The Video Plankton Recorder

by Cabell Davis and Scott Gallager

The VPR is a video-microscope system used for imaging plankton and other particulate matter in the size range from a few micrometers to several centimeters. It consists of four video cameras (with magnifying optics) synchronized at 60 fields per second (fps) to a red-filtered 80 W xenon strobe (pulse duration = 1 microsecond). The current lens on each camera can be adjusted to provide a field of view between 5 mm and 10 cm. Use of higher magnification lenses is currently being explored for viewing protozoans (<1 Ám resolution). The four cameras are set for concentric viewing fields so that a range of up to four magnifications can be viewed simultaneously, allowing a wide size range of plankton to be sampled. Depth of field is adjusted by the lens aperture setting, and the volume sampled in each video field ranges from about 1 ml to 1 liter, depending on lens settings. The cameras have been configured for stereoscopic viewing as well.

The red light beam from the strobe is expanded to 10 cm, collimated, and aimed obliquely to provide dark-field illumination. Strobe to camera distance is 1.0 m with the viewing area at 0.5 m. Video data are telemetered to the surface via fiber optic cable and stored, together with time code overlay, using broadcast quality video tape recorders. Abundance is calculated from the video by counting number per field and dividing by the field volume. Full description of the VPR design is given in Davis et al. (1992a,b). (To minimize potential avoidance problems, the gauze recorder box shown in Davis et al. (1992a) has not been incorporated into the VPR, so that the 1.0 m space between cameras and strobe is free of obstructions).

The VPR can be deployed in towed configuration for mapping plankton distributions, or in a fixed position for viewing plankton swimming behaviors in two or three dimensions. The system has been used in both configurations, and deployment on moorings and ROVs has been proposed.

During towing of the VPR, fluid flow is orthogonal to the camera-strobe axis. The VPR has been towed at speeds of 0.5-3 m s-1 in shelf and oceanic waters near Woods Hole providing data on abundance of planktonic taxa over a continuum of scales from micrometers to kilometers (Davis et al. 1992b). Extensive transects (100 km, 14 h deployments at 2 m s-1) have been made across Georges Bank while "towyoing" the instrument between the surface and bottom. Reconfiguring the tow body for towing at speeds up to 5 m s-1 has been proposed. The VPR is currently rated to 300 m, and pressure casings rated to 2000 m are under construction.

In situ observations of zooplankton swimming behaviors have been made at the WHOI dock and the data are currently being analyzed. In addition, the VPR will be deployed on the ROV JASON for two-camera observations of plankton distributions and swimming behaviors in mid-water and near bottom in the vicinity of hydrothermal vent sites (Guaymas Basin, March 1993).

The VPR was designed to minimize disturbance of the sampled volume in order to reduce possible disruption of the imaged particles or detection and avoidance of the sampler by the plankton. Frontal area is much smaller than that of a comparably sized plankton net, and the imaged volume is located along the forward (upstream) edge of the instrument. Red light was used since zooplankton are known to be phototactic but are least sensitive to long wavelengths of light. The large amount of open space between cameras and strobe (1.0 m) minimizes flow disturbance near the viewing area as determined by dye and avoidance studies in a tow tank (C. Davis and L. Haury, unpublished data). In situ observations made in lower magnification cameras revealed that organisms' trajectories, body orientation, and shape remained constant during transit through these windows, indicating lack of flow distortion or escape response.

Examples of planktonic organisms imaged with the VPR are shown in Figure 1. Pattern recognition algorithms are being developed for automated analysis of the VPR images. We presently use an Imaging Technologies Inc. model 151 Image Processor that does real-time (60 fps) frame grabbing, convolutions, edge detection, and outputs the edge coordinates to a host computer. This procedure follows that described by Berman et al. (1990). New image processing algorithms are being developed in collaboration with pattern recognition experts as part of a two-year ONR contract (1/1/93-12/31/94). The goal is to develop a procedure for accurate automated analysis of zooplankton size and taxonomic composition (first to major taxa and ultimately to genus level). Once accurate algorithms are developed, appropriate hardware for real-time analysis will be configured. The automated system then can be used at-sea to provide real-time acquisition of these data. A moored system also has been proposed which will transmit processed images to shore via satellite.

References

Berman, M. S., Katsinis, C., Jefferies, H. P. Lambert, R. M. 1990. Image analysis techniques for the identification and measurement of marine zooplankton. EOS 71: 94.

Davis, C. S., S. M. Gallager, M. S. Berman, L. R. Haury, and J. R. Strickler. 1992a. The Video Plankton Recorder (VPR): Design and initial results. Arch. Hydrobiol. Beih. 36: 67-81.

Davis, C. S., S. M. Gallager, and A. R. Solow. 1992b. Microaggregations of oceanic plankton observed by towed video microscopy. Science 257: 230-232.


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