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

Abstract

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.