Bioluminescence of the tropical Indian Ocean: a multiple-scale variation.

Large-scale surveys represented by 5800 bathymetric casts in the western Indian Ocean (0-22 o N, 54-58 o E), elucidated the 10-fold variation of the bioluminescent potential (BP) in the upper mixed layer, during the winter (north-east) monsoon season. The mesoscale survey in February 2017 consisted of 26 drift stations (4 o N-3 o S, 65-68 o E) on which 5-10 bathymetric casts were deployed down to 60 m. The maximal BP was associated with the periphery of a cyclonic eddy. The two-fold to three-fold variation of BP characterized the spatial heterogeneity modulated by a detected eddy. High-frequency casts on drift stations resembled the fine-scale heterogeneity in which the three-fold variation was observed within the BP maximum at a 37 ± 13 m depth. The latter one was located above the deep chlorophyll maximum at a 80 m depth. A general decline of the BP variance from the large scale through mesoscale to fine scale, fits that of the zooplankton biomass.