Exploring Sources of Error and Correction Methods for the Fluorometric Quantification of Phytoplankton

Rachel So1, Anna Ryack2, Yuyuan Xie3 and Yaqin Li3, (1)Northwestern University, Environmental Science, Earth and Planetary Sciences, Evanston, IL, United States, (2)Mount Holyoke College, Biological Sciences, Geology, South Hadley, MA, United States, (3)NOAA Fisheries - Milford, Milford, CT, United States
Phytoplankton are found in nearly all aquatic ecosystems, sustaining the food web as the lowest trophic level. They are frequently quantified by measuring chlorophyll a (chla) concentration in the water using chla fluorescence. Here, errors associated with two methods of chla fluorescence measurement are examined: chemically extracted chla , and in vivo fluorescence measurements. For the extraction method, storing samples for an extended period of time could lead to chla degradation. To test the effects of storage temperature and duration on chla degradation, the chla concentrations of a filtered Thalassiosira pseudonana culture were monitored overtime. Samples were stored at two different temperatures (-20ºC and -80ºC) and were measured for their chla concentrations after acetone extraction over a period of nine weeks. Comparisons to values measured before storage showed chla concentrations decreased overtime, suggesting chla degradation. Moreover, it decreased more for samples in -20ºC than in -80ºC. An ANOVA test determined that storage temperatures and durations both had statistically significant effects on chla concentrations. These results indicate storage duration should be minimized, but if necessary, storage at -80ºC is preferred over at -20ºC.

For in vivo measurements, fluorescence from colored dissolved organic matter (CDOM) in natural waters can interfere with chla fluorescence measurements. Black tea was used to simulate CDOM and Thalassiosira weissflogii culture was used as a chla source. For mixtures of tea and culture, chla concentration estimates made using in vivo measurements compared to acetone extraction showed that the former significantly overestimated chla quantity. As such, a method was devised to mathematically correct for CDOM interference in the mixtures using absorbance and in vivo fluorescence measurements from a black tea dilution series. New estimates made using corrected fluorescence values were much closer to actual concentrations, indicating correction for CDOM fluorescence interference is possible.