PATTERN OF PLANKTONIC BIOLUMINESCENCE IN THE NORTHERN SARGASSO SEA - SEASONAL AND VERTICAL-DISTRIBUTION

The vertical structure of bioluminescence potential (BPOT) and flash density (FD) were measured on five cruises to the northern Sargasso Sea in 1987 and 1988. Depth-integrated (0 to 150 m) BPOT did not vary seasonally, remaining within the range 9 to 15 × 1015 photons m-2 in all months sampled. Conversely, depth-integrated FD was significantly higher (> 2 × 105 flashes m-2) during winter (November and March) than during summer (< 9 × 104 flashes m-2 in May and August). The vertical patterns of BPOT and FD were well correlated within a single profile, more highly so in summer than in winter. Despite intracruise variability in the vertical pattern of BPOT and FD, there were clear summer-winter differences in the vertical distribution of BPOT and FD. During winter, BPOT and FD were maximal and relatively uniform throughout the surface mixed layer; for example in November they declined sharply within the thermocline at 130 to 150 m. During summer, BPOT and FD were greatest (12 to 25 × 1013 photons m-3 and 600 to 1 200 flashes m-3, respectively) at subsurface depths. Commonly in summer, the upper depth limit of high BPOT and FD occurred at the base of the surface mixed layer (10 to 40 m) and the lower depth limit was located at the base of the subsurface fluorescence maximum (usually at 100 to 120 m). © 1990 Springer-Verlag.