Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 40

depends on the proportion of separation valves to
linking valves.

Gelatinous stalks. Biofilm diatoms such as
Rhoicosphenia (Fig. 4.67) and Gomphonema
(Fig. 2.29) have gelatinous stalks, which attach the
diatom to the substratum and join small groups of
cells together as a single colony. In these diatoms,
the stalk material is secreted via apical pores. Some
species of Cymbella also produce a gelatinous
tube-like filament.

100 μm

1.10.3 Ecology

Figure 1.16 Spring diatom bloom in a eutrophic lake.
SEM view of lake surface phytoplankton sample
(March), showing numerous stellate colonies of Aste-
rionella (A) with some filamentous Aulacoseira (Au).
The large amount of debris (d) occurs due to the com-
plete mixing of the water column at this time of year. See
also Figs. 2.8 (seasonal cycle), 4.9 (live Aulacoseira )and
4.42 (live Asterionella ).

Diatoms are ubiquitous in both standing and running
freshwaters - occurring as planktonic, benthic, epi-
phytic (on higher plants and other algae) and epizoic
(on animals such as zooplankton) organisms. They
are equally successful as free-floating and attached
forms, where they may be respectively important in
plankton bloom formation and biofilm development.

has elongate cells which are joined at their inner
ends to form stellate colonies. In Tabellaria , cells
are linked valve to valve as short stacks, with con-
nection of the stacks at frustule edges by mucilage
pads to form a zig-zag pattern. Mucilage pads are
seen particularly clearly in Fig. 2.28, where chains
of Tabellaria are attached to Cladophora .

Planktonic diatoms: bloom formation

In many temperate lakes, diatoms such as Asteri-
onella and Tabellaria dominate the phytoplankton
population in Spring and early Summer (Fig. 1.16),
atatimewheninorganicnutrients(N,P,Si)areathigh
concentration, light and temperature levels are rising
and lake turbulence is maintained by moderate wind
action. At this point in the seasonal cycle, diatoms
are able to out-compete other microalgae due to their
tolerance of low temperature and low light condi-
tions, coupled with their ability to grow in turbulent
water. Autumn blooms of Fragilaria and Asterionella
are also a common feature of many lakes, with the
diatoms part of a mixed phytoplankton population.
Diatoms such as Aulacoseira italica are regarded
as meroplanktonic, with major growth in very
early Spring, but spending the rest of the year as
resistant cells on lake sediments. Other diatoms,
such as Asterionella and Tabellaria also form major
blooms in Spring, but are additionally present as
minor constituents within the mixed phytoplankton
population over much of the annual cycle - and are
regarded as holoplanktonic.

Interlinking spines. In diatoms such as Aulaco-
seira , chains of cells are joined valve to valve by
spines. The valves are of two main types (Davey
and Crawford, 1986):

- Separation valves, with long tapering spines and
straight rows of pores

- Linking valves, with short spines and curved
rows of pores

Cells are strongly linked into chains via the short
spines on the linking valves, but weakly linked via
the long spines of the separation valves - where fil-
ament breakage easily occurs (Fig. 4.9). The length
of filaments within a population of Aulacoseira