Environmental Engineering Reference
In-Depth Information
Sample contamination . The phytoplankton sam-
ple tends to become contaminated by zooplankton,
which selectively consume algae and contribute
to overall biomass. Pre-filtration using the zoo-
plankton net removes larger zooplankton such as
Daphnia , but smaller zooplankton such as rotifers
and protozoa pass through. Small particulate inor-
ganic and organic debris may also be retained
with the phytoplankton, some of which (decom-
posing matter) may have an associated bacterial
population. In stratified temperate lakes, debris
contamination is particularly important during
winter months, when the lake is vertically mixed
and sediment at the bottom of the water column
may become resuspended. In streams, contami-
nation of the net sample by non-algal particulate
material is most likely to occur at night-time (sedi-
ment disturbance by the nocturnal benthic commu-
nity) and during winter months (Richardson et al .,
2009).
Van Dorn, Niskin and Nansen samplers (Eaton et al .,
2005).
Phytoplankton samples at vertical points in the
water column (Fig. 2.3b) can be obtained by lowering
a depth sampler such as the Van Dorn model (Fig. 2.5)
Suspension
cord
Weight
messenger
Cable
attachment
Upper
stopper
Sample deterioration . Because of the concentrated
nature of the sample, adverse changes in the water
(e.g. oxygen depletion) during transport may lead
to death of some of the more delicate algae.
Internal tension
band
Problems in quantitation . For various reasons, net
samples cannot be used to make a quantitative
assessment of phytoplankton populations (per unit
volume of water) within the lake. During sample
collection, the vertical position of the net varies
within the surface waters, so the sample is not truly
representative of the entire epilimnion. Even when
trawling is carried with a flow meter, complexities
ofwatercirculationaroundandthroughthenet(see
above) make it difficult to know the actual volume
of water that has been filtered.
Sample
chamber
Cable to lower
stopper
Lower
stopper
Figure 2.5 Van Dorn Volume Sampler - shown in the
open (sprung) position. The sampler is lowered to the
required depth in the water column, then the weight mes-
senger released down the suspension cord. This strikes
the cable attachment, which releases the cables to the
upper and low stoppers. These are then jerked into posi-
tion at either end of the chamber by the internal ten-
sion band, creating a water-tight seal, which encloses the
water sample. The sampler is subsequently pulled to the
surface and the water sample collected.
2.2.2 Volume samplers
Collecting phytoplankton samples within a volume of
lake water has the advantages that biomass or species
populations can readily be determined per unit vol-
ume of medium, and that all sizes of phytoplankton
are obtained within the sample. A variety of collect-
ing vessels are available - including the Kemmerer,
 
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