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<br />BIOLOGICAL METHODS
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<br />rents are winds, flow, solar heating, and tides.
<br />Sunlight influences both the movements of
<br />plankton and primary production. Daily vertical
<br />plankton migrations are common in many
<br />waters. Qoud cover, turbidity, and shading (e.g.,
<br />from ice cover and dense growths of vegetation)
<br />influence the amount of light available to plank-
<br />ton.
<br />Chemical factors, such as salinity, nutrients,
<br />and toxic agents, can profoundly affect plank-
<br />ton production and survival.
<br />The nutrients most frequently mentioned in
<br />the literature as stimulators of algal growth are
<br />nitrogen and phosphorus; however, a paucity of
<br />any vital nutrient can limit algal production. The
<br />third category of chemical factors, toxic agents,
<br />is almost limitless in its components and com-
<br />binations of effects. Toxic compounds may be
<br />synergistic or antagonistic to one another and
<br />may either kill planktonic organisms or alter
<br />their life cycles. Many chemicals discharged in
<br />industrial effluents are toxic to plankton.
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<br />2.1.2 Sampling frequency
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<br />The objectives of the study and time and man-
<br />power limitations dictate the frequency at which
<br />plankton samples are taken. If it is necessary to
<br />know the year-round plankton population in a
<br />body of water, it is necessary to sample weekly
<br />through spring and summer and monthly
<br />through fall and winter. However, more frequent
<br />sampling is often necessary. Because numerqus
<br />plankton samples are usually needed to char-
<br />acterize the plankton, take daily samples when-
<br />ever possible. Ideally, collections include one or
<br />two subsurface samples per day at each river
<br />sampling station and additional samples at
<br />various depths in lakes, estuaries, and oceans.
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<br />2.1. 3 Sampling locations
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<br />In long-term programs, such as ambient trend
<br />monitoring, sampling should be sufficiently fre-
<br />quent and widespread to define the nature and
<br />quantity of all plankton in the body of water
<br />being studied. In short-term studies designed to
<br />show the effects of specific pollution sources on
<br />the plankton, sampling station locations and
<br />sampling depths may be more restricted because
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<br />of limitations in time and manpower.
<br />The physical nature of the water greatly
<br />influences the selection of sampling sites. On
<br />small streams, a great deal of planning is not
<br />usually required; here, locate the stations up-
<br />stream from a suspected pollution source and as
<br />far downstream as pollutional effects are ex-
<br />pected. Take great care, however, in interpreting
<br />plankton data from small streams, where much
<br />of the "plankton" may be derived from the
<br />scouring of periphyton from the stream bed.
<br />These attached organisms may have been ex-
<br />posed to pollution at fixed points for unknown
<br />time periods. On rivers, locate sampling stations,
<br />both upstream and downstream from pollution
<br />sources and, because lateral mixing often does
<br />not occur for great distances downstream,
<br />sample on both sides of the river. In both rivers
<br />and streams, care should be taken to account for
<br />confusing interferences such as contributions of
<br />plankton from lakes, reservoirs, and backwater
<br />areas. Plankton sampling stations in lakes, reser-
<br />voirs, estuaries, and the oceans are generally
<br />located in grid networks or along longitudinal
<br />transects.
<br />The location, magnitude, and temperature of
<br />pollutional discharges affect their dispersal,
<br />dilution, and effects on the plankton. Pollutants
<br />discharged from various sources may be antag-
<br />onistic, synergistic, or additive in their effects on
<br />plankton. If possible, locate sampling stations in
<br />such a manner as to separate these effects.
<br />In choosing sampling station locations,
<br />include areas from which plankton have been
<br />collected in the past. Contemporary plankton
<br />data can then be compared with historical data,
<br />thus documenting long-term pollutional effects.
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<br />2.1.4 Sampling depth
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<br />The waters of streams and rivers are generally
<br />well mixed, and subsurface sampling is suffi-
<br />cient. Sample in the main channel and avoid
<br />backwater areas. In lakes and reservoirs where
<br />plankton composition and density may vary
<br />with depth, take samples from several depths.
<br />The depth at the station and the depth of the
<br />thermocline (or sometimes the euphotic zone)
<br />generally determines sampling depths. In shallow
<br />areas (2 to 3 meters, 5 to 10 feet), subsurface
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