Environmental Engineering Reference
In-Depth Information
140
120
Biovolume
100
80
60
40
Chlorophyll-
a
20
0
1
2
3
4
5
6
7
8
9
10
11
12
WINTER
PHASE
SPRING
BLOOM
CLEAR
WATER
PHASE
SUMMER
BLOOM
AUTUMN
DECLINE
4.5
4
Figure 2.8
Monitoring phytoplan-
kton biomass in lake water -
relationship between chlorophyll-
a
concentration, total biovolume and
Secchi depth. Seasonal changes in
estimated biovolume (expressed as
μm
3
l
−1
× 10
4
) closely follow
chlorophyll-
a
concentration (μgl
−1
).
Water turbidity, measured as Secchi
depth (m), is inversely related to the
above parameters. Reproduced with
permission from Qari, 2006.
3.5
3
2.5
2
1.5
1
0.5
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
sample in a thermostatically controlled oven at 105
◦
C
to constant weight (normally taking about 24 h). To
obtain ash-free dry weight, the sample is heated in a
thermostatically controlled muffle furnace for 1 h (or
to constant weight) at 300-500
◦
C, and the resulting
ash-weight deducted from the dry weight to obtain
the ash-free value.
Limitations in the direct determination of biomass
as dry weight are as follows:
2.3.2 Dry weight and ash-free dry weight
One of the most direct ways to measure algal biomass
is to simply collect phytoplankton in a net or using
a membrane (glass fibre or Millipore© fibre) fil-
ter from a water volume sample and obtain values
for wet weight (inorganic and organic biomass, plus
water), dry weight (inorganic and organic biomass)
or ash-free dry weight (organic biomass only).
Wet weight is not normally determined because
of the variability in removing free water from the
sample.
In practice, dry weight or ash-free dry weight
is determined from at least three replicate water
samples. Dry weight can be measured by drying the
The phytoplankton sample is liable to contain
contaminant non-algal material, such as particu-
late debris and zooplankton - both of which con-
tribute to dry weight. The presence of organic and
inorganic particulate material (from sediments) is
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