Composition and Biomass of Phytoplankton
141
Biovolume
Cell volumes are calculated for each species from formulae for solid geometric shapes
that most closely match the cell shape based on cell dimensions. The extent of change
in cell dimensions caused by preservatives should also be evaluated for important
species by comparison with living specimens [e.g., Borsheim and Bratbak (1987)]. A
microscope fitted with an internally reflecting drawing prism, allowing for simulataneous binocular viewing and projection of an enlarged image of the specimens
on a table top, enhances the accuracy of measurements [cf., Tyler (1971)]. Computers
with appropriate software and other electronic devices are available to digitize and
measure two-dimensional distributions of data such as line graphs and irregular
objects (e.g., Sigma Scan measurement systems, Jandel Scientific, Sausalito, CA).
When counting and evaluating the size of algae, some nondestructive method of
concentrating the organisms in a water sample is usually necessary. Methods
commonly used include sedimentation of algae onto a surface, filtration onto a surface
that can be rendered transparent, and centrifugation. By far the best method is the
sedimentation technique, because the algae settle by gravity onto a glass surface in
a random distribution and are not subjected to potentially disrupting vacuum or
pressure forces.
SEDIMENTATION AND ENUMERATION BY INVERTED MICROSCOPY
Phytoplankton samples to be observed by the sedimentation technique should be
preserved with Lugol's solution and stored in glass or opaque polyethylene bottles,
and in darkness, until analyzed. The absorption of iodine from Lugol's fixative stains
the algae but also adds weight, which serves to accelerate their sedimentation in the
chambers. When too little Lugol's solution is added, gas occlusions of blue-green
algae may not be discharged, and sedimentation of these forms may be incomplete
as a result.
Sedimentation chambers, originally developed by Utermohl in the 1930s [reviewed
in Utermohl (1958)J, are available commercially from a number of manufacturers
(e.g., Zeiss, Germany; Wild Instrument Co., Switzerland; Prior and Co., England;
Unit ron Inc., U.S.A.; and others). Chambers also can be constructed from cast acrylic
tubing (not extruded tubing, which can have ripple imperfections) and large coverslips.
In the latter case, the volume of each chamber must be determined carefully.
Sedimentation chambers are normally manufactured with volumes of 1, 5, 10,25,
50, and 100ml (Figs. 10.1 and 10.2). The chambers and accompanying coverslips
must be cleaned thoroughly to avoid contamination with organisms from previous
samples. The bottom coverslip is mounted carefully in the retention ring and screwed
to the cylindrical chamber for a pressure fitting. (Caution: The coverslips are expensive
and easily broken if overtightened.) Preserved samples in bottles must be mixed
uniformly by very gentle inversion and then poured into the chambers. Samples from
oligotrophic waters often require settling of 50 or 100 ml, while samples from more
productive waters with increased plankton densities are sampled adequately with 5
or 10 ml. The suspected density of organisms should determine the size of the chamber
selected.
Fill the sedimentation chamber to the top edge with sufficient excess to permit the
water to "bead" upward above the edge. Slide the chamber cover cap across the top
of the cylinder to remove any excess water and to enclose the sample of exact volume
141
Biovolume
Cell volumes are calculated for each species from formulae for solid geometric shapes
that most closely match the cell shape based on cell dimensions. The extent of change
in cell dimensions caused by preservatives should also be evaluated for important
species by comparison with living specimens [e.g., Borsheim and Bratbak (1987)]. A
microscope fitted with an internally reflecting drawing prism, allowing for simulataneous binocular viewing and projection of an enlarged image of the specimens
on a table top, enhances the accuracy of measurements [cf., Tyler (1971)]. Computers
with appropriate software and other electronic devices are available to digitize and
measure two-dimensional distributions of data such as line graphs and irregular
objects (e.g., Sigma Scan measurement systems, Jandel Scientific, Sausalito, CA).
When counting and evaluating the size of algae, some nondestructive method of
concentrating the organisms in a water sample is usually necessary. Methods
commonly used include sedimentation of algae onto a surface, filtration onto a surface
that can be rendered transparent, and centrifugation. By far the best method is the
sedimentation technique, because the algae settle by gravity onto a glass surface in
a random distribution and are not subjected to potentially disrupting vacuum or
pressure forces.
SEDIMENTATION AND ENUMERATION BY INVERTED MICROSCOPY
Phytoplankton samples to be observed by the sedimentation technique should be
preserved with Lugol's solution and stored in glass or opaque polyethylene bottles,
and in darkness, until analyzed. The absorption of iodine from Lugol's fixative stains
the algae but also adds weight, which serves to accelerate their sedimentation in the
chambers. When too little Lugol's solution is added, gas occlusions of blue-green
algae may not be discharged, and sedimentation of these forms may be incomplete
as a result.
Sedimentation chambers, originally developed by Utermohl in the 1930s [reviewed
in Utermohl (1958)J, are available commercially from a number of manufacturers
(e.g., Zeiss, Germany; Wild Instrument Co., Switzerland; Prior and Co., England;
Unit ron Inc., U.S.A.; and others). Chambers also can be constructed from cast acrylic
tubing (not extruded tubing, which can have ripple imperfections) and large coverslips.
In the latter case, the volume of each chamber must be determined carefully.
Sedimentation chambers are normally manufactured with volumes of 1, 5, 10,25,
50, and 100ml (Figs. 10.1 and 10.2). The chambers and accompanying coverslips
must be cleaned thoroughly to avoid contamination with organisms from previous
samples. The bottom coverslip is mounted carefully in the retention ring and screwed
to the cylindrical chamber for a pressure fitting. (Caution: The coverslips are expensive
and easily broken if overtightened.) Preserved samples in bottles must be mixed
uniformly by very gentle inversion and then poured into the chambers. Samples from
oligotrophic waters often require settling of 50 or 100 ml, while samples from more
productive waters with increased plankton densities are sampled adequately with 5
or 10 ml. The suspected density of organisms should determine the size of the chamber
selected.
Fill the sedimentation chamber to the top edge with sufficient excess to permit the
water to "bead" upward above the edge. Slide the chamber cover cap across the top
of the cylinder to remove any excess water and to enclose the sample of exact volume