4.1.2 Procedure
This procedure is done by settling 10mL of each sample and counting the total number of
organisms and number of diatom cells within a 10 mm2 area. No identifications are done
at this time but any irregularities such as excessive sediment in the sample are noted. All
information from the 10 mL preliminary count is recorded in a pre-printed data form
(Appendix 1). This includes unusual observations such as poor sample preservation, high
bacterial or fungal populations, occurrence of special or rare phytoplankton taxa ... etc.
Note: The definition of an organism for 10 mL preliminary counts is as follows:
A colony, a filament, or a single cell. The units of a colony or a filament are not counted
as organisms at this time but the whole aggregate is counted as one organism.
Note: 10 mm2 = One transect from edge of chamber to edge of chamber at 250x.
4.2 Determination of Sample Volume Settled
4.2.1 There is no exact limit set for determining the volume needed, each sample is examined for
the number of organisms present, amount of debris in the sample and its distribution
pattern. Large amount of debris often require that smaller then optimal volumes be settled.
4.2.2 Most samples are settled at 10 or 25 mL, with 25 mL being the usual volume. Only when
samples are difficult or impossible to count are 5 mL or 2.5 mL samples used. The 50 mL
samples are used when very low number of organisms are found in the samples.
4.2.3 The volume needed for setting (soft algae analysis) and for digestion (diatom analysis) is
determined from the number of all organisms counted during the 10 mL preliminary
investigation. However, the minimum volume for digestion is recommended to be 500 mL.
For example:
10 mL preliminary counts
1) 101 organisms total
2) 103 diatom cells (Note: 1 cell has two frustules or valves)
Count needed (minimum)
1) 250 organisms total
2) 500 diatom frustules (250 cells)
Final volumes
1) 25 mL sample for sedimentation
2) 500 mL sample for digestion