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

BIOLOGICAL METHODS
rents are winds, flow, solar heating, and tides.
Sunlight influences both the movements of
plankton and primary production. Daily vertical
plankton migrations are common in many
waters. Cloud cover, turbidity, and shading (e.g.,
from ice cover and dense growths of vegetation)
influence the amount of light available to plankton.
Chemical factors, such as salinity, nutrients,
and toxic agents, can profoundly affect plankton
production and survival.
The nutrients most frequently mentioned in
the literature as stimulators of algal growth are
nitrogen and phosphorus; however, a paucity of
any vital nutrient can limit algal production. The
third category of chemical factors, toxic agents,
is almost limitless in its components and combinations
of effects. Toxic compounds may be
synergistic or antagonistic to one another and
may either kill planktonic organisms or alter
their life cycles. Many chemicals discharged in
industrial effluents are toxic to plankton.
2.1.2 Sampling frequency
The objectives of the study and time and manpower
limitations dictate the frequency at which
plankton samples are taken. If it is necessary to
know the year-round plankton population in a
body of water, it is necessary to sample weekly
through spring and summer and monthly
through fall and winter. However, more frequent
sampling is often necessary. Because numerous
plankton samples are usually needed to characterize
the plankton, take daily samples whenever
possible. Ideally, collections include one or
two subsurface samples per day at each river
sampling station and additional samples at
various depths in lakes, estuaries, and oceans.
2.1.3 Sampling locations
In long-term programs, such as ambient trend
monitoring, sampling should be sufficiently frequent
and widespread to define the nature and
quantity of all plankton in the body of water
being studied. In short-term studies designed to
show the effects of specific pollution sources on
the plankton, sampling station locations and
sampling depths may be more restricted because
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of limitations in time and manpower.
The physical nature of the water greatly
influences the selection of sampling sites. On
small streams, a great deal of planning is not
usually required; here, locate the stations upstream
from a suspected pollution source and as
far downstream as pollutional effects are expected.
Take great care, however, in interpreting
plankton data from small streams, where much
of the "plankton" may be derived from the
scouring of periphyton from the stream bed.
These attached organisms may have been exposed
to pollution at fixed points for unknown
time periods. On rivers, locate sampling stations,
both upstream and downstream from pollution
sources and, because lateral mixing often does
no t occur for great distances downstream,
sampIe on both sides of the river. In both rivers
and streams, care should be taken to account for
confusing interferences such as contribmions of
plankton from lakes, rese'rvoirs, and backwater
areas. Plankton sampling stations in lakes, reservoirs,
estuaries, and the oceans are generally
located in grid networks or along longitudinal
transects.
The location, magnitude, and temperature of
poIlutional discharges affect their dispersal,
dilution, and effects on the plankton. Pollutants
discharged from various sources may be antagonistic,
synergistic, or additive in their effects on
plankton. If possible, locate sampling stations in
such a manner as to separate these effects.
In choosing sampling station locations,
include areas from which plankton have been
collected in the past. Contemporary plankton
data can then be compared with historical data,
thus documenting long-term pollutional effects.
2.1.4 Sampling depth
The waters of streams and rivers are generally
well mixed, and subsurface sampling is sufficient.
Sample in the main channel and avoid
backwater areas. In lakes and reservoirs where
plankton composition and density may vary
with depth, take samples from several depths.
The depth at the station and the depth of the
thermocline (or sometimes the euphotic zone)
generally determines sampling depths. In shallow
areas (2 to 3 meters, 5 to 10 feet), subsurface