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

Several sampling methods can be used.
Towing
An outboard motor boat fitted with a small
davit, meter wheel, wire-angle indicator, and
hand-operated winch is desirable. A 3- to 5-kg
weight attached to the line is used to sink the
net. Maintain speed to ensure a wire angle near
60° for easy calculation of the actual sampling
depth of the net. The actual sampling depth
equals the amount of wire extended times the
cosine of the wire angle.
Oblique tow--Make an 8-minute tow at four
levels in the water column (2 minutes at each
level: just above the bottom, 1/3 total depth,
2/3 total depth, and just below the surface) to
estimate zooplankton abundance.
Horizontal tow--Take samples for estimating
zooplankton distribution and abundance within
a particular layer of water with a messengeroperated
net equipped with a flow-through
measuring vane (such as the Clarke-Bumpus
sampler). Each tow lasts from 5 to 8 minutes.
Vertical two--Lower a weighted net to the
desired depth, record the amount of line extended,
and retrieve at a rate of 0.5 to 1.0
meters per second. The volume of water strained
can be estimated. Duplicate vertical tows are
suggested at each station.
To sample most sizes of zooplankters, two
nets of different mesh size can be attached a
short distance apart on the same line.
Net casting
Zooplankton can also be sampled from shore
by casting a weighted net as far as possible,
allowing the net to reach depth, and hauling to
shore at the rate of 0.5 to 1.0 meters per second.
Take several samples to obtain a qualitative
estimate of relative abundance and species
present.
Suggested net sizes are: No. 6 (0.239 mm
aperture) for adult copepods in estuarine and
coastal waters; No. 10 (0.158 mm) for copepodites
in saline water or microcrustacea in fresh
water; and No. 20 (0.076 mm) for rotifers and
nauplii. The No. 20 net clogs easily with phytoplankton
because of its small aperture size.
Rinse messenger-operated samplers with clean
5
PLANKTON PRESERVAnON
water, allow to dry, and lubricate all moving
parts with light machine oil. Clean nylon netting
material thoroughly, rinse with clean water, and
allow to dry (out of direct sunlight) before
storing.
2.3.2 Sample volume
The sample volume varies with the specific
purpose of the study. Twenty-liter surface
samples obtained by bucket and filtered through
a No. 20 net are large enough to obtain an
estimate of zooplankton present in flowing
streams and ponds. In lakes, large rivers, estuaries
and coastal waters, filter 1.5 m 3 (horizontal
tow) to 5 m 3 (oblique tow) of water through
nets for adequate representation of species present.
2.3.3 Sample preservation
For identification and enumeration, preserve
grab samples in a final concentration of 5 percent
neutral (add sodium tetraborate to obtain a
pH of 7.0 to 7.3) formalin. Adding either 70
percent ethanol or 5 percent neutral formalin,
each with 5 percent glycem (glycerol) added, to
preserve the concentrated net samples. Formalin
is usually used for preserving samples obtained
from coastal waters. In detritus-laden samples,
add 0.04 percent Rose Bengal stain to help
differentiate zooplankters from plant material.
For chemical analysis (taken, in part, from
Recommended Procedures for Measuring the
Productivity of Plankton Standing Stock and
Related Oceanic Properties, National Academy
of Sciences, Washington, D.C. 1960), the concentrated
sample is placed in a fine-meshed
(bolting silk or nylon) bag, drained of excess
water, placed in a plastic bag, and frozen for
laboratory processing. If the sample is taken
from an estuarine or coastal station, the nylon
bag is dipped several times in distilled water to
remove the chloride from interstitial seawater
which can interfere with carbon analysis.
3.0 SAMPLE PREPARATION
3.1 Phytoplankton
As the phytoplankton density decreases, the
amount of concentration must be increased and,
accordingly, larger sample volumes are required.