Environmental Engineering Reference
In-Depth Information
Attachment point for tow rope
Circular opening
(a) Plankton net
Canvas or Hessian
Nylon monofilament mesh
Figure 2.4 Trawl net collection of
phytoplankton. (a) Simple conical
phytoplankton net, with collection
jar. (b) General table of commercially
available net mesh sizes and their
classification. The phytoplankton net
would have mesh sizes in the range
53-76 μm. Larger mesh sizes would
collect colonial blue-greens and other
large algae but not the major part
of the net phytoplankton. Each mesh
size will collect the specified group of
organisms, plus all larger size groups,
unless they are prevented from entry
into the net by pre-filtration. Adapted
from Easton et al. , 2005.
Sample jar
(b) Net mesh sizes and classification
Size of
Aperture
(µm)
Approximate
Open area
(%)
Classification
1024
58
Largest zooplankton and ichthyoplankton
752
54
Larger zooplankton and ichthyoplankton
569
50
Large zooplankton and ichthyoplankton
366
46
Large microcrustacea
239
44
Zooplankton - Microcrustacea
158
45
Zooplankton, Microcrustacea, Rotifers
76
45
Net phytoplankton - Macroplankton and
microplankton
64
33
-
53
10
Nanophytoplankton
where there are dense populations of zooplankton
and phytoplankton (particularly large globular algae
such as Microcystis , and filamentous forms such as
Aulacoseira ).
After trawling, the sample is obtained by unscrew-
ing the end fitting and flushing into a container.
Using a standard phytoplankton net mesh size of
53 μm means that all particulate materials over this
size (both phytoplankton and zooplankton) would
be included. To avoid collecting zooplankton and
other large particulate materials, a zooplankton net
is typically placed within the phytoplankton net prior
to trawling - though large-sized algae may also be
retained by the inner net.
is collected as a concentrated sample. This is partic-
ularly useful in oligotrophic waters and during win-
ter in temperate mesotrophic/eutrophic lakes, where
phytoplankton numbers may be very low.
Trawl net samples can be used for bulk analysis of
phytoplankton (e.g. nitrogen, phosphorus and lipid
content), microscopic analysis of the range of organ-
isms present (e.g. light and electron microscopy, X-
rayandinfra-redmicrospectroscopy)orsimplywhere
arapidcollectionprocedureisrequiredtogiveanidea
of the main large-celled and colonial algae present in
the lake.
The major disadvantages of trawl net sampling
are:
Loss of small algae . Algae below the mesh size are
largely excluded from the sample. Exclusion is not
absolute, since the mesh becomes partly occluded
by large-celled and colonial algae, so some smaller
organisms are retained.
Advantages and disadvantages
The major advantage of trawl net collection is that
a large sample of phytoplankton biomass can be
obtained within a short period of time and that it
 
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