Environmental Engineering Reference
In-Depth Information
Reduced levels of dissolved organic carbon, with
derivation largely from external lake (allochtho-
nous) rather than internal (autochthonous) sources.
water bodies being low nutrient and neutral to acidic,
while lowland sites in agricultural areas are typically
eutrophic and alkaline.
Food web changes, including a reduction in the
number of herbivores.
Nutrient status
Many desmids (e.g.
Closterium
)
are particularly characteristic of oligotrophic (low
nutrient) lakes and ponds - often in conditions that
are also slightly acidic (see above) and dystrophic
(coloured water). Desmid diversity is particularly
high in these conditions, sometimes with hundreds of
species occurring together at the same site (Woelk-
erling, 1976). Desmids are also typical of nutrient-
poor streams, where they are permanent residents
of periphyton - making up to 10% of total com-
munity biomass. These desmids are associated with
plants such as the bryophyte
Fontinalis
, achieving
concentrations as high as 10
6
cells g
−1
of substrate
(Burkholder and Sheath, 1984). Desmids often con-
stitute a significant proportion of algal biomass in
peat wetlands (see Section 3.3.2), where they are also
a major aspect of species diversity.
Some species of desmids - such as
Closterium
aciculare
- are more typical of high-nutrient, slightly
alkaline lakes and are indicators of eutrophic con-
ditions. Periphytic algae typical of eutrophic waters
include
Cladophora.
Laboratory studies (Graham
et al
., 1996a, b)
showed that
Mougeotia
was physiologically adapted
to such conditions, with the ability to photosynthe-
sise over a wide range of irradiances (300-2300 μmol
quanta m
−2
s
−1
) and a tolerance to a broad range of
both pH (pH 3-9) conditions and metal concentra-
tions. The physiological adaptations of this alga, cou-
pled with reduced grazing from herbivores, probably
accounts for the extensive growth and domination of
the benthic environment in many acidic waters.
Alkaline and calcium-rich waters
Some
planktonic (
Closterium aciculare
) and benthic algae
(e.g.
Nitella
and
Chara
) are adapted to grow in alka-
line habitats, many of which are also rich in cal-
cium (hard waters).
Nitella
produces lush meadows
on the bottoms of neutral to high pH lakes, and
Chara
can form dense, lime encrusted lawns in shal-
low alkaline waters.
Hydrodictyon
is also typical of
hard-water conditions, occasionally blanketing the
surface of ponds and small lakes, and occurring
widely in larger alkaline standing waters and hard-
water streams. Extensive growths of this alga can
also be observed in agricultural situations such as
irrigation ditches, rice paddy fields and fish farms -
where there is some degree of eutrophication.
Some calcium-loving algae occur in very restricted
environments. The unusual, slow-growing desmid
Oocardium stratum
, for example, occurs in calcare-
ous streams and waterfalls at the top of branched
calcareous tubes, in association with deposits of
'travertine' and 'tufa'.
Acid lakes
Although desmids are typical of neu-
tral to slightly acid lakes, other green algae are
adapted to more extreme conditions. These include
the filamentous green alga,
Mougeotia
, which can
form substantial sub-surface growths in acidified
waters and is widely regarded as an early indicator
of early environmental change (Turner
et al
., 1991).
Where acidification is the result of acid precipitation,
experimentation (Webster
et al
., 1992) or industrial
pollution, the acid conditions also tend to be associ-
ated with
Saline waters
Various green algae are adapted to
salineconditionsinsaltlakesandestuaries,wheresalt
levels range from low (brackish waters) to high. With
some widely occurring genera such as
Scenedesmus
,
brackish conditions are at one end of the contin-
uum of environmental tolerance. Other algae, such as
Dunaliella
,arespeciicallyadaptedtoamoreextreme
Relatively clear waters, due to low levels of
phytoplankton
Increases in the concentration of metals such as Al
and Zn
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