Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 89

The motile algae will migrate through the tissue to the
light,andthetissuecanthenbecarefullyremovedand
placed on a microscope slide for observation. Alter-
natively, the surface layer can be carefully removed
for examination using a fine pipette.

biofilms, and the accumulation of organic debris as
a mature periphyton community develops.
Diatoms play a key role in the establishment and
development of attached communities, with a succes-
sion of different species involved at different phases
of community transition (Table 2.7). Early colonising
(phase I) diatoms attach rapidly to exposed surfaces
and are able to grow rapidly under conditions of high
irradiance. Mid successional (phase II) diatoms are
tolerant of a wider range of irradiances, while late
successional (phase III) species are able to maintain
a high growth rate at very low irradiances (shaded
conditions). Many of the phase III diatoms found in
mature algal communities are unattached, occurring
as tangled cells within the mature filamentous algal
growths. Filamentous blue-green algae are a major
component of these mature communities, tending to
dominate the periphyton community to form dense
algal mats under high light conditions.
A wide range of physicochemical factors control
attached algal community development - including
substrate stability, water flow rate, degree of turbu-
lence, light availability and inorganic nutrient supply.
Biological factors (e.g. constituent organisms, extent

2.8.2 Algal communities

The collection and quantitation of algae from differ-
ent microenvironments also depends on the type of
community that is being sampled. Algae associated
with underwater solid surfaces may be either freely
motile (moving by various mechanisms) or attached.
The development of attached algal communities
follows a general sequence from pioneer colonies
to biofilms and periphyton growths (Fig. 2.24).
The initial colonisation of freshly exposed surfaces
involves the settlement of planktonic algae and
other microorganisms onto a fine layer of organic
material (conditioning layer) that has adsorbed to
the solid surface. The importance of associated
organic material continues with the development of
a mucilaginous matrix during the formation of some

m

c

d

PERIPHYTON
COMMUNITY

PIONEER
COMMUNITY

ALGAL BIOFILM

Figure 2.24 Succession of attached algal communities in streams and shallow parts of lakes. The initial pioneer
community typically develops into a biofilm, which can be a simple monolayer of algae (lower figure) or a mixture
of organisms within a mucilaginous matrix (top figure). A dense mat of filamentous growths (periphyton) can develop
from either of these. Each community has its own region of associated organic material: conditioning layer (c), mucilage
matrix (m) and organic debris (d). Diagrammatic representation (not to scale) with algae shown as solid objects or
elongate filaments.