Adaptation of underwater video for near-substratum current measurement.

We describe a new method to measure current near the substratum. A variety of current meters are available to record flow data relevant to benthic organisms in their natural habitat. Several factors, including accuracy and precision, sample volume, sample rate, deployment constraints, and cost make them more or less appropriate to different applications. Our method, based on tracking particles in videos from a camera mounted near the substratum, provides an inexpensive option for measuring flow. We validate this “current camera” against a conventional electromagnetic meter, and then use it successfully to estimate flows experienced by the nudibranch Tritonia diomedea in its natural habitat. As we implement it, this current meter samples a large volume and provides a measure of bulk flow in the habitat. However, the method is adaptable to videos acquired from other cameras at a variety of size scales and locations. We therefore conclude that our method is an option that should be considered by biologists interested in measuring flow. In a recent field study (1), we used current direction to partially explain navigation behaviors of the nudibranch mollusc Tritonia diomedea Bergh. To avoid interference with the behaviors and our ability to observe them, we chose to measure flow some distance away from the slugs. As a consequence of this constraint, we needed integrated current headings (over time or space) to overcome the heterogeneous nature of flow in this habitat and thus allow us to estimate flow in nearby regions. Furthermore, we needed measurements made near the substratum, to correspond with flow experienced by the benthic slugs. Several current meter options are available. These include acoustic doppler velocimeters (ADVs), acoustic doppler current profilers (ADCPs), and electromagnetic meters. However, none of these are ideal. ADVs have a very small sample volume, near 1 ml (2). ADCPs, although adapted for near-substratum use (3), still have relatively small sample volumes, typically less than 1 liter (4). Electromagnetic meters have either small sample volumes or are compromised when deployed close to the substratum (pers. comm: Marsh-McBirney, Inc., Frederick, MD; Interocean systems, Inc., San Diego, CA). All are expensive. Two other cruder options, mechanical meters and dye tracking, are less accurate. We therefore chose to use particle tracking velocimetry (PTV) in videos from a camera placed to record particle movement near the substratum (Fig. 1). This “current camera” has several advantages. (1) PTV is an established method for determining fluid flow directions (5–7). (2) A large sample volume (Fig. 1) can be created with the appropriate camera field of view. (3) Current meter output is not corrupted by proximity to the substratum. (4) The meter is unobtrusive (camera dimensions: 5 5 8 cm; pole diameter: 2.2 cm) and will have little effect on flow. (5) The system requires the same hardware for deployment and data capture as typically used to record slug behaviors and thus is easily integrated into field studies. (6) All current meters are susceptible to fouling and other disruptions, and a video record gives unequivocal information on the source of anomalous data. The current camera recorded particles moving in flow across a black background (Fig. 1). To convert particle motion into flow measurements, we designed an automated particle tracking velocimetry algorithm (Fig. 2; Movie 1 [http://www.biolbull.org/supplemental/]; code is available in the online supplementary material [http://www.biolbull. org/supplemental/or by contacting the corresponding auReceived 8 June 2005; accepted 11 July 2006. * To whom correspondence should be addressed, at Department of Physiology and Biophysics, Dalhousie University, Halifax, NS, B3H 1X5, Canada. [email protected] Abbreviations: PTV, particle tracking velocimetry; RTF, relative to flow. Reference: Biol. Bull. 211: 101–105. (October 2006) © 2006 Marine Biological Laboratory