Abstract
Postincubation differential filtration (PIDF), preincubation differential filtration (Pre-IDF), and track autoradiography (TA) were compared for estimating cell-specific and total photo-autotrophic picoplankton production. Experiments were performed in Lakes Michigan and Huron and in the Gulf of Mexico. When Synechococcus dominated the photoautotrophic picoplankton community (> 70% of total picoplankton abundance), PIDF estimates of cell-specific and total picoplankton production were $\thicksim 3.0\times (range, 2.0-3.8\times)$ higher than TA estimates PreIDF estimates of cell-specific and total picoplankton production, however, were only slightly higher TA estimates $(mean, 1.4\times; range, 1.4-1.5\times)$ The higher PIDF estimates were attributable to breakage and damage of larger photoautotrophs during postincubation filtration and to retention of this labeled material on the smaller $(0.2\mu m)$ pore size filter. Results from PIDF experiments must be viewed with caution and previous estimates of picoplankton production, cell-specific or total based solely on PIDF may need to be re-evaluated.