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