Feature ArticlesCoastal Zone Research Gets Assistance From an Open System Tow Body Tool
By Christian E. Casagrande
Sea Sciences Inc.
Four ongoing, diverse research projects involving aquaculture, dissolved oxygen mapping, photographing coral ecosystems and satellite sea-truthing required better data collection techniques in order to accomplish set goals.
In each project, the primary tool of choice was the Acrobat™ tow body system designed and manufactured by Sea Sciences Inc. The Acrobat is designed as an open system, meaning that almost any payload can be added or exchanged by the user without compromising hydrodynamic stability. Components of the Acrobat system can be assembled as needed to support changing payloads and depth needs.
Each Acrobat was customized by Sea Sciences Inc. in cooperation with the customer. The payloads are specific for accomplishing unique real-time data collection requirements.
All Acrobat tow bodies are computer controlled with an emergency manual override. Some of the newer Acrobat tow bodies have Ethernet data communications and the necessary power to run a suite of sensors.
All data is received in real time on deck and parsed to the various computers and software controlling the payload and the Acrobat.
Research Purpose and Location. Scientists at the Bedford Institute of Oceanography (BIO) in Dartmouth, Canada, are collaborating with other researchers in Europe to define the interactions between cultured mussels, their food source and other ecosystem components, hoping to ensure the sustainability of this food industry.
The initial research was conducted on Prince Edward Island, primarily in Tracadie Bay, but has since expanded to Norway and Spain.
A large part of Tracadie Bay is used for mussel farming, and the primary mussel food sources are phytoplankton and other natural suspended particulate matter.
Methodology. If phytoplankton are consumed faster by mussels than they can be replaced by tidal flushing and growth, the mussel culture will not reach its full size production potential. More temporal and spatial (horizontal and vertical) data were needed to separate the effect of the mussels from the large natural variability in coastal waters.
BIO opted to use the Acrobat with a payload consisting of a fluorometer, microCTD (conductivity, temperature and depth sensor) and transmissometer. The Acrobat undulated between a set depth and altitude off the seafloor, providing 3D distribution data on phytoplankton concentrations while being towed behind a boat. An Acrobat survey of a mussel farm was conducted rapidly before the phytoplankton distribution could change with tidal flushing.
Results. This rapid, high-resolution mapping capability allowed the researchers to discover that phytoplankton quantity in Tracadie Bay was reduced to a level that placed mussel development at severe risk. This is the first time this depletion mechanism has been detected at the bay scale.
These results will be used to help locate, design and manage mussel farms.
Research Purpose and Location. Since 2007, the Virginia Institute of Marine Science (VIMS), under contract to the commonwealth of Virginia, has been conducting two and 3D surveys of the James, York and Rappahannock rivers, along with an adjacent Chesapeake Bay assessment.
These surveys will determine the scope and extent of coastal and estuarine oxygen depletion, and the data will be used, in part, to help assess the Environmental Protection Agency’s dissolved oxygen (DO) criteria for the Chesapeake Bay.
Methodology. Originally, the program used a basic Acrobat tow body supporting a sensor payload for CTD, DO and colored dissolved organic matter measurements. After some field training in the survey areas by Sea Sciences Inc., VIMS personnel suggested that it might be a good idea to add a small ski-type frame under the basic Acrobat frame. This addition potentially saved the equipment from a number of damaging encounters with the seafloor and has now become a standard option by the manufacturer.
The Acrobat’s performance capabilities were subsequently enhanced with the addition of a specially designed mini-Rosette® water sampler, along with larger wings for better hydrodynamic response. The Rosette, with six 250-milliliter water sampling bottles, was attached to the bottom of the standard Acrobat frame. Water sampling was performed while towing and provided information for sensor calibration and other biological and chemical analyses.
Results. The survey areas exhibit a highly stratified, complex DO structure with saturated/supersaturated water on the surface and numerous hypoxic and near-anoxic waters closer to the bottom, which are separated by a well defined pycnocline. However, the Acrobat’s continuously sampled two and 3D data shows that this stratification is subject to mixing and aeration by the spring-neap tidal cycle.
It became obvious that there exists a major difference between the mainstream model of Chesapeake hypoxia/anoxia and the existence of hypoxia in some sampled areas. It was hoped that as long as the 30-day mean DO criterion is met, then all the others (seven-day average, one-day average, instantaneous minimum) would also remain valid. However, the new data indicate that some areas may need to be re-evaluated.
Mapping Coral Ecosystems
Research Purpose and Location. The U.S. Geological Survey (USGS) developed the Along-Track Reef-Imaging System (ATRIS) to support mapping activities and the ground-truthing of light detection and ranging and acoustic measurements and satellite imagery of submerged coastal environments, such as coral reef ecosystems. The initial ATRIS was a boat-mounted camera system that acquired high-resolution geolocated digital images in waters less than 10 meters deep. To address the need for a greater operating depth range, increased image acquisition rate and the capability for large-scale surveys, the USGS purchased a customized Acrobat tow body.
The basic Acrobat was enhanced with an Ethernet power and data communications system, altimeter, forward-looking obstacle avoidance sonar and roll-pitch-heading sensor. In addition to this payload, a gigabit-Ethernet digital camera was added to provide image acquisition rates of up to nine hertz, exceeding the original 0.3 hertz rate. The enhanced Acrobat system is now known as Deep ATRIS.
Field tests began in 2007 in Narragansett Bay, Rhode Island, and the Gulf of Mexico near Tampa, Florida. To date, reef surveys have been conducted in Biscayne National Park and Dry Tortugas National Park, both in Florida. Upon adding the avoidance sonar in 2009, further field tests were completed in New Bedford Harbor, Massachusetts.
Methodology. The Acrobat/Deep ATRIS system is typically towed between three and five knots and “flown” two to four meters above the bottom. The constant vehicle altitude is automatically maintained by computer software based on soundings from the downward-looking altimeter, but this mode can be overridden by a manual switch in case of an emergency. Pitch and yaw are minimal, providing a stable imaging platform, and changes in seafloor topography result in a corresponding automatic response by the Acrobat/Deep ATRIS.
Although this response is adequate for flat or gently rolling seafloor conditions, operating in coral reef environments is more challenging, as the altimeter does not provide advance warning of potential obstacles, such as coral heads. To reduce the risk of collision, a forward-looking avoidance sonar was added and tested on commercial shipwrecks. Although the design presented a challenge, the problem was simplified by the fact that avoidance is necessary only in the vertical plane. Obstacle detection occurs within 50 meters, at which time vehicle control is governed solely by the forward-looking sonar. Once the obstacle is cleared, the Acrobat/Deep ATRIS resumes its original flight plan.
Results. Field surveys in Biscayne National Park produced 29,845 JPEG images in about two hours and covered 14 kilometers of track lines. The images were viewed, captured and stored in real time with software written by the USGS using National Instruments (Austin, Texas) LabView. A review of the images also showed that marine animal life was indifferent to the passage of the Acrobat/Deep ATRIS tow body. The 2009 Dry Tortugas National Park surveys covered 163 kilometers of seafloor in eight days, acquiring 459,817 color digital images.
Validating Satellite Sensors Research Purpose and Location. In a project initially funded by NASA, WET Labs Inc. (Philomath, Oregon) was given the task of acquiring and analyzing in-water optical and hydrographic data in support of algorithm development and validation of satellite ocean color sensor products. Since satellite sensors degrade in space, validation efforts are an ongoing endeavor worldwide. In October of 2008, a survey was made in the Ligurian Sea off the northwest coast of Italy, and another was done off the coast of Hawaii in 2007.
To this end, WET Labs purchased an Acrobat, which became an important component of a portable data acquisition system called the Dolphin. Also included in the system are a winch, slip rings, a tow cable, deck controllers, power supplies, a CTD, and sensors for inherent optical properties such as backscattering, absorption and attenuation. When required, a water-pumping system can be attached to the Dolphin to collect in-situ samples for standard lab analysis.
Methodology. The Ligurian Sea survey track was designed as an Archimedian, undulating spiral because the gap (in this case 0.5 kilometer) between the arms remains constant as the distance from the center changes. This spacing is one of several optimal sampling patterns that result in easier data interpolation. Other tow patterns, such as the cloverleaf, were used to provide repeated visits to the same geographical position. At the center of the spiral pattern was the European optical mooring BOUSSOLE (Bouée Pour L’acquisition d’une Série Optique à Long Terme), which primarily supports the satellite medium-resolution imaging spectrometer ocean color sensor.
The undulations ranged from the surface to between 30 and 40 meters deep while towing at six knots. At that speed, the complete spiral course was sampled over a period of four hours. The optical backscattering data was collected with a WET Labs ECO-BB sensor integrated in an optical sampling system mounted to the Acrobat.
Results. The Acrobat/Dolphin system has demonstrated the ability to collect data with sufficient accuracy for validation work and satellite ocean color algorithm development. A synoptic, 3D data collection over a large area with recently calibrated sensors increases the number of points available for algorithm development and validation work from a handful to thousands. These far larger data sets may be useful in developing improved, more analytical remote-sensing algorithms for coastal waters in the future.
The author would like to thank Peter Cranford of BIO, Iris Anderson and Mark Brush of VIMS, David Zawada of the USGS and Michael Twardowski of WET Labs Inc. for providing updates on their latest research projects.
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Christian Casagrande, along with his brother Dirk, founded Sea Sciences Inc. in 1997. He has been involved in marine research endeavors for more than 40 years, 15 of which he spent as president of General Oceanics Inc. Subsequently, he worked for Science Applications International Corp. and the University of Rhode Island Graduate School of Oceanography.