Caribbean Journal of Science, Vol. 46, No. 2-3, 267-282, 2012 Copyright 2012 College of Arts and Sciences University of Puerto Rico, Mayagu¨ez
Mapping marine benthic habitats in Martinique (French West Indies) Legrand H.1,4,*, Lenfant P.3, Sotheran I.S.2, Foster-Smith R.L.2, Galzin R.4 and Mare´chal J-P.1 1
Observatoire du Milieu Marin Martiniquais, 3 avenue Condorcet, 97200 Fort de France, Martinique, France Envision Mapping Ltd., Stephenson House, Horsley Business Centre, Horsley, Northumberland, NE15 0NS, UK 3 Laboratoire Ecosyste`mes Aquatiques Tropicaux et Me´diterrane´ens, UMR 5244 CNRS-EPHE- UPVD, Universite´ de Perpignan, 66860 Perpignan, France 4 USR 3278 CNRS-EPHE, CRIOBE-CBETM, Universite´ de Perpignan, 66860 Perpignan, France *Corresponding author: E-mail address:
[email protected] (Legrand H.) Contact address: Observatoire du Milieu Marin Martiniquais 3 avenue Condorcet 97 200 Fort de France Martinique Tel/fax : 033 596 71 96 42 / 033 596 39 42 16 2
ABSTRACT.—Several mapping techniques (aerial photography and acoustics) were combined to produce bathymetric and marine benthic habitats maps of Martinique (FWI). The eastern Atlantic coast is characterised by an outer spur and groove reef and inner lagoon system. The Caribbean coast shows steep slopes with sporadic fringing reefs, except on the south where a well-developed and continuous reef structure exists. The final maps show important differences in the distribution of habitats and communities between the Caribbean and the Atlantic sides. It appears algal dominated communities have replaced most of the historic coral communities of the Atlantic double bank barrier reef. Portions of fringing reefs still remain along the barrier and protect wide areas of seagrass beds and numerous bays and inlets. The largest reef structure is located along the Southern Caribbean coast, and is an active and productive ecosystem. The North Caribbean coast is characterised by non-framebuilding reefs and limited seagrass beds which are restricted due to a steeply sloping seabed. Maps obtained give an overview of the Martinique coastal benthic habitats and their distribution with an overall accuracy of 95.45% for physical substrata and 96.67% for biological communities. In addition to their use in ecological studies, these data provide valuable tools for management and sustainable exploitation of marine resources. KEYWORDS.—Aerial photography, coral reefs, Geoswath Interferometric sonar, habitat mapping, Martinique, RoxAnn AGDS
Introduction The complexity and biodiversity of coral reefs place them among the most productive and biologically rich ecosystems in the world (Yonge 1972; Dubinsky 1990; Done et al. 1996; Spalding et al. 2001). Part of the tropical coastal seascape, they are interconnected through complex physical, biological and biogeochemical interactions with seagrass beds and mangroves (Ogden 1988). Goods and services of coral reefs are numerous and essential for millions of people (Wilkinson 2004). Apart from buffering the shoreline against oceanic currents and waves, they are important spawning, nursery, breeding and feeding areas for a large number of organ267
isms (Ogden 1988; Moberg and Folke 1999). Coral reefs ecosystems sustain the livelihood of communities by supplying about 10% of the fish consumed by humans and support recreation for billions of tourists (Smith 1978; Salvat 1992; Done et al. 1996). Over the course of the last decade, the scientific community has clearly highlighted the dramatic consequences of human and natural disturbances impact on them (Hughes 1994; Hodgson 1999; Gardner et al. 2003; Hughes et al. 2003; Chabanet et al. 2005). It is acknowledged that the consequences have resulted in 20% loss and 50% are threatened of collapse (Wilkinson 2004). The Island of Martinique, located in the Lesser Antilles in the Caribbean Sea, has
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been increasingly subjected to demographic growth and tourism over the last 30 years (Wilkinson 2004) especially on the coastal zone. Indeed, the mountainous character of the island has led to the concentration of residences and economic activities on the coastal area, weakening the littoral ecosystems, with a third of the population living on the coast (Saffache et al. 2004). The marine benthic communities of Martinique have been extensively studied since the end of the 70’s (Adey et al. 1977; Battistini 1978; Chassaing et al. 1978; Laborel 1982; Laborel et al. 1984; Bouchon and Laborel 1986) but the geographic distribution of marine habitats is poorly understood and previous mapping studies have been restricted to localised sites (Manie`re et al. 1993; Chauvaud et al. 1998; Augris et al. 2000). The spatial distribution of marine habitats is a primary dataset required for Martinique to develop long-term management strategies regarding socioeconomic and ecological stakes. Remote sensing and marine habitat mapping methods are widely applied to provide valuable data to enhance the sustainable use and preservation objectives of tropical ecosystems (Sheppard et al. 1995; Green et al. 1996; Davies et al. 1997). Amongst the methods, aerial imagery and acoustic technologies have been shown to be highly suitable to map marine habitats and their associated biotopes (eg. Andre´foue¨t et al. 2003; Foster-Smith and Sotheran 2003; Freitas et al. 2003; Mumby et al. 2004; Kendall et al. 2005; Cassata and Collins 2008; Vela et al. 2008; Walker et al. 2008). The aim of this work is to identify and map the inshore benthic substrata and biological communities within the 0–50 m depth range around Martinique using a combination of aerial photography analysis, acoustic ground discrimination system (AGDS) and interferometric bathymetric sidescan sonar. The data collected have been processed to produce full coverage habitat and sediment maps of the inshore coastal area of the whole island and detailed bathymetry. Materials and Methods The island of Martinique is characterised by a large insular shelf of 1 100 km2 of
which 75% is located on the Atlantic side (Figure 1), expanding up to 25 km from the coast (Durand 1996). The total subtidal area from 0 to 50 m depth of the Island was surveyed and mapped during 2006 and 2007. In tropical regions water clarity, due to oligotrophic conditions, enables the use of aerial photographs to identify benthic habitats to a depth of 10-20 m (Manie`re et al. 1993; Sheppard et al. 1995; Chauvaud et al. 1998). For deeper areas where light penetration is reduced or areas where light penetration is restricted, e.g. for wave action or high sun-glint, require alternative mapping techniques such as vessel based acoustic sonar survey. Two zones have been considered to cover the whole area: the shallow zone ranging from 0 to 10 m and the deeper zone ranging from 10 to 50 m. All the data collected were collated within Geographic Information System (GIS) and maps were produced at a 1:2 500 scale. The final map is a combination of both sets of data. 1. Data collection 1.1. Shallow area.—The 0-10 m zone was visually analysed using eighty-nine aerial photographs from the 2004 IGN aerial campaign. These images cover approximately 150 km2. Fifty-height areas (1:17 000 scale maps) were delimited for the entire coastal zone to work at a more detailed scale (1:2 500). The shallow water benthic community boundaries were first digitised from the georeferenced photographs and integrated into a GIS using ESRI ArcGISÒ 9.1. The delineation of habitats was based upon photo interpretation supplemented by local knowledge of the marine and coastal ecosystems and their distributions. 1.2. Deeper area.—Two acoustic systems were used to map the 10-50 m zone: an interferometric sonar, GeoSwath Plus and an Acoustic Ground Discrimination System (AGDS), RoxAnn. These systems were pole-mounted to the survey vessel and peripheral systems such as a sound velocity profiled, motion reference unit and heading sensor mobilised to the same vessel.
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Fig 1. The study area.
The use of these two systems in conjunction provided an excellent data set for marine habitat mapping (Brown et al. 2005). A full coverage bathymetric data set, along with coincidental sidescan sonar imagery was produced from the GeoSwath interferometric system.
The system produced detailed bathymetric models and sidescan sonar mosaics which are fully georeferenced and can be easily incorporated into GIS and used for habitat mapping. These products provided information on bedform features (Kendall et al. 2005) and major seabed sediment categories.
1.2.1. GeoSwath Interferometric sonar.—The GeoSwath plus system was pole mounted to the side of the survey vessel with the transducer array containing the interferometric transducers, a DMS 2-05 motion reference unit, used to record vessel movement, a sound velocity sensor, used to record the speed of sound at the transducer and a digital altimeter used for determine vertical depth below the transducer for quality assurance during data collection. The vessels position and heading were recorded from a CSI/ Hemisphere Vector GPS. GeoSwath Plus was operating at 250 kHz with a swath width between 25 m and 70 m depending upon the depth of water being surveyed.
1.2.2. RoxAnn AGDS.—A RoxAnnÔ GroundMaster AGDS was also pole mounted to the vessel on the opposite side to the geoswath and set back 2 meters to prevent the two systems interfering. An operating frequency of 50 kHz was used with the echosounders depth range fixed at 0-60 m. It provided information on the physical attributes of the seafloor along the vessel track. Acoustic ground discrimination systems (AGDS) are based on single beam echo sounders and, apart from determining depth, are designed to detect different substrata by their acoustic reflectance properties: rough surfaces produce an echo that decays slowly (a property termed ’backscatter’) whilst flat
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surfaces result in a rapid decay of the signal. These properties can be used to discriminate broad categories of sea floor habitats (Foster-Smith et al. 1999; Foster-Smith et al. 2001; Foster-Smith and Sotheran 2003). The whole area (350 km2) was surveyed using a track spacing of 50-200 m and the majority of the data were collected with overlapping swaths, which therefore provided 100% coverage. 2. Ground-truthing For the shallow areas, a directed sampling strategy was adapted to conduct a ground-truthing campaign. Geographic coordinates were extracted (approximately every 200 m) for each community identified from the photo interpretation. These points were positioned regularly to maximise the study area coverage. A total of 2 284 points were surveyed. For the deeper areas, a drop-down video camera system was used to conduct groundtruthing to gather information on the biology and physical characteristics of the seafloor. The system was designed to allow for rapid deployment and retrieval along with high quality footage and a low impact on the sea floor. It comprised a 550 line CCTV camera housing six ultra-bright LEDs to provide lighting where natural light was insufficient. A total of 487 video records were collected and permitted to classify the acoustic data. At each site the dominant biological community and physical substrata were identified, a general description of the habitat noted and any modifying features recorded, such as evidence of stress within the environment or physical disturbance due to either biogenic or anthropogenic factors. A semi-quantitative abundance score (1-5, present/rare – abundant) for six habitat features, hard corals, soft corals, sponges, macroalgae, algal turf and seagrass were recorded from each video sample. Using these data a standardised community and substratum classification was produced to ensure consistency when assigning habitat to ground-truth samples (Table 1). The substrata and benthic communities were identified and assigned to the classification at each location. These point locations
were imported into GIS and used to confirm and refine the habitat boundaries identified from the photo interpretation. 3. Acoustic data processing and classification The AGDS data were cleaned to remove data dubious quality. Spurious data can be caused by the presence of fish or debris, interrupt failure in the hardware or GPS position skips or errors. Depth spikes (change in depth values ± 2 m over 1-5 recordings) or positional errors (where recorded position and speed fall outside recording operating parameters) were removed using spreadsheet and GIS software. In order to produce images from the AGDS data, which can be classified using image processing, the track points require interpolation to produce a continuous image. Track spacing and data quality enabled a 25 m grid to be generated using an inverse distance weighted algorithm (detailed interpolation parameters are available on request from the authors). Three separate grids representing the variables E1 (roughness), E2 (hardness) and depth were produced. The grids/images were imported into IdrisiÔ for classification. Supervised classification using the maximum likelihood classifier is regarded as a satisfactory means of interpreting multispectral data, and the different acoustic variables have been considered as analogous to electromagnetic data from satellite or airborne sensors. Procedures are described in Sotheran et al. (1997). The swath data required processing to remove the effects of vessel roll, pitch and yaw, speed and direction. This processing was carried out in propriety GeoAcoustic software. Additionally, the data was cleaned and processed to remove depth spikes both across the swath and between subsequent swathes. The cleaned data was then generalised as a grid format at 5 m resolution. The backscatter sidescan data was also processed to compensate for vessel motion and a greyscale mosaic was produced using the same geographic parameters as the bathymetric grids. These mosaics and bathymetric grids can be displayed in GIS software and enabled the data to be examined to identify habitat types and boundaries.
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Table 1. Description of the substrata types and benthic communities encountered in the coastal zone 0-50 m of Martinique. Substrata and benthic communities types
Description
Substrata Rock/Reef
Rocky plateforme/blocks or coral structure constituted by live or dead colonies Fine to coarse sand Consolidated or unconsolidated assemblage of coral fragments, shells, rubbles or calcareous algae Fine sandy sediments Very fine to muddy sediments Rocky or reef areas interspersed by areas of sand
Sand Rubble Silty sand Mud Sand and Rock/Reef Community Bioconstructive coral community Coral community on rock Seagrass bed Mixed community Algal community Sponge and gorgonian community Soft bottom community
Coral bench constituted of an assemblage of hard corals, gorgonians and sponges Rocky substrate sparsely colonized by encrusting hard corals and sponges Any seagrass present, from sparse patches to thicker beds Coral clumps associated with seagrass. This category is generally a transition zone from coral community to seagrass beds Assemblage of algae, from algal film to fleshy macroalgae on variable substrates Assemblage of sponges and gorgonians on variable substrates Infauna on mobile sediments
The use of the detailed bathymetric models and the sidescan mosaics allowed recognising the physical features of the seabed. Combined with the use of the groundtruth data and the habitat map produced from the AGDS data, habitat distribution boundaries can be determined more accurately than using the AGDS data alone. 4. Map accuracies The accuracy assessment for errors of omission and commission were produced for the benthic communities and substrata (Congalton and Green 1999). This process examines the maps produced and compares this to point sample sites collected as part of from the ground-truthing process. The assessment compares the classes mapped to the class which is found at the sample locations. Percentage accuracy for the map and for each class is then produced. Results A total area of 452 km2 was surveyed along 70 km of coastline, extending from
the shore to 50 m deep. Ground-truthing was performed over 2 700 sites and data collected was analysed and entered into a database which was then incorporated into a GIS. 1. Bathymetry An overview of the bathymetry at 5 m resolution produced from the interferometric sonar data for the 10-50 m zone is shown in Figure 2. Three dimensional (3D) bathymetric profiles of selected areas are represented in Figure 3, illustrating the geomorphological characteristics of the seafloor. The bathymetric data are restricted to the area investigated for the acoustic survey and did not concern the shallower zone (less than 5 m) represented in grey color on Figure 2. The bathymetry is highly contrasted between the windward (Atlantic) and the leeward (Caribbean) coast of the Martinique island. The Caribbean coast is characterized by steep slopes with a deeper region (>50 m)
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Fig 2. General bathymetry of Martinique.
very close to the shoreline (
