Future Directions in Bioluminescence Research

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ABSTRACT

Scales of measurements and methods of calibration

Edith A. Widder

Harbor Branch Oceanographic Institution, Fort Pierce, FL 34946

Bioluminescence in the oceans has been measured on a wide range of scales using a varied assortment of instruments. Coarse-scale measurements (1-100 km), using airborne intensified cameras, have been used by fisheries to detect schools of fish at night by the bioluminescence that they stimulate as they swim. Fine-scale measurements (1-100 m) have been made with profiling instruments known as bathyphotometers. Micro-scale measurements (1-100 cm) have been made using intensified video cameras mounted on research submersibles to record species-specific bioluminescent displays. In every case both the scientific and strategic significance of such measurements rests on knowing what it is that is actually being measured.

The difficulties associated with getting a meaningful measure of bioluminescence is most apparent when one reviews the different bathyphotometer designs that have been used by different investigators and the different units that have been used to report measurements of stimulated bioluminescence (Figure 1).

The need for a standardized bathyphotometer design was first formulated within the U.S. Navy oceanographic community. Based on the combined requirements of:

1. defined excitation in order to quantify the stimulus,
2. high flow rates in order to improve sampling statistics, and
3. a long residence time, capable of making a radiometrically calibrated measurement of an entire flash,

a system known as the HIDEX-BP (high intake defined excitation bathyphotometer) was developed (Figure 2) (Widder et al., 1993). The U.S. Navy (Naval Oceanographic Office) now uses the HIDEX-BP for routine monitoring of bioluminescence (see abstract by M. Geiger).

Use of the HIDEX-BP has revealed that dinoflagellates are not always the dominant source of bioluminescence in the water column. Zooplankton, which can evade low-flow bathyphotometers, can be responsible for bioluminescent hot spots. For example, we have recently reported on the discovery of very thin layers (0.5 m) of intense (>2 x 1011 p/l) bioluminescence in Wilkinson Basin, Gulf of Maine (Widder et al., 1999). These layers, which were found as shallow as 6 m and as deep as 215 m, were due to aggregations of the bioluminescent copepod Metridia lucens at density discontinuities in the water column. The composition of these thin layers was determined using microscale mapping with a submersible-based video transect technique (Figure 3). This mapping procedure, which is known as SPLAT (Spatial PLankton Analysis Technique), uses a computer image recognition program to identify organisms based on the temporal and spatial characteristics of their luminescent displays and then maps their coordinates in three-dimensional space (Widder and Johnson, 2000). This discovery has both strategic and scientific significance. From a strategic standpoint the existence of thin layers of intense bioluminescence could have a detrimental impact on covert naval operations and objectives. From a scientific standpoint the existence of food-rich thin layers provides a possible explanation for why measured average in situ food concentrations appear to be inadequate for the daily metabolic requirements of marine grazers. The energy that grazers must expend to locate food-rich micropatches is greatly reduced if those patches are spread out into thin layers because the search strategy can be reduced from three dimensions to one.

Literature cited:

Widder, E.A., J.F. Case, S.A. Bernstein, S. MacIntyre, M.R. Lowenstine, M.R. Bowlby, and D.P. Cook. (1993) A new large volume bioluminescence bathyphotometer with defined turbulence excitation. Deep Sea Res. 40(3): 607-627.

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

Widder, E.A and S. Johnsen (2000) 3D spatial point patterns of bioluminescent plankton: A map of the "minefield" J. Plank. Res. 22(3): 409-420.

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