (fine-grained carbonate sand, sand mixed with coral rubble, coral rubble). Major reflectance features were identified by peaks in fourth derivative reflectance spectra of coral (at 573, 604, 652, 675 nm), algae (at 556, 601, 649 nm) and sand (at 416, 448, 585, 652, 696 nm). Stepwise wavelength selection and linear discriminant function analysis revealed that spectral separation of the communities is possible with as few as four non-contiguous wavebands. These linear discriminant functions were applied to an airborne hyperspectral image of a patch reef in Kaneohe Bay. The results demonstrate the ability of spectral reflectance characteristics, determined in situ, to discriminate the three basic benthic community types: coral, algae and sand. Accepted: 12 January 2000     

Large-scale bleaching of corals on the Great Barrier Reef

 The Great Barrier Reef (GBR) experienced its most intensive and extensive coral bleaching event on record in early 1998. Mild bleaching commenced in late January and intensified by late February/early March 1998. Broad-scale aerial surveys conducted of 654 reefs (∼23% of reefs on the GBR) in March and April 1998, showed that 87% of inshore reefs were bleached at least to some extent (>1% of coral cover) compared to 28% of offshore (mid- and outer-shelf) reefs. Of inshore reefs 67% had high levels of bleaching (>10% of coral) and 25% of inshore reefs had extreme levels of bleaching (>60% of coral). Fewer offshore reefs (14%) showed high levels of bleaching while none showed extreme levels of bleaching. Ground-truth surveys of 23 reefs, which experienced bleaching in intensities ranging from none to extreme, showed that the aerial survey data are likely to be underestimates of the true extent and intensity of bleaching on the GBR. The primary cause of this bleaching event is likely to be elevated sea temperature and solar radiation, exacerbated by lowered salinity on inshore and some offshore reefs in the central GBR. Accepted: 30 July 1998     

Bacterial communities inhabiting the healthy tissues of two Caribbean reef corals: interspecific and spatial variation

Bacterial communities inhabiting healthy tissues of the reef-building corals Diploria strigosa and Montastraea annularis were evaluated across a human-induced environmental gradient along the southern coast of Curaçao, Netherlands Antilles. Variations in bacterial communities inhabiting coral tissues were determined using terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes, and the ¹⁵N value of coral tissue was used to assess the relative amount of human contaminants at each reef locality. Bacterial communities of D. strigosa were more variable than M. annularis, but there were no systematic differences in the populations of healthy M. annularis and D. strigosa. The ¹⁵N value of coral tissues showed as much as a 1.5 increase in the impacted versus the non-impacted localities. While M. annularis showed no significant variation in bacterial community structure due to local reef conditions, the bacterial communities of D. strigosa showed dramatic shifts in community structure. The most abundant bacterial taxa inhabiting D. strigosa display increased dominance at impacted localities. By linking variations in microbial communities with an understanding of variations in local environmental conditions, this study provides a means of assessing potential factors that may impact the microbial habitat of coral tissues as well as overall reef health.     

A 200-year coral stable oxygen isotope record from a high-latitude reef off Western Australia

 A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition^*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral δ¹⁸O. At 29′S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral δ¹⁸O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral δ¹⁸O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral δ¹⁸O demonstrate a high variability in spectral composition through time. Oscillations in the 5–7-y, 14–15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral δ¹⁸O suggests a rise in SST by 0.6 ′C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4 ′C since AD 1795 is inferred from the coral record. Accepted: 3 July 1998     

Spongivory on Caribbean reefs releases corals from competition with sponges

Competition for space is an important process on tropical coral reefs. Few studies have examined the role sponges play in community structure despite the fact that many sponges are competitively superior to reef-building corals in space acquisition. Surveys conducted throughout the Florida Keys indicated that Chondrilla nucula was involved in about 30% of all coral-sponge interactions; this sponge has also been observed in 40–50% of coral-sponge interactions on other Caribbean reefs. C. nucula is also the top prey item of the Hawksbill turtle, and among the preferred prey of several spongivorous fish. I examined how predation influenced sponge competitive abilities (particularly those of C. nucula), and whether this type of indirect effect had important consequences for community dynamics in the Florida Keys. Exclusion of sponge predators (primarily angelfish) resulted in increased sponge overgrowth, with a subsequent greater loss of coral cover, compared to uncaged pairwise interactions. When caged, the corals Dichocoenia stokesii and Siderastrea sideraea lost significantly greater surface area and number of polyps to the sponge C. nucula compared to uncaged interactions. For caged interactions involving the sponge Ectyoplasia ferox, there was a trend for greater loss of  S. sideraea surface area and polyps compared to uncaged interactions. Predation had a greater affect on C. nucula than on any of the other sponges examined. Predator exclusion experiments performed with naturally occurring coral-sponge interactions demonstrated a significant decrease in total coral cover compared to uncaged controls. It is proposed that indirect effects arising from spongivory (especially consumption of  C. nucula) may have large community consequences. Species diversity on Caribbean reefs may be maintained, at least in part, by spongivores. Received: 28 January 1998 / Accepted: 22 June 1998     

Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef

 Experimental studies of the upper thermal limits of corals from Orpheus Island, an inshore reef in the central Great Barrier Reef, show that Acropora formosa has a 5-day 50%-bleaching threshold of between 31 and 32 °C in summer, only 2 to 3 °C higher than local mean summer temperatures (29 °C). Summer bleaching thresholds for Pocillopora damicornis and A. elseyi were 1 °C higher (between 32 and 33 °C). The winter bleaching threshold of Pocillopora damicornis was 1 °C lower than its summer threshold, indicating that seasonal acclimatisation may take place. This seasonal difference raises the possibility that at least some corals may be capable of short-term thermal acclimatisation. Neither P. damicornis nor A. elseyi showed habitat-specific (reef flat versus reef slope) differences in bleaching thresholds. Further, colonies of P. damicornis collected from sites 3 km apart also showed no difference in bleaching threshold despite populations of this species responding differently at these two sites during a natural bleaching event. The bleaching thresholds determined in this study are best considered as the maximum tolerable temperatures for local populations of these species because they were determined in the absence of additional stressors (e.g. high light) which often co-occur during natural bleaching events. We consider the 5-day 50% bleaching thresholds determined in these experiments to be fair indicators of upper thermal limits, because >50% of a sample population died when allowed to recover in situ. We found a delay of up to a month in the bleaching response of corals following thermal stress, a result that has implications for identifying the timing of stressful conditions in natural bleaching events. Accepted: 26 May 1999     

Sea surface temperatures and coral reef bleaching off La Parguera, Puerto Rico (northeastern Caribbean Sea)

 Much recent attention has been given to coral reef bleaching because of its widespread occurrence, damage to reefs, and possible connection to global change. There is still debate about the relationship between temperature and widespread bleaching. We compared coral reef bleaching at La Parguera, Puerto Rico to a 30-y (1966–1995) record of sea surface temperature (SST) at the same location. The last eight years of the La Parguera SST record have all had greater than average maximum temperatures; over the past 30 y maximum summer temperature has increased 0.7 °C. Coral reef bleaching has been particularly frequent since the middle 1980s. The years 1969, 1987, 1990, and 1995 were especially noteworthy for the severity of bleaching in Puerto Rico. Seven different annual temperature indices were devised to determine the extent to which they could predict severe coral bleaching episodes. Three of these, maximum daily SST, days >29.5 °C, and days >30 °C predict correctly the four years with severe bleaching. A log-log linear relationship was found between SST and the number of days in a given year above that SST at which severe coral beaching was observed. However, the intra-annual relationship between temperature and the incidence of bleaching suggests that no one simple predictor of the onset of coral bleaching within a year may be applicable. Accepted: 17 March 1998     

Food ration and condition affect early survival of the coral reef damselfish, Stegastes partitus

The supply of larvae is a major determinant of population and community structure in coral reef fishes. However, spatial and temporal variation in condition (i.e. quality) of potential recruits, as well as their density (i.e. quantity), may influence survival and growth of juveniles. We conducted an experiment to test whether recent feeding history could affect growth, condition and post-recruitment survival in a Caribbean damselfish, Stegastes partitus. Fish were collected soon after settlement, and fed either low or high rations in aquaria for 7 days. Fish fed the high ration grew faster in aquaria and were in a better condition (higher total lipids and Fulton’s condition factor) at the end of the feeding period. Subsequently, we released 50 fish in 25 pairs (one fish subjected to low rations, the other to high rations) on a Bahamian coral reef and monitored survival for 10 days. Survivorship of high-ration fish was double that of low-ration fish (80 vs 40% over 10 days). However, low-ration fish that survived 10 days were of similar condition and grew at similar rates to high-ration fish, suggesting that short-term ration differences may not persist in surviving fish. Laboratory experiments showed that low-ration fish were taken by piscivorous fishes before high-ration fish, indicating that differential predation may account for survival differences. This study highlights the potential of feeding history and condition to affect the relationship between patterns of larval arrival at reefs, and subsequent juvenile and adult population densities. Received: 1 March 1999 / Accepted: 15 July 1999     

Nocturnal orientation to reefs by late pelagic stage coral reef fishes

 The nocturnal orientation behaviour of the late pelagic stages of two reef fish families (Apogonidae and Pomacentridae) was examined using behavioural cages deployed in the field. The behavioural cages enabled the fish to choose between swimming towards or away from the reef in response to natural cues. Overall, 55% of fish displayed a choice in the experiments, however, the proportion varied between the two families, with 67% of pomacentrids and 27% of apogonids displaying a choice. In both families, of the fish which displayed a choice, the proportion of fish swimming towards the reef was significantly greater than 50%, as random movement would predict (64% of pomacentrids and 67% of apogonids swam towards the reef). This proportion did not vary significantly among four field sites with different current regimes and geographic locations. The results suggest that the late pelagic stages of reef fish display nocturnal orientation behaviour, possibly in response to sound, which may aid in their settlement on reefs. Accepted: 18 August 1997     

Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals : evidence for photoinhibition and photoprotection

 Diurnal patterns of photoinhibition have been identified in seven species of shallow water reef corals from the Andaman Sea, off the west coast of Thailand, using pulse amplitude fluorometry. Photochemical efficiency (F_v/F_m) and quantum yield (ΔF/F_m∑) of symbiotic dinoflagellates within the corals declined after dawn to reach a minimum between midday and early afternoon, recovering to former dawn levels by early evening. Parallel studies on the xanthophylls diadinoxanthin (Dn) and diatoxanthin (Dt), and their inter-conversion, also revealed a strong diurnal pattern as well as inverse correlations between the xanthophyll ratio Dt/(Dn+Dt) and F_v/F_m and ΔF/F_m′. These findings suggest a photoprotective function for these pigments. Accepted: 18 March 1999     

First report of folliculinid ciliates affecting Caribbean scleractinian corals

This is the first report of a ciliate of the genus Halofolliculina infecting hard coral species of six families (Acroporidae, Agaricidae, Astrocoeniidae, Faviidae, Meandrinidae and Poritidae) and milleporids in the Caribbean. Surveys conducted during 2004–2005 in Venezuela, Panama and México confirmed that this ciliate affects up to 25 scleractinian species. The prevalence of this ciliate at the coral community level was variable across sites, being most commonly found at Los Roques, Venezuela, and at Bocas del Toro, Panama (prevalence 0.2–2.5%), but rarely observed in the Mexican Caribbean. Ciliates were more prevalent within populations of acroporids (Acropora palmata, Acropora cervicornis and Acropora prolifera) in Los Roques. Recent observations also corroborate the presence of these ciliates in Curacao and Puerto Rico. Our observations indicate that ciliates affecting corals have a wider distribution than previously thought, and are no longer exclusively found in the Indo-Pacific and Red Sea.     

Direct measurements of carbon and carbonate export from a coral reef ecosystem (Moorea Island , French Polynesia)

The export of carbon and carbonate from coral reefs was investigated through a multidisciplinary investigation of the hydrological, geochemical, sedimentological and biological features of Tiahura reef on the northwestern coast of Moorea Island (French Polynesia). The hydrology of the fore-reef is characterised by prevailing longshore western currents and a strong thermocline. As revealed by turbidity structures (benthic and intermediate nepheloid layers) and by the amount of particles collected by near-bottom sediment traps, horizontal and downslope advections of particles dominate over offshore vertical transport. The exported material is rich in carbonate (ca. 80%) and poor in organic matter (ca. 4%). Sedimentation rates at 430 m depth, i.e. definitive export, reached 209.6 mg m^-2 d^-1 (dry weight). Estimates of carbon and carbonates export for Tiahura reef also reported here represent respectively 47% and 21% of the organic and inorganic carbon produced within the reef.     

Patch dynamics of coral reef macroalgae under chronic and acute disturbance

The patch dynamics (colonisation rate, growth rate, and extinction rate) are quantified for two dominant species of macroalgae on a Caribbean forereef in Belize: Lobophora variegata (Lamouroux) and Dictyota pulchella (Hörnig and Schnetter). Measurements were taken on time scales of days, weeks, months, and years during which three hurricanes occurred. All patches were followed on naturally occurring ramets of dead Montastraea annularis. The first hurricane (Mitch) caused massive coral mortality and liberated space for algal colonisation. The cover of Lobophora increased throughout the study and herbivores did not appear to limit its cover within a 4 year time frame. In contrast, the cover of D. pulchella fluctuated greatly and showed no net increase, despite an increase in parrotfish biomass and settlement space. Variation in the overall percent cover of an alga is not indicative of the underlying patch dynamics. The steady rise in the cover of Lobophora took place despite a high turnover of patches (12–60% of patches per year). The patch dynamics of Dictyota were slower (7–20%), but a greater patch density and threefold higher lateral growth rate led to greater fluctuations in total cover. The dynamics of algal patches are size-specific such that larger patches are less likely to become extinct during hurricanes.     

Review of coral reef ecosystem remote sensing

Coral reef communities face unprecedented pressures at local, regional and global scales as a consequence of climate change and anthropogenic disturbance. Remote sensing, from satellites or aircraft, is possibly the only means to measure the effects of such stresses at appropriately large spatial scales. In the past 30 years, remote sensing of coral reefs has made rapid progress. However, the current technology is still not mature enough to monitor complicated coral reef ecosystems. Compared with foreign research in this field, our work lags far behind. There are still deficiencies in many aspects, such as basic data collection, theoretical research and platform construction. In our nation, it is even unclear how coral reefs disperse and where they may be unhealthy. In this paper, general characteristics of coral reef ecosystems and spectral features of different reef benthos have been summarized, based initially on a review of relevant literature in recent years. Based on the spectral separability of different reef types or benthos, remote sensing can be used to monitor two aspects of coral reefs: (1) Measurement of the ecological properties of reefs. (2) Health assessment of the coral reef ecosystem. In the first part, optical remote sensing methods are widely used to map reef geomorphology and habitats or biotopes. The investigation of geomorphologic zonation has proven to be one of the most successful applications, as different geomorphologic zones are associated with characteristic benthic community structures and occur at spatial scales of tens to hundreds of meters, they are amenable to remote detection by moderate to high resolution sensors. With more and more attention on the ecological problems of coral reefs, a number of studies have used high resolution sensors to map reef communities. The number of classes distinguishable depends on many factors, including the platforms, resolution (spectral, spatial and temporal resolution) and environmental conditions (water depth, water clarity, surface roughness, etc.). Compared with deep water color remote sensing, or terrestrial remote sensing, three techniques for the measurement of reef ecological properties are examined in this paper: (1) Coral reef classification system using remote sensing. (2) Techniques of sea surface correction and water column correction. (3) Techniques of coral reef information extraction from images. In terms of the complexity of coral reef ecosystems, the current techniques still need further improvement or optimization. In the health assessment of coral reef ecosystems, there are two ways to carry out the monitoring using remote sensing: (1) Monitoring the pigment or symbiotic zooxanthellae contents in corals. (2) Measuring the environmental properties of reefs. The first way is theoretically feasible, but difficult to achieve in practice. Currently, most reef health assessments are carried out by measuring environmental parameters, including sea surface temperature, solar radiation, ultraviolet radiation, water color, wind speed and direction, rainfall, ocean acidification, sea level, etc., of which sea surface temperature has been routinely measured by NOAA to monitor coral bleaching. In addition to the contents above, this article puts forward five main prospects for development in the future: (1) Establishment of a coral reef classification system using remote sensing. (2) Satellite launch for monitoring coral reefs. (3) Theoretical and methodological development. (4) Establishment of a spectral database for different reef benthos. (5) Integrated application of multi-source remote sensing data. It is hoped that the information provided here will be a reference for subsequent similar studies.     

Bathymetry,water optical properties,and benthic classification of coral reefs using hyperspectral remote sensing imagery

The complexity and heterogeneity of shallow coastal waters over small spatial scales provides a challenging environment for mapping and monitoring benthic habitats using remote sensing imagery. Additionally, changes in coral reef community structure are occurring on unprecedented temporal scales that require large-scale synoptic coverage and monitoring of coral reefs. A variety of sensors and analyses have been employed for monitoring coral reefs: this study applied a spectrum-matching and look-up-table methodology to the analysis of hyperspectral imagery of a shallow coral reef in the Bahamas. In unconstrained retrievals the retrieved bathymetry was on average within 5% of that measured acoustically, and 92% of pixels had retrieved depths within 25% of the acoustic depth. Retrieved absorption coefficients had less than 20% errors observed at blue wavelengths. The reef scale benthic classification derived by analysis of the imagery was consistent with the percent cover of specific coral reef habitat classes obtained by conventional line transects over the reef, and the inversions were robust as the results were similar when the benthic classification retrieval was constrained by measurements of bathymetry or water column optical properties. These results support the use of calibrated hyperspectral imagery for the rapid determination of bathymetry, water optical properties, and the classification of important habitat classes common to coral reefs.     

Herbivory by crabs and the control of algal epibionts on Caribbean host corals

Summary A short-term experiment was conducted to examine the relationships among the branching coral Porites porites, algal epibionts, and a facultative crab associate Mithrax sculptus in Belize, Central America. Initial field observations suggested that coral colonies supporting resident crabs generally had lower algal cover than colonies without crabs. The hypothesis was tested that Mithrax significantly depresses host coral algal cover and thereby indirectly affects host survivorship and growth. Crab accessibility to an array of coral colonies, similarly covered with algal epibionts, was manipulated in three treatments. Results strongly support the hypothesis, with significant differences in algal cover (primarily Dictyota spp.) noted among treatments after only one month. Caged heads with crabs included and uncaged natural controls allowing crabs free access averaged less than 10% cover, whereas mean algal cover exceeded 75% where crabs were excluded. The uncaged treatment, in which crabs were allowed free access to Porites heads was not significantly different from the crab inclusion treatment. Collectively, these results demonstrate that under natural conditions, crabs can have pronounced effects on host corals by reducing fouling algal epibionts. Furthermore, these facultative coral associates may have more important, albeit localized effects on Caribbean corals than has been suggested previously.