Research Projects

Optical Sampling Techniques for Zooplankton

Most censuses of ocean life have been performed using net sampling. While this has been successful for crustaceans and fish, it does not work well at all for fragile gelatinous organisms. Net sampling also destroys all information about how the individuals are distributed on a small scale. Our lab has collaborated with Larry Madin (Woods Hole Oceanographic Insitution) and Edith Widder (Harbor Branch Oceanographic Insitution) to develop techniques to sample the distribution of gelatinous and small organisms.

SPLAT

Many of the most important species in the water column are too small to be detected by camera. However, if they are bioluminescent (as many are), one can record their flash quite easily. By using an intensified video camera mounted on a midwater submersible it is possible to record bioluminescent organisms that are mechanically stimulated to luminesce during horizontal transects. The unique temporal and spatial characteristics of these displays permits identification of many sources to the species level and the exceptional signal-to-noise ratio afforded by a self-luminous source means that even microscopic organisms (e.g. a 50 *m dinoflagellate) can be identified in a field of view of 1 meter. The analysis of this video data has recently been simplified with the development of a computer image recognition program that can identify bioluminescent displays based on their spatial and temporal characteristics. The resulting data set, which is basically a set of points distributed in space, is known as a spatial point pattern. Well-established procedures for the statistical analysis of 2D spatial point patterns can easily be extended to three-dimensional sets. Known as the spatial plankton analysis technique (SPLAT), this procedure not only provides a new perspective on the nature of the bioluminescence light field, it also provides information about the internal organization of plankton aggregations, about which very little is known.

LAPIS

LAPIS (Large Area Plankton Imaging System) is currently under development at the Woods Hole Oceanographic Institution. It is essentially a 'stealth' video array that can be towed horizontally or vertically at slow speeds, creates little or no optical or hydrodynamic evidence of its presence, illuminates a volume of water up to one cubic meter, and obtains images of organisms or other objects ranging from 0.5 cm to 1 m or more in size. Organisms are illuminated and filmed as the pass through a sheet of red light produced by a bank of red LEDs. Red LEDS are highly efficient and invisible to the target organisms, which are with rare exception, blind to red light. The red pigmentation of mesopelagic crustacea and of many medusae (and ctenophores, annelids, chaetognaths), while providing camouflage in ambient blue light, will actually make them more visible in the red illumination of LAPIS (Johnsen submitted). LAPIS is designed for full ocean depth (6000 m) and so can be used for exploration at depths below the reach of current submersibles.

Publications:

Widder, E. A., and S. Johnsen (2000). 3D spatial point patterns of bioluminescent plankton: a map of the minefield. Journal of Plankton Research 22: 409-420.

Widder, E. A., Johnsen, S., Bernstein, S. A., Case, J. F., and D. J. Neilson (1999). Thin layers of bioluminescent copepods found at density discontinuities in the water column. Marine Biology (Berlin) 134: 429-437