The responses of the Aqua Troll 500 Instrument to turbidity (NTU) and the fluorescence (RFU) from three photosynthetic pigments contained in Trichodesmium, namely chlorophyll a (Chl a), phycocyanin (PC) and phycoerythrin (PE) were measured over a wide range of concentrations of Trichodesmium. The responses were measured on samples of cultured biomass (measured by filaments/litre, F/L) diluted with filtered seawater (FSW) and unfiltered seawater (UFSW). The results show positive correlations between all “readings” and Trichodesmium concentrations above the nominated “satellite threshold detection” (STD) range of 100,000 –200,000 F/L and strong significant-positive linear correlations between the PE “readings” and Trichodesmium concentrations above 40,000 F/L. The instrument should be able to measure responses to all parameters in the fine layer structure of surface accumulations of Trichodesmium without overly disturbing that layer and hence provide quantitative data for calibrating satellite images. The PE “readings” show a positive curvilinear correlation with Trichodesmium concentrations in the low measurement range in UFSW; these results suggest an instrument detection limit is at or below ~12,000 F/L. The ability to measure such low concentrations means that the measurement of depth profiles of Trichodesmium below the surface accumulations and in pre-bloom conditions should be possible.