Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 95

Surface structure of biofilms can be viewed using
the environmental scanning electron microscope
(ESEM). This allows direct visualisation of the
biofilm in the natural wet state.

extended growths and dense area coverage. Vari-
ous studies have been carried out on diatom popu-
lations within periphyton and mature biofilms for the
determination of bioindices and assessment of water
quality (see Section 3.4.5). These require consistent
protocols for sampling and analysis at different sites
in the study area (see Section 2.8.1).

Matrix variation with depth in the biofilm can be
observed using the scanning confocal laser micro-
scope(SCLM).Usingarangeoflectinstolabeldif-
ferent polysaccharide components, Droppo et al .
(2007) used this approach to demonstrate a highly
stratified matrix structure, with a change in carbo-
hydrate composition through the matrix as differ-
ent organisms contributed to the matrix along the
temporal succession.

Experimental conditions

Development and growth of periphyton can be stud-
ied experimentally by placing fresh surfaces into the
stream or other shallow water environment. Glass
slides are particularly useful for this, since they can
be exposed for set periods of time, then brought
back to the laboratory and directly examined by light
microscopy. Etched rather than smooth slides are
often used, as some algae appear not to stick to the
smooth surface for attachment.
The periphyton sampler shown in Fig. 2.27 (con-
taining glass slides) is designed to be placed mid-
stream, and has floating baffles and a current deflector
to reduce the full force of the current. Other mate-
rials can also be used for algal colonisation. Hill
et al . (2008) studied the growth of periphyton by
placing ceramic tiles in both indoor (experimental)
and outdoor (natural) streams. Periphyton was col-
lected by carefully removing the tiles, then placing
them in Petri dishes which were frozen and stored
at −85 ◦ C. Periphyton was later harvested by brushing
the thawed tiles, collecting the slurry on preweighed
filter membranes and then drying at 60 ◦ C for 24 h to
determine dry mass.

The extracellular polysaccharide fibrils that consti-
tute the bulk of the bioilm matrix can be viewed by
cryoscanning electron microscope (Defarge et al .,
1996).

2.10 Periphyton - algal mats

The development of periphyton (extended filamen-
tous growths) represents an end point in attached
community development (Fig. 2.24) and is typical
of shallow waters in lakes and streams. The substra-
tum can be either inorganic (e.g. stones) or organic
(e.g. macrophyte surfaces) - Table 2.10.

2.10.1 Inorganic substratum

The composition of mature biofilm and periphyton
populations can be studied in terms of existing nat-
ural communities or by inserting fresh surfaces into
environmental or laboratory conditions.

2.10.2 Plant surfaces

Natural communities

Epiphytic algae are common in many shallow river
and standing waters attached to plant surfaces -
including macroalgae, mosses and higher plants
(Table 2.10). As with other attached communities
(Fig. 2.1), these algae interact with planktonic popu-
lations in terms of recruitment and loss. This section
commences with some examples of epiphytic com-
munities, followed by an account of experimental
procedures.

Maturenaturalalgalcommunitiesonrocksandstones
(Fig. 2.23) are often dominated by mats of filamen-
tous blue green algae (such as Phormidium ), with
large attached diatoms that radiate out from a basal
attachment point (e.g. Synedra ) or project out on
stalks (e.g. Gomphonema ). As with mature terres-
trial communities, competition for light results in