fluorometers (i.e. bbe lOcells, TD BallastCheck-2™, Walz WATER-PAM and Hach BW680), which 
had highly concordant results in agreement with previous work (e.g. Gollasch and David, 
2012b), differed in their sensitivity (Figures 1-3). In particular, the Hach BW680 was less 
sensitive than the bbe lOcells and TD BallastCheck-2™. In total, 14 out of 32 samples were 
below the detection limit of the Hach BW680 in open ocean oligotrophic waters where viable 
organisms were detected by microscopy; five of these samples had concentrations > 10 
individuals/ml. In contrast, bbe lOcells and TD BallastCheck-2™ reported measured values for 
all samples analysed. The sensitivity differences between these instruments are likely explained 
by differences in their methodology. More specifically, both the Hach BW680 and the bbe 
lOcells determine relative 'active' chlorophyll biomass estimates based on the F v value, which is 
calculated as the difference between the F m and F 0 measurements. This can reduce the 
sensitivity of the instrument output, because the error in each measure is combined. The bbe 
lOcells measurement increases signal strength to avoid sensitivity loss by concentrating 
samples onto a selective filter. 
For both size classes, the concentrations estimated by the Satake Pulse Counter were 
lower than measured by microscopy. There are multiple possible explanations for these 
differences. Firstly, the Satake Pulse Counter would likely register a group of colonial organisms 
as one pulse instead of many pulses; this could lead to discrepancies if the Satake Pulse Counter 
registers one large pulse (> 50 size class) when the microscopist counts multiple cells (likely 10- 
50 size class), or if one pulse is registered where the microscopist determines all cells within the 
colony are smaller than 10 pm. Alternatively, it is possible that differences in methods to 
estimate organism size could lead to discrepancies. The Satake Pulse Counter determines