140
Exercise 10
meaningful data than single depth samples and are a practical necessity when
investigation horizontal variations in populations, as in large reservoirs where marked
physical and chemical gradients exist.
Plankton nets are not recommended for either qualitative or quantitative analyses
of phytoplankton. A large percentage of important algal species is much smaller than
the mesh opening dimensions of nets even of the finest mesh size. Furthermore, the
volume of water passing through most nets is very difficult to measure.
Preservation
Whenever possible, phytoplankton species should be examined while alive,
particularly delicate species of flagellated algae. Algae may be maintained for several
hours without appreciable deterioration when kept cold during transportation to
the laboratory. Usually, however, preservation is necessary for longer storage
periods.
Samples can be preserved in a 0.5 to 2% buffered formalin solution, although
formaldehyde tends to cause rupture, deformation, and shrinkage of delicate algae.
A better preservative is Lugol's solution, added to samples to yield a 1 % final
concentration (1: 100). The absorption of iodine from Lugol's solution by the cells
also promotes settling when the sedimentation-inverted microscopy technique,
discussed below, is used. A small amount of glycerol also is added commonly to
samples preserved in Lugol's solution to prevent drying during long periods of storage.
A 3% solution of glutaraldehyde in final concentration, neutralized to pH 7 with
NaOH, is also a good preservative that results in minimal shrinkage and distortion
of cells. See "Apparatus and Supplies" (p. 162) for further details.
QUANTITATIVE EVALUATION OF NUMBERS AND BIOMASS OF SPECIES
Detailed analyses of phytoplanktonic populations require estimation of numbers and
volume of each species. Phytoplankton consist of individual cells, filaments, and
colonies. It is best to count cells, although this procedure is impractical in the case
of many multicellular colonies. When colonies of species are counted, it is important
to determine the average number of cells per colony. The number of cells per colony
can vary spatially within a lake and seasonally with changes in the population vigor
of a species. In filamentous algae, the average length of the filaments should be
determined.
Cell numbers often do not represent true biomass because of considerable variation
in sizes of cells among algal species. This disparity can be evaluated by multiplying
the number of cells of a given species by its average cell volume and then summing
these volumes over all species. Cell volume is estimated from knowledge of mean cell
dimensions and correspondence of cellular shape to geometric solids or combinations
of simple solids [spheres, cones, turncated cones cylinders, etc.,; cr., Sicko-Goad, et aI.,
(1977)]. The volume of physiologically inert wall material ranges from nil, in some
flagellates, to over 20% of the total cell volume in certain diatoms (Sicko-Goad, et aI.,
1977). Although a number of tables have been published [e.g., Table 8.1 in Wetzel
(1983, 150)] listing the cell volumes of various algal species, these values must be
viewed as being only approximate. Cell dimensions of a species can vary greatly from
season to season or from lake to lake. The cell volume of each important algae should
be determined for each sample.
Exercise 10
meaningful data than single depth samples and are a practical necessity when
investigation horizontal variations in populations, as in large reservoirs where marked
physical and chemical gradients exist.
Plankton nets are not recommended for either qualitative or quantitative analyses
of phytoplankton. A large percentage of important algal species is much smaller than
the mesh opening dimensions of nets even of the finest mesh size. Furthermore, the
volume of water passing through most nets is very difficult to measure.
Preservation
Whenever possible, phytoplankton species should be examined while alive,
particularly delicate species of flagellated algae. Algae may be maintained for several
hours without appreciable deterioration when kept cold during transportation to
the laboratory. Usually, however, preservation is necessary for longer storage
periods.
Samples can be preserved in a 0.5 to 2% buffered formalin solution, although
formaldehyde tends to cause rupture, deformation, and shrinkage of delicate algae.
A better preservative is Lugol's solution, added to samples to yield a 1 % final
concentration (1: 100). The absorption of iodine from Lugol's solution by the cells
also promotes settling when the sedimentation-inverted microscopy technique,
discussed below, is used. A small amount of glycerol also is added commonly to
samples preserved in Lugol's solution to prevent drying during long periods of storage.
A 3% solution of glutaraldehyde in final concentration, neutralized to pH 7 with
NaOH, is also a good preservative that results in minimal shrinkage and distortion
of cells. See "Apparatus and Supplies" (p. 162) for further details.
QUANTITATIVE EVALUATION OF NUMBERS AND BIOMASS OF SPECIES
Detailed analyses of phytoplanktonic populations require estimation of numbers and
volume of each species. Phytoplankton consist of individual cells, filaments, and
colonies. It is best to count cells, although this procedure is impractical in the case
of many multicellular colonies. When colonies of species are counted, it is important
to determine the average number of cells per colony. The number of cells per colony
can vary spatially within a lake and seasonally with changes in the population vigor
of a species. In filamentous algae, the average length of the filaments should be
determined.
Cell numbers often do not represent true biomass because of considerable variation
in sizes of cells among algal species. This disparity can be evaluated by multiplying
the number of cells of a given species by its average cell volume and then summing
these volumes over all species. Cell volume is estimated from knowledge of mean cell
dimensions and correspondence of cellular shape to geometric solids or combinations
of simple solids [spheres, cones, turncated cones cylinders, etc.,; cr., Sicko-Goad, et aI.,
(1977)]. The volume of physiologically inert wall material ranges from nil, in some
flagellates, to over 20% of the total cell volume in certain diatoms (Sicko-Goad, et aI.,
1977). Although a number of tables have been published [e.g., Table 8.1 in Wetzel
(1983, 150)] listing the cell volumes of various algal species, these values must be
viewed as being only approximate. Cell dimensions of a species can vary greatly from
season to season or from lake to lake. The cell volume of each important algae should
be determined for each sample.