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Coral reefs in Fatu Huku Island, Marquesas Archipelago, French Polynesia
CHAPTER 31
Coral reefs in Fatu Huku Island, Marquesas Archipelago, French Polynesia Antoine Collin1,2, Jean Laporte3, Benjamin Koetz4, Franc¸oisRe´gis Martin-Lauzer3 and Yves-Louis Desnos4 1
EPHE, PSL Research University, Dinard, France 2LabEx CORAIL, Moorea, French Polynesia 3 ARGANS Ltd, Plymouth, United Kingdom 4ESA-ESRIN, Frascati, Italy
Abstract
Fatu Huku is an uninhabited coral reef island in the southeastern Marquesas Islands (9 260 10vS, 138 550 26vW), French Polynesia. The island is 1.3 km2, with a lagoon of 23.7 km2. The deepest part of the lagoon is measured at 36.6 m, while outer reefs recorded a deepest point of 114.7 m. Distinct geomorphic features identified from satellite imagery include passes, channels, single and triple barrier reefs, fringing reefs, ledge, and bulge. In 2016 a digital depth model constructed using water depths and video (R2 5 0.66 and r 5 0.81) was used to calibrate and validate four habitats: deep water, reef flat, coral reefs, and sand. The classification of habitats was implemented with a satisfactory overall accuracy of 0.75. The relationships between benthic habitats and morphometry (bathymetry, slope, hotspots, and ecotones) are discussed.
Keywords: Coral reefs; Marquesas; Pleiades-1 imagery; satellite-derived bathymetry; acoustic; video
Introduction Geomorphic feature type The study site is located in the lagoon of Fatu Huku Island (9 260 10vS, 138 550 26vW), in the Marquesas Archipelago (French Polynesia, Fig. 31.1AC). This archipelago is the northernmost of the four NeogeneQuaternary linear intraplate chains of French Polynesia. It originated from the Marquesas hotspot, an upwelling magma that underlies the Pacific Plate. The northwestern islands are the oldest in the archipelago, and were formed around 5.5 million years ago (e.g., Eiao), while the younger southeastern islands formed around 1.1 million years ago (e.g., Fatu Hiva, Maury et al., 2014). Most Marquesas Islands are composed of shield volcanoes, subject to zonal collapses due to caldera formation. The total
Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00031-2 © 2020 Elsevier Inc. All rights reserved.
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Figure 31.1 Fatu Huku’s (A) location in Marquesas Islands (French Polynesia), (B) natural-colored satellite Pleiades-1 imagery, and (C) scenic view of the central island.
volume of Marquesas volcanic formations was quantified at 42,500 km3 above the 3000 m isobaths (Maury et al., 2014). In contrast to all other Marquesas Islands, which lack coral reefs, Fatu Huku Island has a lagoon with a well-defined reef structure, including an heterogeneous mosaic of passes (north and west), channels (north and southeast of the central island), fringing (around the island), barrier (northwest, northeast, east, and south of the lagoon) and outer reefs over 23.7 km2 (Fig. 31.1B).
Depth range A hydrographic campaign was carried out on February 25 and 26, 2016 using a 200-kHz echosounder, provided with a 10 Hz GNSS geolocation (Lowrance HDS 7 Gen 3, Fig. 31.2A). An array of 78,433 soundings, across eight predetermined transects (red lines in Fig. 31.2B), was characterized by an average depth of 20.7 m, a standard deviation of 8.7 m, a skewness of 3.1 m, and a kurtosis of 22.0 m. The deepest sounding was measured at
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Figure 31.2 (A) Subsurface waterborne device to observe water depths and benthic features. (B) Actual transects with their bathymetric profiles.
114.7 m (36.6 m in the lagoon over the channel south of the central island) over the northwestern outer reef slope, while the shallowest surveyed point of 8.4 m was located over the northwestern barrier reef. The 1.3 km2 island (Fig. 31.1C), is topped by a plateau of an overraised atoll with a peak reaching 361 m. The island receives a low range of precipitation (8001000 mm per year), which limits the amount of sediment entering the lagoon.
Oceanography Subject to microtidal fluctuations (,1 m tidal range), the site is under the predominant influence of the Pacific South Equatorial Current (PSEQ), forming long swells coming from
536 Chapter 31 east, in agreement with the trade winds, blowing from east to west. The site is hydrodynamically characterized by high energy, favoring a significant erosion of the shoreline. Located in the equatorial belt, yearly surface water temperatures range from 27 C to 29 C. However, water temperature measurements were greater than 31 C during the February 2016 survey (Collin et al., 2016). Even though the South Pacific Gyre is characterized by oligotrophic waters, the Marquesas Islands show a constant higher density of phytoplankton, due to the dynamic interactions of the PSEQ and the Marquesas topobathymetry, called the Marquesas Island mass effect (Martinez and Maamaatuaiahutapu, 2004). This results in a turbidity ranging from 0.04 to 0.08 m21 (diffuse attenuation coefficient at 490 nm, https://oceancolor.gsfc.nasa.gov/cgi/l3), and a salinity around 36m.
Naturalness, condition, and trend Fatu Huku Island is uninhabited and has not been modified by local anthropogenic activities nor human threats, such as the introduction of black rats (Rattus rattus) which are a common pest on other Pacific islands. Fatu Huku hosts a small number of the vulnerable Marquesas ground doves (Gallicolumba rubescens) (Meyer et al., 2005) and has recorded no historical bird (Trevino et al., 2007). The area is home to two rare crab species: Chaceon poupini and Naxioides vaitahu The isolation of the archipelago has resulted in a high number of endemic species and consequently all the islands are considered to be of ecological significance (Poupin et al., 2012). The naturalness is considered to be very good (score 8), classified as a coral reef lagoon that is structurally and functionally intact and able to support all dependent species. Due to the remoteness of the island, it has been very little studied, so a trend cannot be determined, but it is assumed to be stable.
General information on data reported in the case study Alongside the acoustic survey, geolocated spectral data has been acquired from the benthopelagos using a high-resolution underwater video camera (GoPro Hero 3 1920 3 1080 with 24 frames per second). A total of 208 minutes of video data was captured from a subsurface device mounted on a small boat (Fig. 31.2A). Spaceborne Pleiades-1 imagery was taken on January 9, 2015 at 19 hours 44 minutes 44 seconds UTC over Fatu Huku. Launched in 2011, Pleiades-1 leverages four spectral bands in the optical electromagnetic spectrum (blue, green, red, and near-infrared) at 2-m spatial resolution. Clouds and their shadows were masked out. Standard geometric and radiometric corrections were applied to the multispectral imagery by first implementing a rational polynomial coefficient-based orthorectification (Fig. 31.3A), and second converting the 12-bit digital number into at-sensor radiance (W m22 st21 µm21) and, in turn, into water-leaving reflectance (unitless, given reflectance 5 radiance/irradiance, Fig. 31.3B, see Collin et al., 2018 for further details).
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Figure 31.3 Preliminary procedures applied to natural-colored Pleiades-1 imagery in order to (A) mask out clouds and their shadows, (B) correct for atmospheric attenuation, and (C) remove the specular reflection (deglinting).
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Geomorphic features and habitats Description of geomorphic features and habitats mapped The lagoon is surrounded by a number of barrier systems, including a triple barrier reef in the southeastern corner. The barriers range in depth from 9 to 18 m. A 30-m-deep channel (2.7 km long and 0.5 km wide) opens to the north. The southern channel, which is 2.0 km long, 1.0 km wide, and 36.6 m deep, shoals at the western end (1617 m deep). This restricts the connection between the channel and the 0.8 km wide western pass. The lagoon also has a number of other features—a gently sloping bulge that extends from the center of the north eastern barrier; a ledge 2.2 km long, 1 km wide, and 1415 m deep that connects the northwestern barrier with the central islands; a number of scattered patch reefs and fringing reefs (Fig. 31.3).
Information on scale, grid size, and data used for mapping The methodology mostly gathers geolocated waterborne acoustic and spectral (video) data with spaceborne spectral wavebands. The latter dataset spans 67.5 km2 (i.e., 4046 3 4195 pixels with 2-m pixel size) and constitutes the spatial reference (WGS84 datum, UTM 7 South projection) to assess Fatu Huku’s geomorphic habitats and related biological communities.
Methods used to derive habitat maps The use of the spaceborne imagery for the mapping of open lagoon areas in ocean can often be impeded by significant swell, resulting in complex light refraction patterns, such as the sun glint. A robust deglinting procedure was therefore applied so that the three visible bands could be corrected for specular reflection (Fig. 31.3C). The standardized method relied on the regression of the visible bands by the near-infrared reflectance over deep water (no benthic reflectance) areas (Hedley et al., 2005): R0vis 5 Rvis 2 avis ðRnir 2 minnir Þ
(31.1)
where R0vis is the deglinted reflectance band vis, Rvis the glinted reflectance band vis, avis the slope of the regression between Rvis, and Rnir reflectance bands. The classification of the benthic composition was achieved using ground truth video frames associated with Pleiades-1 pixels. Out of 104 min across four exploitable transects, 354 frames were retrieved at a rate of one frame every 30 sec and utilized to classify transect pixels into four classes: deep water, reef flat (pavement), coral reefs, and sand pixels (Fig. 31.4A). Each class was randomly subdivided into a training and a validation dataset composed of 100 and 50 pixels (closely extended from the geolocated frame deemed as a seed), respectively. Training pixels were implemented as input data to aid the maximum
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Figure 31.4 (A) Classification map of the four main habitats derived from of the maximum likelihood classifier applied to the deglinted reflectance Pleiades-1 imagery and 100 calibration pixels per class, with (B) associated confusion matrix based on 50 validation pixels per class.
likelihood algorithm establish specific deglinted reflectance signatures for each class and then assign remaining pixels to the most likely class. Validation pixels were used to build a confusion matrix leading to the quantification of the classification process accuracy, based on overall accuracy (Congalton and Green, 2008; Fig. 31.4B).
Biological communities Description of benthos associated with habitats The benthic composition was derived from the high-resolution camcorder mounted on the waterborne device (Fig. 31.2A), following a transect (Fig. 31.5A). Since the camera featured a wide angle and the device was at the subsurface, the size of the geolocated frame varied with the water depth. We therefore conducted an optical correction of the selected frames (N 5 354), taking into account the wide field-of-view (14 mm) and the acoustic depth measurement. The frame coverage was estimated through a virtual grid of 25 evenly distributed squares superimposed on the geometrically corrected frames (Fig. 31.5B). Each
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Figure 31.5 (A) Zoom in the natural-colored Pleiades-1 deglinted reflectance imagery showing a portion of the southernmost transect, geolinked with (B) the subsurface video frames capturing benthic features.
of the 25 subframes was assigned with the visually inspected dominant benthic variable: Porites, Montipora, Acropora, Synarea, brown and green macroalgae, red calcareous algae, coral pavement, coral coarse sand, coral sand, and coral fine sand. Then the whole frame was attributed to the majority benthos and thus classified into one of the four habitat classes. The most extensive coral variable was Porites (43%), followed by Synarea (28%), Montipora (18%), and Acropora (11%). Fleshy brown and green macroalgae were systematically associated with and dominated by coral coarse sand, whereas red coralline algae covaried with the dominant reef pavement. Coral sand was the most prominent sediment feature (59%), followed by coral coarse sand (22%) and coral fine sand (19%). Importantly, the optical limitation of the frame-based categorization could not distinguish microbenthos (such as the cyanobacteria) that certainly covered the reef pavement.
Surrogacy Methods applied to measure statistical relationships between physical surrogates and benthos As a powerful predictor of the coral reef ecology (Collin and Planes, 2012), the bathymetry was mapped using Pleiades-1 visible deglinted bands by implementing the ratio transform
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Figure 31.6 (A) Scatterplot of the observed acoustic and the modeled spectral (Pleiades-1) depths, resulting in the (B) calibration of the digital depth model.
(Stumpf et al., 2003) and calibrating the obtained ratio with acoustic measurements ranging from 8.4 to 24 m depth. The bathymetry was then solved as follows: Z 5s
lnðkRvisA Þ 2i lnðkRvisB Þ
(31.2)
where s and i are the slope and intercept of the best calibration model (Fig. 31.6A) and k is a constant to ensure the positivity of the natural logarithm. The digital depth model was generated with a 2 m 3 2 m pixel size (Fig. 31.6B). The slope and local spatial statistics were derived from the bathymetry in order to quantify their relationships with benthic habitat classes. The slope (in degrees) consists of the variation in bathymetry across a kernel of three pixels. By convention a 0-degree slope is a horizontal plane. The Getis-Ord Gi index identifies hotspots, such as areas of very high or very low values that occur near one another. The Local Geary’s C index identifies edges, such as areas of high variability between a pixel value and its neighboring pixels. Specific morphometric patterns can be deduced from the statistical relationships between bathymetry and its derivatives within the Fatu Huku’s marine habitats (Fig. 31.7). Coral
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Figure 31.7 Relationships between three habitat classes with (A) bathymetry, (B) slope, (C) Getis-Ord Gi index, and (D) Geary’s C index.
reefs constitute the shallowest habitat (16.03 6 0.33 m), characterized with a medium slope average (22.48 degrees 6 1.23 degrees), a medium concentration of similar water depths (0.05 6 9.1023), and a low degree of depth edge (0.59 6 0.12). Those statistical findings underline the fact that coral reefs, that is, living coral colonies, are evenly located and patched on the shallow barrier reefs, slightly curved, and not along abrupt reef ecotones in Fatu Huku. Reef flat (pavement) is located at medium water depth (17.59 6 0.41 m), with the most gentle slope (20.97 degrees 6 1.2 degrees), the lowest concentration of similar depths (0.04 6 1.1022), and a medium level of depth edge (1.11 6 0.15). Those results robustly match the seascape ecology description of reef pavement, namely some large flats (deprived of hotspots) outlined by medium ecotones, and lying below coral reefs. Sandy environments exemplify the deepest habitat (18.34 6 0.27 m), associated with the most abrupt slope (23.97 degrees 6 1.24 degrees), and both the highest concentration of similar depths (0.06 6 1.1022), and the highest score of depth edge (1.39 6 0.1). Sand habitats therefore gather plains and sharp dunes’ hotspots below reef flats (consequently, coral reefs), and exhibit many transitional ecotones.
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Acknowledgments Authors strongly acknowledge the European Space Agency and ARGANS-ACRI for funding the scientific campaign. Catamaran Itemata team and Nancy Lamontagne are warmly thanked for the fieldwork.
References Collin, A., Planes, S., 2012. Enhancing coral health detection using spectral diversity indices from worldview-2 imagery and machine learners. Remote Sens. 4 (10), 32443264. Collin, A., Laporte, J., Koetz, B., Martin-Lauszer, F.R., Desnos, Y.L., 2016. Mapping bathymetry, habitat and potential bleaching using Sentinel-2. In: 13th International Coral Reef Symposium 19-24 June, 51 Remote Sensing of Coral Reefs: Transitioning from Developmental to Operational, pp. 416430. Collin, A., Hench, J.L., Pastol, Y., Planes, S., Thiault, L., Schmitt, R.J., et al., 2018. High resolution topobathymetry using a Pleiades-1 triplet: Moorea Island in 3D. Remote Sens. Environ. 208, 109119. Congalton, R.G., Green, K., 2008. Assessing the Accuracy of Remotely Sensed Data: Principles and Practices. CRC Press. Hedley, J.D., Harborne, A.R., Mumby, P.J., 2005. Simple and robust removal of sun glint for mapping shallowwater benthos. Int. J. Remote Sens. 26 (10), 21072112. Martinez, E., Maamaatuaiahutapu, K., 2004. Island mass effect in the Marquesas Islands: time variation. Geophys. Res. Lett. 31 (18), L18307. Maury, R.C., Guille, G., Guillou, H., Chauvel, C., Legendre, C., Rossi, P., et al., 2014. Geology of Marquesas Islands: collapsed intra-oceanic shield volcanoes generated by an atypical hotspot. Ge´ol. France 1, 111135. Meyer, J.-Y., Thibault, J.-C., Butaud, J.-F., Coote, T., Florence, J. 2005. Sites de conservation importants et prioritaires en Polyne´sie franc¸aise. Contribution a` la Biodiversite´ de Polyne´sie franc¸aise N 13. Sites Naturels d’Inte´reˆt Ecologique V. De´le´gation a` la Recherche, Papeete. Poupin, J., Corbari, L., Pe´rez, T., Cheveldonne´, P., 2012. Deep-water decapod crustaceans studied with a remotely operated vehicle (ROV) in the Marquesas Islands, French Polynesia (Crustacea: Decapoda). Zootaxa 3550, 4360. Stumpf, R.P., Holderied, K., Sinclair, M., 2003. Determination of water depth with high-resolution satellite imagery over variable bottom types. Limnol. Oceanogr. 48, 547556. Trevino, H.S., Skibiel, A.L., Karels, T.J., Dobson, F.S., 2007. Threats to Avifauna on Oceanic Islands. Conserv. Biol. 21 (1), 125132.
Coral reefs in Fatu Huku Island, Marquesas Archipelago, French Polynesia Antoine Collin1,2, Jean Laporte3, Benjamin Koetz4, Franc¸oisRe´gis Martin-Lauzer3 and Yves-Louis Desnos4 1
EPHE, PSL Research University, Dinard, France 2LabEx CORAIL, Moorea, French Polynesia 3 ARGANS Ltd, Plymouth, United Kingdom 4ESA-ESRIN, Frascati, Italy
Abstract
Fatu Huku is an uninhabited coral reef island in the southeastern Marquesas Islands (9 260 10vS, 138 550 26vW), French Polynesia. The island is 1.3 km2, with a lagoon of 23.7 km2. The deepest part of the lagoon is measured at 36.6 m, while outer reefs recorded a deepest point of 114.7 m. Distinct geomorphic features identified from satellite imagery include passes, channels, single and triple barrier reefs, fringing reefs, ledge, and bulge. In 2016 a digital depth model constructed using water depths and video (R2 5 0.66 and r 5 0.81) was used to calibrate and validate four habitats: deep water, reef flat, coral reefs, and sand. The classification of habitats was implemented with a satisfactory overall accuracy of 0.75. The relationships between benthic habitats and morphometry (bathymetry, slope, hotspots, and ecotones) are discussed.
Keywords: Coral reefs; Marquesas; Pleiades-1 imagery; satellite-derived bathymetry; acoustic; video
Introduction Geomorphic feature type The study site is located in the lagoon of Fatu Huku Island (9 260 10vS, 138 550 26vW), in the Marquesas Archipelago (French Polynesia, Fig. 31.1AC). This archipelago is the northernmost of the four NeogeneQuaternary linear intraplate chains of French Polynesia. It originated from the Marquesas hotspot, an upwelling magma that underlies the Pacific Plate. The northwestern islands are the oldest in the archipelago, and were formed around 5.5 million years ago (e.g., Eiao), while the younger southeastern islands formed around 1.1 million years ago (e.g., Fatu Hiva, Maury et al., 2014). Most Marquesas Islands are composed of shield volcanoes, subject to zonal collapses due to caldera formation. The total
Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00031-2 © 2020 Elsevier Inc. All rights reserved.
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Figure 31.1 Fatu Huku’s (A) location in Marquesas Islands (French Polynesia), (B) natural-colored satellite Pleiades-1 imagery, and (C) scenic view of the central island.
volume of Marquesas volcanic formations was quantified at 42,500 km3 above the 3000 m isobaths (Maury et al., 2014). In contrast to all other Marquesas Islands, which lack coral reefs, Fatu Huku Island has a lagoon with a well-defined reef structure, including an heterogeneous mosaic of passes (north and west), channels (north and southeast of the central island), fringing (around the island), barrier (northwest, northeast, east, and south of the lagoon) and outer reefs over 23.7 km2 (Fig. 31.1B).
Depth range A hydrographic campaign was carried out on February 25 and 26, 2016 using a 200-kHz echosounder, provided with a 10 Hz GNSS geolocation (Lowrance HDS 7 Gen 3, Fig. 31.2A). An array of 78,433 soundings, across eight predetermined transects (red lines in Fig. 31.2B), was characterized by an average depth of 20.7 m, a standard deviation of 8.7 m, a skewness of 3.1 m, and a kurtosis of 22.0 m. The deepest sounding was measured at
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Figure 31.2 (A) Subsurface waterborne device to observe water depths and benthic features. (B) Actual transects with their bathymetric profiles.
114.7 m (36.6 m in the lagoon over the channel south of the central island) over the northwestern outer reef slope, while the shallowest surveyed point of 8.4 m was located over the northwestern barrier reef. The 1.3 km2 island (Fig. 31.1C), is topped by a plateau of an overraised atoll with a peak reaching 361 m. The island receives a low range of precipitation (8001000 mm per year), which limits the amount of sediment entering the lagoon.
Oceanography Subject to microtidal fluctuations (,1 m tidal range), the site is under the predominant influence of the Pacific South Equatorial Current (PSEQ), forming long swells coming from
536 Chapter 31 east, in agreement with the trade winds, blowing from east to west. The site is hydrodynamically characterized by high energy, favoring a significant erosion of the shoreline. Located in the equatorial belt, yearly surface water temperatures range from 27 C to 29 C. However, water temperature measurements were greater than 31 C during the February 2016 survey (Collin et al., 2016). Even though the South Pacific Gyre is characterized by oligotrophic waters, the Marquesas Islands show a constant higher density of phytoplankton, due to the dynamic interactions of the PSEQ and the Marquesas topobathymetry, called the Marquesas Island mass effect (Martinez and Maamaatuaiahutapu, 2004). This results in a turbidity ranging from 0.04 to 0.08 m21 (diffuse attenuation coefficient at 490 nm, https://oceancolor.gsfc.nasa.gov/cgi/l3), and a salinity around 36m.
Naturalness, condition, and trend Fatu Huku Island is uninhabited and has not been modified by local anthropogenic activities nor human threats, such as the introduction of black rats (Rattus rattus) which are a common pest on other Pacific islands. Fatu Huku hosts a small number of the vulnerable Marquesas ground doves (Gallicolumba rubescens) (Meyer et al., 2005) and has recorded no historical bird (Trevino et al., 2007). The area is home to two rare crab species: Chaceon poupini and Naxioides vaitahu The isolation of the archipelago has resulted in a high number of endemic species and consequently all the islands are considered to be of ecological significance (Poupin et al., 2012). The naturalness is considered to be very good (score 8), classified as a coral reef lagoon that is structurally and functionally intact and able to support all dependent species. Due to the remoteness of the island, it has been very little studied, so a trend cannot be determined, but it is assumed to be stable.
General information on data reported in the case study Alongside the acoustic survey, geolocated spectral data has been acquired from the benthopelagos using a high-resolution underwater video camera (GoPro Hero 3 1920 3 1080 with 24 frames per second). A total of 208 minutes of video data was captured from a subsurface device mounted on a small boat (Fig. 31.2A). Spaceborne Pleiades-1 imagery was taken on January 9, 2015 at 19 hours 44 minutes 44 seconds UTC over Fatu Huku. Launched in 2011, Pleiades-1 leverages four spectral bands in the optical electromagnetic spectrum (blue, green, red, and near-infrared) at 2-m spatial resolution. Clouds and their shadows were masked out. Standard geometric and radiometric corrections were applied to the multispectral imagery by first implementing a rational polynomial coefficient-based orthorectification (Fig. 31.3A), and second converting the 12-bit digital number into at-sensor radiance (W m22 st21 µm21) and, in turn, into water-leaving reflectance (unitless, given reflectance 5 radiance/irradiance, Fig. 31.3B, see Collin et al., 2018 for further details).
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Figure 31.3 Preliminary procedures applied to natural-colored Pleiades-1 imagery in order to (A) mask out clouds and their shadows, (B) correct for atmospheric attenuation, and (C) remove the specular reflection (deglinting).
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Geomorphic features and habitats Description of geomorphic features and habitats mapped The lagoon is surrounded by a number of barrier systems, including a triple barrier reef in the southeastern corner. The barriers range in depth from 9 to 18 m. A 30-m-deep channel (2.7 km long and 0.5 km wide) opens to the north. The southern channel, which is 2.0 km long, 1.0 km wide, and 36.6 m deep, shoals at the western end (1617 m deep). This restricts the connection between the channel and the 0.8 km wide western pass. The lagoon also has a number of other features—a gently sloping bulge that extends from the center of the north eastern barrier; a ledge 2.2 km long, 1 km wide, and 1415 m deep that connects the northwestern barrier with the central islands; a number of scattered patch reefs and fringing reefs (Fig. 31.3).
Information on scale, grid size, and data used for mapping The methodology mostly gathers geolocated waterborne acoustic and spectral (video) data with spaceborne spectral wavebands. The latter dataset spans 67.5 km2 (i.e., 4046 3 4195 pixels with 2-m pixel size) and constitutes the spatial reference (WGS84 datum, UTM 7 South projection) to assess Fatu Huku’s geomorphic habitats and related biological communities.
Methods used to derive habitat maps The use of the spaceborne imagery for the mapping of open lagoon areas in ocean can often be impeded by significant swell, resulting in complex light refraction patterns, such as the sun glint. A robust deglinting procedure was therefore applied so that the three visible bands could be corrected for specular reflection (Fig. 31.3C). The standardized method relied on the regression of the visible bands by the near-infrared reflectance over deep water (no benthic reflectance) areas (Hedley et al., 2005): R0vis 5 Rvis 2 avis ðRnir 2 minnir Þ
(31.1)
where R0vis is the deglinted reflectance band vis, Rvis the glinted reflectance band vis, avis the slope of the regression between Rvis, and Rnir reflectance bands. The classification of the benthic composition was achieved using ground truth video frames associated with Pleiades-1 pixels. Out of 104 min across four exploitable transects, 354 frames were retrieved at a rate of one frame every 30 sec and utilized to classify transect pixels into four classes: deep water, reef flat (pavement), coral reefs, and sand pixels (Fig. 31.4A). Each class was randomly subdivided into a training and a validation dataset composed of 100 and 50 pixels (closely extended from the geolocated frame deemed as a seed), respectively. Training pixels were implemented as input data to aid the maximum
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Figure 31.4 (A) Classification map of the four main habitats derived from of the maximum likelihood classifier applied to the deglinted reflectance Pleiades-1 imagery and 100 calibration pixels per class, with (B) associated confusion matrix based on 50 validation pixels per class.
likelihood algorithm establish specific deglinted reflectance signatures for each class and then assign remaining pixels to the most likely class. Validation pixels were used to build a confusion matrix leading to the quantification of the classification process accuracy, based on overall accuracy (Congalton and Green, 2008; Fig. 31.4B).
Biological communities Description of benthos associated with habitats The benthic composition was derived from the high-resolution camcorder mounted on the waterborne device (Fig. 31.2A), following a transect (Fig. 31.5A). Since the camera featured a wide angle and the device was at the subsurface, the size of the geolocated frame varied with the water depth. We therefore conducted an optical correction of the selected frames (N 5 354), taking into account the wide field-of-view (14 mm) and the acoustic depth measurement. The frame coverage was estimated through a virtual grid of 25 evenly distributed squares superimposed on the geometrically corrected frames (Fig. 31.5B). Each
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Figure 31.5 (A) Zoom in the natural-colored Pleiades-1 deglinted reflectance imagery showing a portion of the southernmost transect, geolinked with (B) the subsurface video frames capturing benthic features.
of the 25 subframes was assigned with the visually inspected dominant benthic variable: Porites, Montipora, Acropora, Synarea, brown and green macroalgae, red calcareous algae, coral pavement, coral coarse sand, coral sand, and coral fine sand. Then the whole frame was attributed to the majority benthos and thus classified into one of the four habitat classes. The most extensive coral variable was Porites (43%), followed by Synarea (28%), Montipora (18%), and Acropora (11%). Fleshy brown and green macroalgae were systematically associated with and dominated by coral coarse sand, whereas red coralline algae covaried with the dominant reef pavement. Coral sand was the most prominent sediment feature (59%), followed by coral coarse sand (22%) and coral fine sand (19%). Importantly, the optical limitation of the frame-based categorization could not distinguish microbenthos (such as the cyanobacteria) that certainly covered the reef pavement.
Surrogacy Methods applied to measure statistical relationships between physical surrogates and benthos As a powerful predictor of the coral reef ecology (Collin and Planes, 2012), the bathymetry was mapped using Pleiades-1 visible deglinted bands by implementing the ratio transform
Coral reefs using spaceborne imagery
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Figure 31.6 (A) Scatterplot of the observed acoustic and the modeled spectral (Pleiades-1) depths, resulting in the (B) calibration of the digital depth model.
(Stumpf et al., 2003) and calibrating the obtained ratio with acoustic measurements ranging from 8.4 to 24 m depth. The bathymetry was then solved as follows: Z 5s
lnðkRvisA Þ 2i lnðkRvisB Þ
(31.2)
where s and i are the slope and intercept of the best calibration model (Fig. 31.6A) and k is a constant to ensure the positivity of the natural logarithm. The digital depth model was generated with a 2 m 3 2 m pixel size (Fig. 31.6B). The slope and local spatial statistics were derived from the bathymetry in order to quantify their relationships with benthic habitat classes. The slope (in degrees) consists of the variation in bathymetry across a kernel of three pixels. By convention a 0-degree slope is a horizontal plane. The Getis-Ord Gi index identifies hotspots, such as areas of very high or very low values that occur near one another. The Local Geary’s C index identifies edges, such as areas of high variability between a pixel value and its neighboring pixels. Specific morphometric patterns can be deduced from the statistical relationships between bathymetry and its derivatives within the Fatu Huku’s marine habitats (Fig. 31.7). Coral
542 Chapter 31
Figure 31.7 Relationships between three habitat classes with (A) bathymetry, (B) slope, (C) Getis-Ord Gi index, and (D) Geary’s C index.
reefs constitute the shallowest habitat (16.03 6 0.33 m), characterized with a medium slope average (22.48 degrees 6 1.23 degrees), a medium concentration of similar water depths (0.05 6 9.1023), and a low degree of depth edge (0.59 6 0.12). Those statistical findings underline the fact that coral reefs, that is, living coral colonies, are evenly located and patched on the shallow barrier reefs, slightly curved, and not along abrupt reef ecotones in Fatu Huku. Reef flat (pavement) is located at medium water depth (17.59 6 0.41 m), with the most gentle slope (20.97 degrees 6 1.2 degrees), the lowest concentration of similar depths (0.04 6 1.1022), and a medium level of depth edge (1.11 6 0.15). Those results robustly match the seascape ecology description of reef pavement, namely some large flats (deprived of hotspots) outlined by medium ecotones, and lying below coral reefs. Sandy environments exemplify the deepest habitat (18.34 6 0.27 m), associated with the most abrupt slope (23.97 degrees 6 1.24 degrees), and both the highest concentration of similar depths (0.06 6 1.1022), and the highest score of depth edge (1.39 6 0.1). Sand habitats therefore gather plains and sharp dunes’ hotspots below reef flats (consequently, coral reefs), and exhibit many transitional ecotones.
Coral reefs using spaceborne imagery
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Acknowledgments Authors strongly acknowledge the European Space Agency and ARGANS-ACRI for funding the scientific campaign. Catamaran Itemata team and Nancy Lamontagne are warmly thanked for the fieldwork.
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