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B oluminescence is die production and emission of visible light by living organisms. Although a
handful of studies have been carried out to identify light producing marine organisms, their
spatial and temporal distribution pattern and other aspects related to bioluminescence in the
Iridian Ocean, no such studies have been conducted in the coastal waters around Sri Lanka.
Variations in biolumihescence intensity with respect to depth and vertical temperature profiles in
ti e east coast of Sri Lanka were studied from 9 * to 11* November 2015 using the Research
Vessel (r/v) Samudrika. The intensity of light produced by bioluminescence organisms was
measured using Recoverable BathyphotoMeter (RBPM). The RBPM recorded light intensity while
free falling at a speed of about 1 m/s. Several RBPM profiles in the upper 125 m were collected at
sjx different sampling sites encompassing a region 81° 38'E, 82'02'E and 7°56'N, 8°00'N. All
RBPM measurements were taken during night in a dark environment (2000 h to 0430 h) to
minimize interferences from other light sources. The temperature, density and salinity profiles of
each sampling site were collected using a conductivity temperature depth (CTD) profiler. The
generalized additive model (GAM) was used to examine the relationships between, the
bioluminescence intensity with the depth and vertical temperature profile. Bioluminescence was
recorded from all the sampling sites with varied intensity. The recorded bioluminescence
intensity rangedkfrom 25 to 200 counts and the highest intensity of 200 counts, was observed in
the sampling site 4 at the depth range of 20 to 40 m. According to the GAM analysis,
bioluminescence intensity decreased with increasing depth and the highest intensity was in the
sliallow depths range from 20 - 40 m. An average thermocline depth of all the sampling sites
w as~70 m except for site 6 where it was ~ 9 0 m. This study proved that the bioluminescence
intensity was comparatively higher above the thermocline than the below it. This was further
proven by GAM output which, clearly showed that the highest bioluminescence intensity was in
the 26 - 29 °C temperature range and it decreased with decreasing water temperature.
