Environmental Engineering Reference
In-Depth Information
Assessment of the intensity of the biomass green
coloration by comparison to a standard colour
chart. The phytoplankton colour index (PCI) can
be directly related to chlorophyll- a concentrations
obtained by conventional extraction and fluorimet-
ric analysis procedures (see below).
Algal groups Fluorimetry can detect and quan-
tify the biomass of different algal groups in relation to
their fluorescent emissions (from characteristic pig-
ments) at specific wavelengths. This approach was
used by Serra et al . (2007) to monitor the vertical
distribution of four dominant groups of algae (green
algae,cyanophytes,diatomsandcryptophytes)within
the water of a stratified reservoir. The fluorimeter
was operated at four different excitation wavelengths
and was calibrated by the manufacturer using model
algal species to convert fluorescence measurements
to chlorophyll- a concentrations, from which biomass
could be derived.
Potential limitations with this technique are:
PCI gives an immediate and in situ assessment of
biomass. It has an advantage over algal count pro-
cedures, since small phytoplankton cells that can-
not be counted under the light microscope contribute
to the coloration of the filtering silk. PCI analyses
are particularly useful for low nutrient (oligotrophic)
waters where large areas are being analysed, and
show good correlation with surface chlorophyll con-
centrations as determined by direct measurement and
satellite methods. This approach has been used by
various Water Authorities as part of lake manage-
ment, providing a rapid and rough guide to phy-
toplankton levels. In this context, PCI does have a
major disadvantage in not indicating which species
are present and thus what potential problem algae are
there.
Crosscontamination . Specificity of emission is not
absolute. Chrysophytes, for example, emit fluores-
cence at a similar wavelength to cryptophytes, and
(in the study of Serra et al ., 2007) would thus con-
tribute to the signal obtained from the dominant
cryptophyte community.
Photochemical quenching . Under high irradiance
values, excess light energy is dissipated as heat.
This 'photochemical quenching' occurs in surface
waters, down to a depth where light intensity is
about 10% of the surface water value and leads
to an underestimate of chlorophyll concentration -
which needs to be corrected.
Laboratory pigment analysis
Laboratory procedures for measuring pigments are
accurate and straightforward, and can be carried out
both in vivo (direct measurement from algae in con-
tainer) and by extraction (from water samples).
Pigment analysis of laboratory samples can be
carried out by fluorimetry (Table 2.1), spectropho-
tometry or high-performance liquid chromatography
(HPLC). The latter techniques require filtration and
pigment extraction prior to measurement of pigment
concentrations.
Colorimetric analysis One of the simplest ways
of estimating chlorophyll concentration is to make
a direct colorimetric assessment of net biomass.
Although this has been used particularly for marine
phytoplankton (McQuatters-Gollop et al ., 2007), it
also has potential for large freshwater bodies where
an extensive area of lake is being analysed. The tech-
nique is as follows:
Filtration Water samples collected in the environ-
ment are normally brought back to the laboratory and
filtered as rapidly as possible to remove all the algae -
including the smallest organisms. Filtration is nor-
mally carried out using either a cellulose acetate filter
membrane (0.45 μm) or a glass fibre filter (Whatman
GF/C, 1.2 μm). Samples should not be exposed to
high light or temperature prior to filtration (to min-
imise pigment destruction), and the filtration process
Net-haulcollectionofsamplesusingarecorderthat
is towed behind the ship. With marine systems, this
is carried out over a speed of 28-37 km h −1 ,ata
mean depth of ∼10 m, over a distance of 18 km.
Phytoplankton biomass is collected on a fine silk
mesh.
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