Page 115 - Freshwater-Biology-and-Ecology-Handbook

Figure 2.16
Set of sieves for sorting an invertebrate
sample. Left to right: mandatory 500
μm for sorting, advisable 250 μm
for catching fine waste to prevent
blocking the sink, and optional coarse
sieve for separating larger debris and
specimens to aid sorting.

12.4.1 Sieving

Before sorting, the sample must be passed through a set of without silt traps from becoming blocked by silt, but material
sieves under running tap water to gently rinse out the fine on this sieve is not part of the sample. The condition of the
silt. A 1 mm sieve with either a 4 mm or 8 mm sieve above sieves is not as critical for washing invertebrate samples as
it is recommended, depending on the nature of stream it is for sediment particle size analysis. Holes in fine mesh
being sampled. The finest sieve must be 500 μm mesh size sieves should be repaired by filling with solder.
(half the aperture of the nets used for sample collection)
and anything retained on it is considered to be a part of After rinsing and removing the fine sediments, large organic
the sample. Coarser sieves are merely to help sorting but material such as whole leaves, twigs, algal or macrophyte
the 500 μm sieve is critical to the procedure. It is advisable mats that were not removed in the field should be rinsed,
to place a 250 μm mesh sieve below it, to prevent drains inspected for attached animals, and discarded.

12.4.2 Sorting tray

This must be white and have a completely flat bottom
surface. Figures 2.17 and 2.18 show sorting trays in use.
Mark the tray with a grid of thin lines to divide it into 12
or 16 equal-sized areas. Gridlines help you to estimate
abundances and sort methodically. Apply the lines with
an indelible marker pen. Pale or mid-blue lines are better
than black because dark coloured animals remain clearly
visible on them.
Smaller trays of about 35 x 25 cm are recommended
for general use. They focus attention better than larger
trays. They are also more comfortable to use because it
is not necessary to lean so far over them, improving staff
posture and concentration. Larger trays, about 45 by 35
cm, are useful for sorting stones and larger fragments of
debris. Some people prefer larger trays for all sorting.

Figure 2.17
Biologist sorting a sample in a white sampling tray. Good lighting is essential. Notice how little material is in the tray.

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