A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 217

143 (141) (a) Cells not markedly elongate but
square to rectangular. Frustules have four
spines, one arising from each corner and
either projecting in the direction of the
long axis of the cell or at a shallow
angle. Many girdle bands visible. Four
discoid to plate-like chloroplasts per cell.
......................... Acanthoceros

Tabellaria forms zig-zag, almost linear
or sometimes stellate colonies. Cells rect-
angular to oblong in shape and usually
seen in girdle view - making the promi-
nent septa easy to see. In valve view
they are elongate, slightly capitate with a
slightly inflated mid-region. Chloroplasts
elongate and lying between the septa.
Tabellaria fenestrata (33-116 μm long,
4-10 μm wide) and T. flocculosa (6-
130 μm long,

Acanthoceros cells(upto40× 80 μm
in size) are normally solitary with one to
many plate-like/disc-shaped chloroplasts.
Found in the plankton of enriched lakes
usually in slightly alkaline conditions.
Bacillariophyta. Plate XX.

5 μm wide) are the most
frequent species. Widespread in plank-
ton (oligotrophic/mesotrophic waters) or
attached to stones or submerged vegeta-
tion. Reported to produce fishy tastes in
drinking water if present in large numbers
(Palmer, 1962). Bacillariophyta Plate IV.
Figs. 2.28 and 4.11.

<

(b) Cells square to shallow rectangular in
shape, up to 15 × 30 μm in size, containing
1 or 2 cup-shaped chloroplasts. Few girdle
bands visible. Spines project at 45 ◦ angle
from each corner. . ......... Chaetoceros

(b) Valves isopolar with costae but no
septa......................... Diatoma

(see also key No. 29)

Chaetoceros cells are usually joined
together into filaments by the interlock-
ing of the spines. Found in waters with
a high conductivity, especially in coastal
regions, and in brackish waters. Bacillar-
iophyta. Plate XX.

Diatoma cells often form ribbon or
zig-zag-shaped (occasionally stellate)
colonies. In girdle view, the valves are
rectangular to oblong with small plate-
like or discoid chloroplasts. There are dis-
tinct thickened transverse bars (costae)
across the valves. Valve ends sometimes
capitate, rostrate or bluntly rounded. Cells
up to 120 μm long. Epiphytic or plank-
tonic, in littoral regions of lakes or slow
flowing rivers. Can impart an odour to
drinking water (Palmer, 1962). Bacillar-
iophyta. Plate V. Fig. 4.12.

144 (139) (a) Cells cuneate, heteropolar and form-
ing fan-shaped colonies . ...... Meridion

(see also key No. 77)

(b) Cellsnotasabove.............. 145

145 (144) (a) Cells with either costae or septa
present........................... 146

Williams (1985) separates two subgenera.
One with uniseriate rows of striae, promi-
nent internal ribs and scattered spines
near polar areas. Chloroplasts plate-like
or discoid. A second subgenus, Odon-
tidium , with a more diffuse area of
markings and lobed chloroplasts is also
recognised.

(b) Cells with neither costae nor septa
present........................... 147

146 (145) (a) Valves swollen in the middle and
with slightly swollen poles. Isopolar
............................ Tabellaria

(see also key No. 29)