Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 16

Symbiotic association with aerobic bacteria - bac-
terial symbionts at the heterocyst surface main-
tain the local reducing atmosphere required for
nitrogen fixation and are also an important source
of inorganic nutrients in surface-depleted waters
(Sigee, 2004).

of life for about 1.5 billion years. As a result of
this long evolutionary history, they have adapted
to (and frequently dominate) all types of fresh-
water environment - including extreme conditions
(thermal springs, desiccating conditions), brackish
(semi-saline) conditions, high and low nutrient envi-
ronments and planktonic/benthic habitats. The ability
of blue-green algae to dominate freshwater environ-
ments is particularly important in standing waters,
where algal blooms may result due to eutrophication
(Section 3.2.3).

Species preferences

Many blue-green algae can tolerate a wide range of
environmental conditions, so strict ecological prefer-
ences are unusual in this group. Microcystis aerug-
inosa (Fig. 4.34), for example, can form massive
growths in eutrophic lakes, but low numbers may also
occur in oligotrophic waters (Reynolds, 1990). The
distinction between planktonic and benthic organ-
isms is also frequently not clear-cut. Microcystis is
regarded as a typical planktonic alga, but may also
occur as granular masses on lake bottoms, and (as
with the majority of temperate lake algae) over-
winters on lake sediments rather than in the water
column. Many benthic algae become detached and
either remain loose on the substratum or rise in the
water column and become free-floating. Colonies of
Gloeotrichia , for example, often detach from sub-
strates and become planktonic (Fig. 4.22), where they
grow and may reach bloom proportions.
In spite of these cautions, many blue-green algae
can be characterised in relation to particular envi-
ronments and whether they are typically attached
or planktonic. Some examples are shown in Table
1.6, for both 'normal' and 'extreme' environments.
As with other algae, ecological preferences typi-
cally relate to multiple (rather than just single) envi-
ronmental factors. Colonial blue-green algae, which
grow particularly well in high-nutrient lakes, are also
suitedtohardwaters(highMg,Ca)andtothealkaline
conditions typical of these environments. Conversely,
oligotrophic lakes, which support unicellular rather
than colonial blue-greens, tend to be soft, slightly
acid waters.

Algal blooms

In mid to late summer, eutrophic temperate lakes
frequently develop massive populations of colonial
blue-green algae. These may rise to the surface of the
lake, forming a thick layer of algal biomass at the top
of the water column, out-competing other algae and
having major impacts on zooplankton and fish popu-
lations (Sigee, 2004). The ability of blue-greens to
out-compete other freshwater algae has been
attributed (Shapiro et al ., 1990) to a range of charac-
teristics, including
Optimum growth at high temperatures - summer
temperatures.
Tolerance to low light - importantwithinthedense
algal bloom, and enabling algae to survive lower
in the water column.
Tolerance of low N/P ratios - allowing continued
growth when N becomes limiting.
Depth regulation by buoyancy - avoiding pho-
toinhibition during the early phase of population
increase, and allowing algae to obtain inorganic
nutrients from the hypolimnion when the epil-
imnion becomes depleted in mid to late summer.
Resistance to zooplankton grazing - limiting the
impact of herbivory on algal growth.

Tolerance of high pH/low CO 2 concentrations -
allowing continued growth of blue-greens (but not
other algae) at the lake surface during intense
bloom formation .

1.3.4 Blue-green algae as bioindicators

As with other algal groups, the presence or absence
of particular species can be a useful indicator of