Sea-Floor Survey Off Key Largo, Florida, Using Along-Track Reef-Imaging System (ATRIS)


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Sea-Floor Survey Off Key Largo, Florida, Using Along-Track Reef-Imaging System (ATRIS)

By Emily Klipp and Monica Palaseanu-Lovejoy
March 2008

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Above: ATRIS image acquired in the Florida Keys reef tract during September 2007 survey, showing branching gorgonians. [larger version]

Detailed imagery of the Florida reef tract off Key Largo collected in September 2007 will help scientists better understand local coral reefs and associated habitats. The imagery was collected by the U.S. Geological Survey (USGS)'s Decision Support for Coastal Parks, Sanctuaries, and Preserves project (principal investigators: Amar Nayegandhi, John Brock), which uses remote-sensing techniques to inventory coastal resources and monitor coastal change.

The Florida reef tract extends along the Atlantic Ocean side of the upper, middle, and lower Florida Keys. The islands of Key Largo and Elliot Key in the upper Keys separate the northernmost part of the reef tract from the shallow inland lagoons of Barnes Sound, Card Sound, and Biscayne Bay. Coral cover in the reefs off the middle and lower Keys has declined over the past several decades, but patch reefs off the upper Keys have been less affected. Recent surveys there have shown comparatively greater richness and diversity of coral and gorgonian species. The patch reefs typically consist of clusters of one or more massive colonies of head corals (so-called for their bulbous shape), an area of head corals scattered among abundant octocorals (which have eightfold symmetry, in contrast to true corals' sixfold symmetry, and include sea plumes, sea whips, and gorgonians), and occasionally a coral-rubble apron.

Above: Florida Keys, showing location of study area. [larger version]

Above: Digital elevation model (DEM) of study area acquired with Experimental Advanced Airborne Research Lidar (EAARL) and overlain by preplanned track lines used to guide ATRIS survey. Black track lines were plotted in the office; pink track lines were plotted in the field when surveying proceeded more quickly than expected, allowing collection of additional data. [larger version]

A 2002 survey of the submerged topography of the coral-reef tract off Elliot Key and Key Largo by the Experimental Advanced Airborne Research Lidar (EAARL) showed that patch-reef clusters are the primary source of benthic topographic complexity. In general, the greater the topographic complexity, the greater the diversity and abundance of plants and animals living in the reef. The lidar (light detection and ranging) survey also showed topographic complexity in the coral-rubble-and-sand zone at the patch-reef margins.

Above left: Hydrographic survey pole, which supports an acoustic transducer at its lower end and a GPS antenna at its top to collect data for bathymetric mapping. [larger version]

Above right: ATRIS camera in waterproof housing attached to a motorized pole mounted on the vessel. A global-positioning-system (GPS) antenna is mounted at the top of the camera pole (not shown). [larger version]

In September 2007, a USGS survey crew collected high-resolution imagery and bathymetric data off Key Largo to get a more detailed look at sea-floor structure and benthic habitat in that area. The crew consisted of Phil Thompson, Keith Ludwig, and Jerry Butcher of the USGS Florida Integrated Science Center, and Monica Palaseanu-Lovejoy and Emily Klipp, both working as contractors for the USGS through Jacobs Technology, Inc. The team used a digital elevation model (DEM) of submerged topography produced from EAARL data to plan track lines, a vessel-mounted echosounder to collect bathymetric data, and an along-track reef-imaging system (ATRIS) to collect high-resolution imagery.

The ATRIS system is a boat-based sensor package for mapping shallow-water (less than 10-m depth) benthic environments (see ATRIS - Boat-Mounted, Georeferenced, Digital Imaging System). The ATRIS captures real-time, high-resolution digital images at a rate of approximately 1,000 images per hour while the vessel is traveling at a speed of about 1 knot. Simultaneously, navigation software guides the vessel along preplanned transect lines and continuously logs the vessel's position. A precision bathymetric sounder records camera-to-sea-floor distances that allow the geometric scaling of each individual image. Acquisition time and geographic coordinates are recorded for every ATRIS image; the "georeferenced" images are later uploaded into a Linux-based program—ATRIS Data Analysis and Processing Tool (ADAPT)—specifically written for the ATRIS. ADAPT allows the user to classify the images into categories, plot the vessel track, and accomplish further qualitative analysis.

Before surveying, the team plotted transects along and across linear topographic features identified on the EAARL DEM of submerged topography. The DEM shows a consistent pattern of smooth areas landward of the reefs and rough, jagged areas seaward of the reefs. A goal of the ATRIS survey was to map submarine topography and determine habitat composition (such as sand, seagrass, or type of coral, some identified to species level). Preliminary interpretation indicates that the survey area consists of three types of habitat: coral reefs to the southwest, seagrass and sand to the northeast, and seagrass and isolated head corals in between. The georeferenced ATRIS images collected off Key Largo will be analyzed for habitat type relative to lidar-sensed topographic complexity. Results will be compared with previous, independently derived habitat maps of the study area. In addition to providing information about benthic habitats, the ATRIS data will expand our understanding of reef development and sea-level changes during the Holocene.

Along-Track Reef-Imaging System (ATRIS) Used to Survey the Sea Floor in Dry Tortugas National Park, Florida
September 2005