Radiotracer kinetics using carrier-free 32P-PO4 were conducted on samples of water from Heart Lake, Ontario. Results obtained using 0.45-μm membrane filters were compared with those for 0.1 μm at vacuums of 400 mm Hg and to those for 0.45-μm filters using very low-pressure (4 mm Hg) filtration. The difference between 0.45 and 0.1 can reach 8–20% of the total radioactivity during the first 10 min of the experiment. After 60 min the fraction removed by 0.1, but not 0.45-μm filters, declines to only 1% of the total radioactivity, but this may represent as much as 50% of that which goes through 0.45 μm. The low-pressure filtration techniques provided similar results to those for normal filtration when kinetics were monophasic. Later in the season, the low-pressure method was shown to provide confusing artifacts that were explained by the hypothesis that tiny filaments extend from the surfaces of some species of aquatic algae and bacteria and are often dislodged during filtration.
