It's the number of cells in your average dinoflagellate
This group of unicellular organisms, with cell dimensions between 5 m and 2 
mm, ... Only a small number of studies have detected circadian regulation of .... to 
natural daynight cycle (solid line shows 7-point running average, .... upon the 
bioluminescent community composition, its influence on the diurnal... [DOC] [TLE] Modelling dinoflagellates as an approach to the seasonal ... [PAR] In situ measurement of bioluminescence offers a number of applications for ... or 
forecasting changes in its spatial distribution on time scales of the order of days ... 
p < .05) between BPOT and dinoflagellate cell abundance (Tokarev et al., 1999).