Environmental Engineering Reference
In-Depth Information
Entry of phytoplankton into rivers can occur from
standing waters or stream inlets, and assessment
of river phytoplankton dynamics needs to take into
account both irregularities of entry and displacement
by current. Studies by Welker and Walz (1998) on
grazing of phytoplankton in the River Spree (Ger-
many), for example, took both of these factors into
account by adopting a Lagrangian sampling strategy.
This involved timed collection of phytoplankton in
the middle of the river to achieve successive sampling
of the same water parcel as it travelled downstream.
Using this approach, these authors demonstrated
a major removal of phytoplankton by freshwater
mussels (unionid bivalves) after discharge of algae
into the river from a highly productive shallow lake.
collection. Trawl nets of different mesh sizes can
be classified in relation to the minimum size
range of organisms collected (Fig. 2.4b) and vary
fromlargestzooplankton/ichthyoplanktonnets(mesh
aperture 1024 μm) to phytoplankton (53-76 μm) and
nanoplankton (
50 μm) nets.
A typical phytoplankton net mesh size of
about 50 μm will collect the majority of algae
in the microplankton and macroplankton range
(Table 1.1) - referred to collectively as 'net phy-
toplankton'. Small unicellular algae (nanoplankton)
can be collected by smaller mesh sizes (commer-
cially available nets go down to 10 μm), but problems
with clogging and reduced water low mean that these
smaller organisms are best collected as bulk samples
(see Section 2.2.2) followed by sedimentation or cen-
trifugation (see Section 2.5.1).
In practice, routine collection of net zooplankton
and phytoplankton involves using separate zooplank-
ton and phytoplankton nets (mesh sizes about 250 μm
and 50 μm, respectively). Using the two nets in com-
bination has the advantages that both samples are
collected from the same volume of lake water, and
that zooplankton is largely removed from the phy-
toplankton sample - reducing sample contamination
and algal ingestion during transport to the laboratory.
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2.2 Mode of collection
Phytoplankton samples can be directly collected from
the water body in three main ways - via a phytoplank-
ton trawl net, volume sampler or as an integrated sam-
ple. In addition to this, samples of sedimenting phy-
toplankton can be passively collected over a period
of time using sediment traps. In all cases, material for
live examination should be brought back to the labo-
ratory as quickly as possible and examined straight-
way (see Section 2.5.1). For chemically preserved
material, the fixative should be added immediately
(on the boat) after collection.
Application and quantitation
The phytoplankton net can be hauled horizontally,
vertically or obliquely depending upon the type of
sample required. A vertical haul will collect a com-
posite community within all or part of the water
column, whereas a horizontal haul through surface
waters will collect a composite sample across an area
of lake.
The volume of water passing through the net can
be very approximately estimated from the area of the
net aperture and the distance (vertical or horizontal)
travelled. With horizontal trawls, more precise vol-
umes can be determined using a flow meter (Eaton
etal ., 2005). The speed of the horizontal trawl should
be sufficiently slow to allow the net to sink to a depth
of about 0.5-1 m below the water surface and also to
prevent the formation of a bow wave that reduces the
efficiency of collection. Problems may arise with net
clogging in the case of small mesh-size nets, and also
2.2.1 Phytoplankton trawl net
Plankton nets vary in design from basic tow-nets
(conical or with truncated neck) to more complex
devices fitted with flow meters and special open-
ing/closing mechanisms (Eaton et al ., 2005). In their
simplest form (Fig. 2.4), they consist of long cones
with a circular opening at their mouth and some form
of collecting chamber at the narrow end.
Mesh size
The type of netting and mesh size determine filtra-
tion efficiency (extent of removal of different par-
ticle sizes), clogging tendencies, velocity of water
flow within the net and condition of sample after
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