chamber with a syringe (less than 10 mL) or macropipettor (10 mL or more). The
sample bottle should be inverted at least once between each addition. This is done
because larger organisms settle quickly and may remain in the bottle if the sample
is simply poured. The chamber is topped with a round glass top plate.
5.1.2.2 Sample Settling
Algae are allowed to settle onto the base of the settling chamber. Since oil
immersion may be used in the course of identification, the coverglass at the bottom
of the chamber should not be thicker than 0.2 - 0.3 mm in thickness (or No. 1
coverglass). The time recommended for complete sedimentation varies with the
height of the chamber, i.e. 8 cm/day to 4 cm/day depending on accuracy required
in enumeration (Furet & Benson-Evans, 1982).
Approximate settling times necessary are as follow:
100 mL --------------- 100 hours
50 mL --------------- 50 hours
25 mL --------------- 25 hours
10 mL --------------- 10 hours
5 mL --------------- 5 hours
2 mL --------------- 2 hours
5.1.2.3 Sedimented Sample Analysis
Only "live" forms (chloroplast containing organisms) are counted and identified at
500x. Higher magnification may be used for identification when necessary.
5.1.2.3.1 The chamber of settled material is scanned and the dominant (four
or five most common organisms) as well as subdominant taxa are
determined. This is to give the analyst an idea of the sample
composition as well as to insure that the sample is evenly settled.
5.1.2.3.2 Enumeration and identification are done by scanning parallel
strips of 10 mm per strip (each strip has a width of 0.2 mm which
gives an area of 2 mm2). A minimum of three strips (30 mm or
6 mm2) is required, including no less than 250 "live" organisms.
If 250 organisms are not observed within the three strips,
identification and enumeration are continued in strips until at least
250 are counted. The area counted is recorded as it is needed for
cells per mL calculation.
5.1.2.3.3 The number of "live" cells are identified and enumerated to the
lowest taxonomic rank possible. All "emptied" lorica from
Chrysophyta are also identified and enumerated.