In situ field measurements of aquatic animal-fluid interactions using a Self-Contained Underwater Velocimetry Apparatus (SCUVA)

We describe the development of a Self-Contained Underwater Velocimetry Apparatus (SCUVA) that enables a single SCUBA diver to make in situ digital particle image velocimetry (DPIV) measurements of animal-fluid interactions in real time. The device is demonstrated in a study of the dynamics of Aurelia labiata jellyfish swimming in the coastal waters of Long Beach, California. We analyze the DPIV measurements by computing the kinetic energy in the flow field induced by an animal’s swimming motions. As a proof-of-concept, we compare these results with an existing theoretical model and find that the results are consistent with one another. However, SCUVA provides details regarding the temporal evolution of the energetics during the swimming cycle, unlike the model. These results suggest the usefulness of SCUVA as a method to obtain quantitative field measurements of in situ animal-fluid interactions. *Corresponding author e-mail: [email protected] Acknowledgments The authors gratefully acknowledge field support provided by Jifeng Peng of Caltech and Mike Schaadt of the Cabrillo Marine Aquarium (San Pedro, CA). This work is supported by the NSF Ocean Sciences DivisionBiological Oceanography (OCE-0623475 awarded to J.O.D.). K.K. is supported by a National Defense Science and Engineering Graduate Fellowship. Limnol. Oceanogr.: Methods 6, 2008, 162–171 © 2008, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS detection confirmed a sufficient presence of seeding particles to perform DPIV in ocean water (Katz et al. 1999). Submersible PIV systems have previously been designed to measure turbulence levels in the bottom ocean boundary layer, and to provide data on mean vertical velocity profiles and the time evolution of the mean velocity (Bertuccioli et al. 1999; Nimmo Smith et al. 2002). Free-falling platforms using a combination of measurements from planar laser-induced fluorescence and stereoscopic PIV have been used to observe and quantify microscale turbulence structures in the upper ocean (Steinbuck et al. 2004). In addition to these larger (apparatus weight on the order of 103 kg) and more expensive submersible DPIV devices, it has recently been proposed to use a small-scale submersible PIV system to characterize naturally occurring flows close to the shore (Clarke et al. 2007). Existing field DPIV devices lack the functionality required to collect quantitative measurements of animal-fluid interactions. For example, these devices typically have physical connections (e.g., cables) between the submerged device and the surface, which limits the area that can be measured to a fixed radius around a surface connection point. Additionally, these devices are usually unable to actively track the movement of animals in real-time due to their large size and lack of agility and controllability. Hence, existing systems are typically kept in a stationary position, towed behind a vessel, or programmed to execute predefined sweeps for data collection. A self-contained, portable device that can actively track animals independent of any connection to the surface and that is able to provide quantitative measurements of the flow field surrounding an animal has not previously been developed to the best of the authors’ knowledge. Here, we describe the development of a self-contained underwater velocimetry apparatus, or SCUVA, that achieves the goal of real-time, quantitative field measurements of aquatic animal-fluid interactions. To demonstrate the developed method, we conducted a preliminary investigation of the potential role of animal-fluid energy interactions in ocean mixing, a topic of increasing study that has been limited by the need for in situ field data at the scale of individual animals. Mixing against ocean stratification requires an input of mechanical energy whose sources are traditionally attributed primarily to winds and tides (Munk and Wunsch 1998). Munk (1966) found that the production of energy by marine organisms was of the same order of magnitude as tidal energy dissipation but later deemed them as negligible contributors to abyssal mixing. Biological sources of ocean mixing continued to be overlooked until rates of kinetic energy production were calculated for representative species of schooling animals (Huntley and Zhou 2004). The biological rate of kinetic energy production of a broad range of schooling animals was found to be on the order of 10–5 W kg–1, which was later confirmed by microscale shear measurements of a large concentration of krill (Kunze et al. 2006). These findings suggest that biosphere input to the ocean mixing energy budget may impact mixing at the same level as winds and tides, whose respective rates of kinetic energy production are of the same order (Dewar et al. 2006). However, the issue of biogenic mixing remains largely unresolved. To assess the potential of SCUVA to inform the ongoing debate regarding biogenic turbulent mixing, we study the dynamics of Aurelia labiata swimming in coastal regions near Long Beach, California. SCUVA measurements of Aurelia labiata are used to directly quantify the kinetic energy in the flow field induced by the swimming motions of individual medusae. The results are compared with the semi-empirical model predictions of Huntley and Zhou (2004). Materials and procedures SCUVA components—Particle illumination by SCUVA is provided by a continuous 300 mW, 532 nm solid-state laser. The output laser beam is collimated into a planar sheet by a planoconcave cylindrical lens (effective focal length = –6 mm). Using the current optical configuration, the size of the illuminated region can be adjusted from 15 cm W × 15 cm H to as large as 60 cm W × 60 cm H. Still smaller or large viewing windows can be achieved by modifying the camera lens and laser position. The laser and optics are mounted in a waterproof housing at the end of a retractable arm that locks to ensure that the camera is focused on the plane of the laser sheet at all times (Fig. 1). The arm is collapsed while the SCUBA diver swims to the measurement site and subsequently extended to initiate measurements. The flow field illuminated by the laser sheet is imaged by a high-speed camera (Photron APX-RS). The camera records images at a maximum resolution of 1024 × 1024 pixels at speeds up to 3000 frames s–1. Higher speed recordings can be made at reduced spatial resolution. An electronic shutter enables exposure times as short as 1 μs, ensuring that particles can be imaged without any blurring. Recorded images are stored internally on an 8 GB hard drive. This capacity enables collection of 6144 full-resolution frames before the device must be surfaced to upload the stored measurement data. Control of the camera is achieved by a single START-STOP push-button control integrated into the right handle of the Katija and Dabiri In situ field measurements using SCUVA 163 Fig. 1. SCUVA shown with laser arm extended and laser sheet activated