QUANTITATIVE CENTRIFUGATION TO EXTRACT BENTHIC PROTOZOA FROM FRESH-WATER SEDIMENTS

Two methods for extracting protists from freshwater sediment are descr ibed: (i) an adapted isopycnic centrifugation technique for sandy and gyttja-like sediments and (ii) a rate zonal centrifugation technique f or sediments rich in particulate organic material (litter-like sedimen ts). The recoveries of protists during isopycnic centrifugation in med ia of several densities were compared. No significant losses in sodium diatrizoate and Percoll were recorded. After known amounts of nanofla gellates were added to azoic sediments, the protists were extracted an d counted. For sandy sediments, we found 100% recovery, and for the gy ttja-like sediments we found a maximum recovery of 94%. The recovery o f protozoa extracted from litter-like sediments, characteristic of lit toral systems, depends on a given centrifugal force, on time, and on t he dimensions of the flagellates. A recovery model which takes into ac count cell dimensions and centrifugation characteristics gives the min imum expected recovery.