Environmental Engineering Reference
In-Depth Information
Because of the above limitations, phytoplankton
biomass is normally measured in relation to pig-
ment concentration rather than dry weight. Contami-
nant non-algal material, the major problem with dry
weightdeterminations,isnotrecordedinchlorophyll
-
a
estimations of biomass.
2.3.3 Pigment concentrations
Another approach for estimating phytoplankton
biomass is to determine the concentration of a
biomass-related constituent such as pigment (e.g.
chlorophyll-
a
) or organic carbon, expressed per unit
volume or per unit surface area of water. Using
chlorophyll-
a
, the level of phytoplankton can either
be monitored directly as aquatic pigment concen-
tration (e.g. Fig. 2.8) or be converted to estimated
biomass, with an appropriate conversion factor.
There are various limitations in the use of pigment
concentrations to assess biomass.
There is no precise relationship between pig-
ment concentration and biomass. Pigment concen-
trations in algae vary between species and also
within species in relation to external (light, tem-
perature, nutrient availability) and internal (algal
physiology) parameters. In general, chlorophyll-
a
content varies from 0.9% to 3.9% ash-free dry
weight (Reynolds, 1990), and different authors
have used different values to estimate biomass.
Assuming a mean value of 1.5% ash-free dry
weight, algal biomass can be estimated by mul-
tiplying the chlorophyll
-a
content by a factor of
67 (Eaton
et al
., 2005). Chlorophyll-
a
can also be
related to carbon content. Welker & Walz (1998),
for example, converted their river chlorophyll con-
centrations to phytoplankton biomass assuming a
carbon/chlorophyll-
a
ratio of 20 and a carbon con-
tent of 50% dry weight.
Figure 2.9
Monitoring buoy. Remote-sensing station
used for continuous recording of lake parameters on
Rostherne Mere, UK. Aerial sensors record air tempera-
ture, wind speed/direction, solar radiation and incident
photon flux density. Underwater probes monitor con-
ductivity,watertemperature,chlorophyll-
a
concentration
and turbidity (see Fig. 2.10) to a depth of 25 m.
particularly high during winter, when the lake may
be fully mixed. This occurs at a time when the
phytoplankton population in the epilimnion is sea-
sonally low, and both of these factors lead to a
highproportionofparticulatedebrisinphytoplank-
ton samples at this time of year. Wetzel (1983)
has estimated that this particulate material may
contribute over 80% of the total seston in certain
situations.
Pigments of photosynthetic bacteria may also con-
tribute to the total chlorophyll estimation, so that
the final concentration will not only relate to algae.
The drying process can result in appreciable loss of
volatile organic compounds, leading to an under-
estimate of dry weight. This effect can be reduced
by drying at lower temperature (80-90
◦
C).
Any contaminant zooplankton in the sample may
also contain ingested algae, which will also
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