Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 57

Table 2.1 Fluorimeter Detection of Algal Pigment Concentrations and Aquatic Turbidity.

Measured Parameter Total Algal Biomass Cyanophyta Cryptophyta Turbidity

Pigment/ turbidity Chlorophylls- a and - b Phycocyanin Phycoerythrin Particle concentration

Excitation wavelength a 470 (30) nm 590 (30) nm 530 (30) nm 470 (30) FTU

Emission wavelength a

685 (30) nm 645 (35) nm 580 (30) nm 470 (30) FTU

Concentration range 0.03-100 μgl −1 0.03-100 μgl −1 0.03-100 μgl −1 0.04-100 FTU

a Wavelength values are given as centre wavelength and bandwidth.
FTU, fluorimeter turbidity unit.

water column or as part of an automated remote
recording system. The monitoring buoy illustrated
in Fig. 2.9 is permanently positioned at a deep water
site within a eutrophic lake and makes periodic depth
recordings of chlorophyll- a , turbidity and water tem-
perature. Equipment should be calibrated against
known chlorophyll- a concentrations, and periodi-
cally checked. The sensitivity scale should also be
adjusted so that low phytoplankton levels (few cells
per litre) can be detected as well as blooms.
Such in situ profiling of lake parameters from a
monitoring buoy provides a long-term record of phy-
toplanktonbiomassinrelationtobothshort-term(e.g.
diurnal fluctuations) and seasonal (relation to lake
stratification, peaks of algal population) changes. The
development of new high-resolution bio-optical sen-
sors (Wolk et al ., 2004) also permits highly detailed
phytoplankton profiles to be obtained, giving infor-
mation on small-scale patterns of distribution (see
Section 2.1.1). Environmental information collected
by the remote-monitoring buoy is recorded and stored
within a data logger and can be accessed via the GSM
network.
Simultaneous recording of chlorophyll- a concen-
tration and turbidity (Fig. 2.9) provides a useful
comparison of these two parameters for measuring
phytoplankton biomass. In this capacity the instru-
ment is acting both as a fluorimeter (measuring
chlorophyll- a concentration) and as a nephelometer
(measuring particulate concentration) - where the
generation and collection of scattered light at the
same wavelength (470 nm) provide information on
aquatic turbidity. During the period of phytoplank-
ton decline (Fig. 2.10), the decrease in chlorophyll- a
concentration (approximately 5-1 arbitrary units) is

accompanied by a corresponding decrease in turbid-
ity. There is a distinct lag phase between the two sets
of data, however, consistent with a period of phy-
toplankton senescence (see Section 2.6.2) in which
cells are losing their chlorophyll but remaining in
suspension prior to sedimentation from the surface
waters.
Fluorimetric analysis of phytoplankton within the
water column is also relevant to river systems. Sher-
man et al . (1998), for example, used this approach
to monitor biomass changes in a turbid-river weir
pool that was dominated by the diatom Aulacoseira .
Mean vertical water-column chlorophyll concentra-
tions were computed at successive sites in the river,
overatimeperiodthatcoincidedwiththedownstream
transport of a parcel of water between sites. The
decrease in mean chlorophyll concentration between
sites was largely attributed to algal sedimentation.
Based on a range of assumptions, the sediment rate
( w )of Aulacoseira was calculated from the fluores-
cence data using the following equation (Condie and
Bormans, 1997):

w = Δ CH
C o Δ t

(2.1)

where Δ C is the observed change in abundance
(chlorophyll- a decrease 10.9-9.4 mg m −3 ), C o the
initial algal concentration in the surface layer (10.9
mg m −3 ), Δ t the time interval (0.722 days) and H the
mean water depth (5 m).
Using the above equation and values, Sherman
et al . (1998) calculated a sedimentation value of 0.95
md −1 , which (within the limits of their assumptions)
meant that the entire water column would have been
cleared of Aulacoseira in 5 days under stratified con-
ditions.