Extraction method influences the recovery of phytoplankton pigments from natural assemblages

Phytoplankton pigment samples are routinely collected by filtering water and storing filters at −20°C before extraction and fluorometric analyses. Although pigment loss on frozen filters has been demonstrated for phytoplankton cultures, its effect on taxonomically diverse, natural assemblages is unknown. We examined the effect of freezing filters on pigment measurements from weekly field samples collected for 1 y from Narragansett Bay, RI. Over 1000 filters representing the total and 20 µm size class, such as during the winter‐spring bloom peak when 62% of Chl a was lost from frozen samples. Freezing filters reduced analytical precision, increasing the coefficient of variation of Chl a measurements from 3.4% using immediate extraction to 13.1% using frozen filter storage. Correction factors created from regression analyses were applied to the year‐long frozen storage dataset and brought the annual average Chl a concentration to within 4% of the value determined using the immediate extraction technique. The application of correction factors to the Narragansett Bay Long‐Term Monitoring Program from 1999–2007 resulted in an average Chl a concentration that was 94% greater than the historical average.