Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 53

monthly intervals. Algal remains recovered from sed-
iment traps broadly reflected phytoplankton peri-
odicity and showed that turbulent resuspension of
sedimented material occurred infrequently.
The use of traps to collect phytoplankton (sedi-
menting algae) can be compared with recruitment
traps to collect benthic algae (rising from the sedi-
ments) - Fig. 2.22, Section 2.7.1. In both cases, the
traps are emptied at regular intervals and there are
potential problems with algal decomposition and the
dynamics of collection.

2.3.1 Turbidity

Turbidity is caused by suspended particulate and
colloidal matter in water, including both inor-
ganic (e.g. dispersed sediment particles) and organic
(e.g. phytoplankton, zooplankton) material. Turbid-
ity can be monitored manually using a Secchi
disk or electronically via a fluorimeter probe as a
nephelometer.

Secchi disk

2.3 Phytoplankton biomass

Turbidity is frequently monitored as Secchi depth,
which involves lowering a black/white sectored Sec-
chi disk from the side of the boat until it can no longer
beseen,thenraisingthediskuntilitisjustvisible.The
distance from disk to surface is the 'Secchi depth'.
For consistency over a period of time, the operation
should be carried out on the sun-facing side of the
boat and by the same operator. Although Secchi depth
appears to be rather a crude estimate of turbidity, and
includes both phytoplankton and non-phytoplankton
particulates, there is a good inverse correlation with
chlorophyll- a measurements (Fig. 2.8). In many
standing waters, determination of Secchi depth has
been found to be a simple and reliable approach
to monitoring changes in seasonal phytoplankton
biomass.

Determination of phytoplankton biomass is impor-
tant since it provides information on the primary pro-
ductivity of the aquatic system, and the amount of
organic material that is available for consumption by
zooplankton and the rest of the food chain.
Algal biomass can be determined as two major
parameters.

Total biomass. Where the overall value for the
entire phytoplankton population is being evaluated
by direct measurements from the lake water.

Species and group biomass. Where the biomass for
individual species and higher taxonomic groups is
being indirectly estimated from population counts
and biovolumes (see Section 2.5.4).

Nephelometer

Total phytoplankton biomass can be directly esti-
mated in three main ways - turbidity measurements,
determination of fresh/dry weight and analysis of
aquatic pigment concentration. Some of these mea-
surements can be carried out from the boat and
are part of the on-site collection of ecological data.
These in situ methods include direct measurement
of water turbidity (Secchi depth), use of fluorime-
ters to determine pigment/particulate concentrations
and colorimetric assessment of chlorophyll concen-
tration. Other techniques for determining pigment
concentration and fresh/dry weight involve collection
ofwatersampleswithsubsequentlaboratoryanalysis.

Fluorimeter probes can function as nephelometers,
collecting reflected light from suspended matter to
determine the particulate concentration of phyto-
plankton and other ine materials in the water column.
Nephelometer operation, calibration and interpreta-
tion of data are detailed by Eaton et al . (2005). These
instruments have the advantage that they can operate
as part of a remote-sensing system (Fig. 2.9), allow-
ing turbidity to be measured on a continuous long-
term basis. Fluorimeters are normally used to monitor
chlorophyll- a concentration and turbidity simultane-
ously (Fig. 2.10) and are described in more detail in
Section 2.3.3.