Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 90

Table 2.7 Diatom Succession in Algal Biofilms.

Species Growth Form Growth Dynamics Light Adaptation

Early colonisers (phase I) Attached diatoms High irradiance species

High G max and I s values

Gomphonema angustatum Short chains of cells Settlement from high

population level in water
column

Meridion circulare Attached rosettes Fast immigration due to rapid

reproduction after
attachment

Surirella ovata Single cells or pairs Fast immigration due to rapid

attachment

Mid-successional species
(phase II)

Attached diatoms Low irradiance species

Low G max and I s values

Cymbella sp. Gomphonema
olivaceum

Vertical growth via long
mucilaginous stalks

High growth rate maintained
in dense algal conditions

Late-successional species
(phase III)

Unattached, tangled
diatoms

Low irradiance species
Moderate G max and very
low I s values

Fragilaria vaucheriae Unicellular High growth rate

Aulacoseira varians Chain forming maintained in dense algal

Cocconeis placentula Prostrate conditions

The transition from early colonising diatoms to mid- and late successional species is related to growth form, growth dynamics and light
adaptation (see text). Light adaptation parameters: G max - estimated maximum growth rate; I s - estimated light intensity at half G max .

of grazing by invertebrates) are also a key deter-
minant. The sequence shown in Fig. 2.24 gives an
indication of general trend (from pioneer commu-
nity to mature periphyton) and may terminate as a
mature community at any point in the succession,
depending on conditions. The intermediate biofilm
community varies considerably, ranging from a
mucilaginous biofilm (typically rich in bacteria, but
containing algae) to a non-mucilaginous community
composed largely of attached algae - particularly
diatoms. In conditions which promote active algal
growth, the sequence may thus progress from pioneer
to periphyton community, with little development of
a mucilaginous biofilm.
Factors affecting community development can be
studied under controlled laboratory conditions. Gain-
swain et al . (2006), for example, looked at the influ-
ence of substrate particle size - fine material (

biofilm development and phosphorus release in a lab-
oratory fluvarium. Progression to periphyton (sub-
stantial filamentous mass) only occurred with gravel
and stone substrate - the fine material was not suffi-
ciently stable to support attached filamentous algae.

2.9 Algal biofilms

Biofilms are monolayer communities of microorgan-
isms occurring at a physical interface, which in fresh-
water systems includes water/air boundaries (neuston
biofilm at the water surface) and substratum/water
boundaries - such as the surfaces of fine sediments,
rocks and plants. Microalgae are important con-
stituents of biofilms, both in the developing biofilm
and in the mature community. This may remain at
an early stage of development, such as the diatom
biofilm on estuarine sediments (Sutherland et al. ,

< 2

mm), gravel (2-20 mm) and stones ( > 20 mm) - on