How Does the Species Used for Calibration Affect Chlorophyll a Measurements by In Situ Fluorometry?

We determined how the species used for calibration affects the accuracy of in situ chlorophyll a (chl a) measurements by fluorometry using single-species cultures and natural phytoplankton populations from Winyah Bay, South Carolina, USA. When a diatom was used for calibration, chl a in a dinoflagellate culture was overestimated by 66 +/- 7%, whereas concentrations of a cryptophyte, chlorophyte, and cyanobacterium were underestimated by 16 +/- 20%, 40 +/- 7%, and 71 +/- 33%, respectively. In natural populations, the combination of species-specific and environmentally induced variation in the ratio of fluorescence to chl a (F Chl(-1)) led to an overestimate by the in situ fluorometer of 40-169% for an April experiment and an underestimate of 4-50% in July. Even when field samples were dominated by diatoms, environmental effects resulted in highly variable predictions of chl a. Thus, while a carefully selected calibration species can improve estimates of in vivo chl a in the laboratory, calibration of in situ fluorometers should be done with natural communities collected from the site of interest.