A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 155

(b) Cell walls with granulated markings and
often with spines at end (often best seen at
end of a filament). Forming a continuous fil-
amentandnotinpairs......... Aulacoseira

In Aulacoseira , the cells are rectangular in
shape and are linked together to form long
filaments. The surface of the silica cell wall
has characteristic markings of rows of dots
(punctae) and the ends of the cells bear a
ring of spines, one or two of which may
be quite long that link the cells together. A
sulcus can usually be observed between the
mantle and girdle band. Chloroplasts disc-
shaped and usually golden-brown. Several
species are found in freshwaters. Some are
more common in eutrophic lakes, for exam-
ple, A. ambigua (cells 4-17 × 5-13 μmin
size) , A. granulata (cells 4-30 × 5-24 μm
in size), A. granulata var. angustissima has
narrow, long cells with an obvious long spine
protruding from the end cell and A. italica
(cells 6-23 × 8-20 μm in size) and oth-
ers in less enriched waters, for example, A.
islandica (cells 3-28 × 4-21 μm in size) and
A. subarctica (cells 3-15 × 2.5-18 μmin
size) . Aulacoseira granulata can cause filter
clogging problems in water works (see also
Melosira ). Bacillariophyta. Figs. 1.3, 4.8
and 4.9.

25 μm

10 μm

Figure 4.8 Aulacoseira. Top: Planktonic filament.
Cells with numerous disk-shaped chloroplasts. Bottom:
Fragmented filament. Acid digest, lake sediment sam-
ple. A terminal spine (arrow) is visible in both cases.
Images reproduced with permission from G. Ji (top) and
M. Capstick (bottom).

28 (26) (a) Cells with internal septa or costae (see
below)............................... 29

Costae are silica thickenings on the inside
of the valve, orientated at right angles to the
apical (long) axis. Septa are silica plates that
run across the inside of the cell from the
girdle bands partially or wholly dividing the
cell into chambers. The septa may protrude
for different lengths into the cell.

valve faces to form a ribbon-like chain.
Valves fusiform in shape, sometimes slightly
swollen at the centre (gibbous) or with
swollen (capitate) ends or narrow and rect-
angular in girdle view. Usually seen in gir-
dle view in non-acid-cleaned material. Valve
surface with fine striae. Two plate-shaped
chloroplasts present. Fragilaria crotonen-
sis can be found in meso- to eutrophic
waters, especially where phosphorus levels
are high, sometimes forming blooms. Size,

(b) Cells without internal septa
.............................. Fragilaria

In Fragilaria , the pennate or elongate frus-
tules are joined by the central part of their