Environmental Engineering Reference
In-Depth Information
star-shaped colony being slightly larger than
theouterone................
Asterionella
Asterionella
frustules are long (40-130 μm),
straight and narrow - up to 20 times as
long as wide. The narrow central part of the
frustule does have fine transverse striae but
these are difficult to see. There are usually
two or more chloroplasts per cell.
Asteri-
onella
can be abundant in the plankton of
lakes especially in the spring (spring diatom
bloom) and to a lesser extent in the sum-
mer and autumn. Can impart a geranium
or spicy odour to drinking waters (Palmer,
1962). Large growths of
Asterionella
can
cause problems of filter clogging in water
treatment works supplied by lakes or reser-
voirs. Bacillariophyta. Plate XI. Figs. 1.16
and 4.42.
100 μm
(b) Frustules not swollen at each end,
colonieseitherstellateorzig-zag.......
79
79
(78) (a) Frustules with neither internal septa nor
costae..........................
Synedra
Synedra
has elongate, linear, isopolar valves.
Thereisanarrowpseudorapheandinetrans-
verse striae. Colonies of
Synedra
may be
stellate or short chains but it can also be
present as single cells or as an attached epi-
phyte. Widely distributed and may be abun-
dant. Can produce both unwanted taste and
odour problems in drinking waters and, when
in large numbers, clog filters (Palmer, 1962).
Bacillariophyta. Plate XXVI. Figs. 2.23 and
4.43.
(see also key No. 191)
10 μm
Figure 4.42
Asterionella
. Top: Stellate planktonic
colony, double spiral. Cells with 5-10 brownish chloro-
plasts.Bottom:Aciddigest,lakesedimentsample.Single
cell showing frustule with rounded ends. Images repro-
duced with permission from G. Ji (top) and M. Capstick
(bottom).
(b) Frustules have either internal septa or
costae................................
80
NB.
Although the swollen centre can be
clearly seen in valve view if you are observ-
ingastellatecolonyyouseethecellsingirdle
view and the swelling is not visible.
Tabel-
laria
cells, unlike those of
Asterionella
with
whose colonies they may be confused, do not
have swollen and rounded ends to the cells.
However, shorter cells and colonies of
Aste-
rionella
do appear (sometimes less than a
80
(79) (a) Frustules with internal longitudinal septa
but no costae. There are small polar and
larger central swellings when seen in valve
view..........................
Tabellaria
(see also key No. 29)
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