ZOOPS- O2: A broadband echosounder with coordinated stereo optical imaging for observing plankton in situ

Here we describe the configuration, calibration, and initial results from the combination of two recently developed underwater instruments that measure acoustic reflectivity and, simultaneously, the location, pose and size of millimeter-sized plankton relative to the sonar beam. The acoustic system, ZOOPS (ZOOPlankton Sonar), uses a broadband chirp signal that operates with a single monostatically configured transducer in the 1.5-2.5 MHz frequency range. We demonstrate that the system can record, with adequate signal-to-noise levels, identifiable reflections from single copepods with lengths as small as 360 μm. To simultaneously identify taxa and measure orientation, a pair of “O-Cam” microscopes were stereoscopically calibrated and geometrically co-registered with the orientation and range-resolved acoustic transmissions of the sonar beam. The system's capability is demonstrated via the in situ measurement of acoustic reflectivity as a function of orientation for 224 individual pelagic copepods comprising three orders of free-living taxa. Comparison with a well-known model, the Distorted Wave Born Approximation (DWBA), using a spheroidal formulation, yields both differences and similarities between the in situ field data and the model's predictions.