Bioluminescence depth measurements

Since 1979, we have measured marine bioluminescence in diverse ocean waters ranging from tropical waters off St. Croix, V.I. to 73° N latitude to observations made under the pack ice of the Beaufort Sea, and finally from the submersibles Alvin and Johnson Sea Link to depths of 3650 m. This chapter describes the instrumentation developed for this purpose and presents examples of our measurements to date. 

Our primary measurement technique was to pull seawater through a 25-mL volume chamber with a pump. The organisms emit light when stimulated by the tur-bulently flowing seawater. This light is viewed by a photomultiplier tube (PMT). Two in situ measurement systems were used on surface ships. The on-board bioluminescence detector pulls seawater from below the ship s hull for continuous realtime measurements of surface bioluminescence a bathyphotometer was used on station to depths of 100 m. An additional laboratory system was used to measure bioluminescence flashes from individual plankters isolated from plankton tows and pumped collections. 

Other Parameters Measured Simultaneously with Bioluminescence. Measurements of several other parameters were obtained from the seawater after it had traversed the bioluminescence detector. When working on station with the bathy-photometer, which was equipped with a pressure transducer, temperature and beam transmittance were measured at depth while seawater was pumped by the submersible pump at depth to shipboard with 110 m of 2.54-cm ID hose. Sea surface temperature was obtained continuously from a probe at the intake near the sea chest. The seawater, obtained from either the sea chest or the bathyphotometer, was pumped through a Turner Designs fluorometer to measure chlorophyll fluorescence, and past a pH probe (31) and a conductivity cell when available. Samples of seawater were frozen for subsequent nutrient analysis (NO , NH4OH, P04 , and NO2). Plankton filtrates from 20 to 100 L (depending on plankton abundance) of seawater were collected from a 100-L effluent tank fitted with plankton net collection cups of 20-fxm mesh porosity. The filtrate was split, filtered onto Whatman GF/C 4.25-cm filter discs, and frozen for subsequent carbon and nitrogen determinations. The other half of the sample was preserved in 5% buffered formaldehyde solution for taxonomic analysis. 

Data obtained from the pumped (closed) deep bathyphotometer and the open bathyphotometer mounted on the Alvin submersible show that skylight penetrates to 200 m on a moonless night and to 500-700 m in daylight conditions. Therefore, bioluminescent measurements at depths to about 200 m at night and 500 m in the daylight can reasonably be done only with a closed system. (These depths can vary considerably with the turbidity of the seawater.)... 

The optical background noise was also measured at 3000 m depth in Capo Passero. Data collected in Spring 2002 and 2003, for several months, show that optical background induces on 10 PMTs (0.5 s.p.e.) a constant rate of 20 4- 30 kHz (compatible with the one expected from 40K decay), with negligible contribution of bioluminescence bursts. These results were confirmed by biological analysis that show, at depth> 2500, extremely small concentration of dissolved bioluminescent organisms [39]. 

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