Environmental Engineering Reference
In-Depth Information
Environmental changes
Lake d'Orta had been
polluted with copper, other metals (Zn, Ni, Cr) and
acid (down to pH4) for a period of over 50 years
- commencing in 1926 and reaching maximum pol-
lutant levels (30-100 μgCul
−1
) between 1950 and
1970. Lake sediment cores collected after 1990 were
analysed for fossil remains of diatoms, thecamoe-
bians (protozoa) and cladocerans (zooplankton), all
of which showed a marked reduction in mean size
during the period of industrial pollution.
1994
1970
1951
1928
1859
Diatom response to pollution
The initial
impact of pollution, recorded by contemporary analy-
ses, was to dramatically reduce populations of phyto-
plankton, zooplankton, fish and bacteria. Subsequent
sediment core analyses of diatoms showed that heavy
metal pollution:
1790
~1750
Caused a marked decrease in the mean size of indi-
viduals. The proportion of cells with a biovolume
of
10
2
μm
3
increased from under 10% of the total
population in 1920 to over 60% in 1950.
<
~1600
The decrease in mean diatom size was caused
primarily by a change in taxonomic composition
fromassemblagesdominatedby
Cyclotellacomen-
sis
(10
2
-10
3
μm
3
) and
Cyclotella bodanica
(10
3
-
10
4
μm
3
) to populations dominated by
Achnanthes
minutissima
(
8
12
16
Length (µm)
Figure 3.7
Changes in the average valve length of the
diatom
Achnanthes minutissima
measured along a sedi-
mentcorecollectedinLaked'Orta(Italy).Thesizeofthis
heavy metal-tolerant diatom showed a marked decrease
during the period of industrial contamination (shaded
area), falling to 9 μm by the end (1970) of the most acute
period of toxicity. Figure adapted and redrawn from Cat-
taneo
et al.
, 1998.
10
2
μm
3
).
<
The change in mean size was also caused by a
shift in the size within a single taxon -
A. minutis-
sima
. The mean length of this diatom decreased
significantly from about 14 μm before pollution
to a minimum value of 9 μm during 1950-1970
(Fig. 3.7).
Achnanthes minutissima
as a bioindicator
Dominance of Lake d'Orta under conditions of heavy
metal pollution by
A. minutissima
is in agreement
with the known ability of this diatom to withstand
strong metal stress. Other studies (Takamura
et al
.,
1990) have shown it to be dominant in streams
subject to heavy metal contamination. The diatom
is cosmopolitan, however, often common in benthic
assemblages of neutral waters, and its domination
of a particular environment is not therefore directly
indicative of heavy metal pollution. In spite of this,
the presence of dominant populations (coupled with
a decrease in mean cell size) is consistent with severe
environmental stress - and would corroborate other
environmental data indicating heavy metal or acid
contamination.
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