A three-axis fast-tow digital Video Plankton Recorder for rapid surveys of plankton taxa and hydrography

A new Video Plankton Recorder (VPRII) has been developed for rapid surveys of plankton and seston in the size range of 100μm–1cm. The VPRII system includes: 1) a high-resolution digital camera (1Mpixel, 10-bits, 30Hz frame-rate), 2) a fast towfish capable of tow speeds up to 12 knots and 3-axis motion for automatic undulation and ship-wake avoidance, small diameter tow cable and winch for deployment on coastal vessels, and 3) new interface software (Visual Plankton) for automatic identification of plankton to major taxa and visualization of these taxa together with hydrographic data in real time. Camera and strobe optics are laboratory-adjusted to select the field-of-view (5–20mm), and depth-of-field is objectively calibrated using a tethered organism (e.g., copepod) and automatic focus-detection software. The VPRII towfish comprises a fuselage, a fixed main wing, and three servo-controlled tail fins: port and starboard for dive, climb, and roll control and rudder for lateral movement. Placement of the strobe (starboard wing-tip), camera (fuselage nose), and cantilevered tow-bridle minimize disturbance of the imaged volume. Compared with typical net surveys in shelf areas, the VPRII counts more plankton per station, quantifies ubiquitous fragile forms, automatically identifies plankton to major taxa and measures their size, quantifies scales of patchiness down to a few cm, and displays high-resolution distributions of plankton taxa and hydrography while underway. The VPRII is available to researchers via the Woods Hole Oceanographic Institution ship instrumentation pool. Acknowledgments We thank the officers and crew of the R/V Oceanus for their support during field tests of the new VPR system. Engineers Pierre Tillier, Ken Peal, Nick Witzel, Ed Hobart, and Steve Faluotico made valuable contributions to the design and construction of the new VPR. Others involved in this project include Bob McCabe, Dick Edwards, Carlos Medeiros, Matt Naiman, Tracey Sutton, Andy Girard, Geoff Ekblaw, Craig Marquette, Paul Fucile, Frank Bahr, Al Bradley, Ned Forrester, Gary Stanbrough, Charlie Clemshaw, Ken Doherty, David Schroeder, and many more. This work was supported by NSF Grant OCE-9820099. Limnol. Oceanogr.: Methods 3, 2005, 59–74 © 2005, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS quantify coarse taxonomic composition (and species in some cases) of plankton at sea in real time (Davis et al. 2004). Despite these advances, significant problems have remained with the VPR system and with towed plankton samplers in general. In order to sample vertically as well as horizontally, samplers are often towyoed by hauling in and paying out cable with a winch. Such towyoing typically requires a larger cable (e.g., 0.68-inch or 17 mm) to facilitate proper level-winding and avoid cable cross-over on the winch, and it also causes significant cable wear. Moreover, the towyoing method requires a constantly alert winch operator who must be diligent to avoid hitting the bottom with the instrument during payout or twoblocking the instrument into the towing sheave during haulback. Use of a large cable also requires a large winch, which precludes use on smaller coastal vessels. In addition to problems associated with towyoing, towed instruments typically are deployed off the stern of the vessel, which means sampling in the wake, where small-scale plankton distributions are destroyed. To avoid sampling in the wake, the VPR has been towed off the side of the fantail from the crane boom (Davis et al. 1996), but logistically this method is more difficult than stern deployments, especially in higher sea states. The VPR has been deployed on a V-fin and towyoed at ship speeds up to 8 knots (4 m/s), but this speed is slower than typical transit speeds of research vessels. This speed reduction is a significant problem, since it requires more ship time to conduct a given survey. It also precludes opportunistic sampling on research vessels of opportunity (e.g., those in transit). Several undulating instruments exist that do not require towyoing, including Seasoar, Scanfish, Aquashuttle (Chelsea Technologies Group) and the miniBat (Guildline Instruments Inc). These instruments have been used extensively to collect oceanographic data, and they work well. These vehicles undulate, and some can be towed very fast (12.5 m/s). Since these vehicles have a single-axis dynamic control surface, they do not have dynamic lateral control capability and, therefore, cannot be flown off to the side out of the ship’s wake. They could conceivably be retrofitted with a fixed rudder to tow them off to the side, out of the ship’s wake. However, this configuration would cause the vehicles to tilt horizontally at a substantial angle when near the surface so that vertically oriented plankton such as copepods would be imaged end-on rather than from the side, making identification more difficult. In general, these vehicles were not designed to be towed laterally, and lack of roll stability would be a problem in such cases. Instrument packages are mounted in or on the fuselage of these undulating vehicles, which is problematic for the VPR imaging system, because the wake of the bridle and tow cable would disturb the imaged volume and cause avoidance by zooplankton. (The bridle is fixed in the vertical center-plane of the towbody on these vehicles). A three-axis towed vehicle, Triaxus (MacArtney A/S), has been developed and used for oceanographic sampling. This vehicle has vertical as well as lateral motion control and can be used to sample off to the side of the ship out of the wake. This vehicle has a box-kite shape with the control fins at the edges of the box. Like other samplers, the tow bridle is attached at the center of the front of the towfish and therefore the cable/bridle wake would interfere with the VPR’s image volume. There is no “undisturbed” region on this vehicle where the VPR could be mounted. Aside from deployment and towing difficulties, the VPR image acquisition and processing system, as previously reported (Davis et al. 1996; Tang et al. 1998), was a prototype system and required significant user input. Also, the original VPR used analog video cameras that have a relatively low resolution and dynamic range. Video fields were digitized at 256 × 512 pixels and 8 bits per pixel (i.e., 256 gray levels). This lower pixel resolution limits the image volume and contrast range that can be sampled compared to more recent higher resolution cameras. These analog cameras also required an analog-to-digital frame grabber to capture the images, and electrical interference can be introduced to the analog signal prior to digitization, resulting in noisy images. Finally, although the image processing and analysis methods developed for the VPR allowed for automatic identification of plankton taxa and plotting of their distributional patterns in real time, this prototype system required significant user knowledge and setup time and was cumbersome to use. To solve these problems, we have designed, built, and tested a VPRII system that (1) has a high-resolution digital video camera, (2) has a computer-controlled maneuverable three-axis towfish that can be undulated and flown off to the side of the vessel away from the wake, (3) can be towed at high speeds (e.g., 5 to 6 m/s), (4) minimizes disturbance of the image volume by locating the tow bridle on the opposite side of the towfish, (5) uses a small-diameter tow cable and small winch, so that it can be deployed from both small (15 m) coastal vessels as well as large research ships, (6) has improved user-friendly image processing and data analysis/display software for observing abundance patterns of plankton. The new system has been made freely available to the general oceanographic community when using Woods Hole Oceanographic Institution (WHOI) research vessels. Materials and procedures The VPRII system includes a towfish with flight control and data acquisition computers and shipboard computers for supervisory control and data logging, processing, and visualization (Fig. 1). The towfish is similar to a fixed wing aircraft, with tail rudder and port and starboard tail fins that are used as ailerons and elevators (Figs. 1, 2). It is towed by a rigid bridle arm extending from the port side of the nosecone. Using this configuration, the towfish can be launched through the stern A-frame and flown off to starboard, out of the ship’s wake. The Davis et al. New Video Plankton Recorder