Biological field and laboratory methods for measuring the quality of ...

BIOLOGICAL METHODS
As a rule of thumb, concentrate samples when
phytoplankton densities are below 500 per ml;
approximately 6 liters of sample are required at
that cell concentration. Generally, 1 liter is an
adequate routine sample volume.
The following three methods may be used for
concentrating preserved phytoplankton, but
sedimentation is preferred.
3.1.1 Sedimentation
Preserved phytoplankton samples can often be
settled in the original storage containers. Settling
time is directly related to the depth of the
sample in the bottle or settling tube. On the
average, allow 4 hours per 10 mm of depth.
After settling, siphon off the supernatant
(Figure 1) or decant through a side drain. The
use of a detergent aids in settling. Exercise
caution because of the different sedimentation
rates of the diverse sizes and shapes of phytoplankton.
3.1.2 Cen trifugation
During centrifugation, some of the more
fragile forms may be destroyed or flagella may
become detached. In using plankton centrifuges,
many of the cells may be lost; modern
continuous-flow centrifuges avoid this.
3.1.3 Filtration
To concentrate samples by filtration, pass
through a membrane filter. A special filter
apparatus and a vacuum source are required.
Samples containing large amounts of suspended
rnaterial (other than phytoplankton) are
difficult to enumerate by this method, because
the suspended matter tends to crush the phytoplankters
or obscure them from view. The
vacuum should not exceed 0.5 atmospheres.
Concentration by filtration is particularly useful
for samples low in plankton and silt content.
3.2 Zooplankton
The zooplankton in grab samples are concentrated
prior to counting by allowing them to
settle for 24 hours in laboratory cylinders of
appropriate size or in specially constructed
settling tubes (Figure 1).
6
50.8 eM
Figure 1. Plexiglas plankton settling tube with
side drain and detachable cup. Not
drawn to scale.
Take care to recover organisms (especially the
Oadocera) that cling to the surface of the water
in the settling tube.
4.0 SAMPLE ANALYSIS
4.1 Phytoplankton
4.1.1 Qualitative analysis ofphytoplankton
The optical equipment needed includes a good
quality compound binocular microscope with a
mechanical stage. For high magnification, a substage
condenser is required. The ocular lens
should be 8X to 12X. Binocular eyepieces are
generally preferred over a monocular eyepiece
because of reduced fatigue. Four turret-mounted
objective lenses should be provided with magnifications
of approximately 10, 20, 45, and