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Enumerating plankton cells, especially ciliates, usually requires settling of samples in order to concentrate the cells. Poorly settled samples could introduce large errors into plankton counts. The time sufficient to settle all ciliates has, however, never been established in the literature. Here, using both theoretical and empirical studies, we suggest improvements of the current method, which mostly relies on experience to determine settling times. Ciliate density was used to calculate the theoretical settling time of fixed ciliates with the Stokes equation. To determine ciliate density (g mL −1 ), we modified and established a density gradient centrifugation method. We found that ciliate density was in the range 1.02–1.08 g mL −1 . Additionally, empirical sinking rates were gathered semi-automatically with a digital camera system. The theoretical and experimental settling times were in the same range, though there were differences for some species. From this, we recommend working with the empirical sinking rates that are more reliable: 0.5 and 1.7 mm min −1  for fixed marine (at salinities of 16 and 40, respectively) samples and 2.4 mm min −1  for fixed freshwater samples. Using these rates potentially saves up to 95% of the time for settling compared to old, experience derived times. Although ciliate density was significantly correlated with settling rates, there was no correlation with particle size and shape.

Concentration of fixed plankton samples via settling: how long is long enough?