Unusual or Extreme Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 303

rooted freshwater macrophyte (Sand-Jensen et al., 1999). The cyanobacte-
ria Nostoc commune is probably the most common primary producer
found in aquatic high arctic aquatic habitats (Sheath and Müller, 1997).
High-altitude ponds or lakes are similar to polar lotic habitats because
they are ice-free for only a few months a year. The lakes and ponds in high
altitudes at temperate or tropical latitudes experience very high levels of light
in the summer, and the zooplankton in these habitats are often red or orange
because they contain carotenoids that protect against damage by UV-B.
Simple microbial communities can be associated with snowfields that oc-
cur in high mountainous or polar regions. Snow algae were noted first by Aris-
totle, and detailed study began in the early 1800s. The microbial primary pro-
ducers in snowfields can include chlorophytes, euglenoids, chrysophytes,
cyanobacteria, and diatoms. The primary producers can support a community
of fungi, bacteria, rotifers, protozoa, and some invertebrates (Hoham, 1980).
Increased photosynthesis by the snow algae leads to greater bacterial produc-
tivity (Thomas and Duval, 1995). This productivity can be transferred to the
terrestrial food web that includes small mammals and birds (Jones, 1999).
The most common algae in snow generally are single-celled green al-
gae (chlorophytes). The most obvious sign of these algae is the pink (“wa-
termelon”) snow associated with the psychrophilic Chlamydamonas nivi-
alis, a green alga that can acquire a strong reddish color produced by high
levels of carotenoids. These pigments are produced by C. nivialis and other
species of Chlamydamonas to protect the cells from ultraviolet irradiance
(Bidigare et al., 1993). The irradiance is extremely high at the snow sur-
face because of the high altitude and the reflective properties of snow.
A problem in the life cycle of C. nivialis is how to inhabit the upper,
lighted portions of snowfields when they are buried each winter. The spores
of the alga rest in the soil over the winter and hatch and swim to the sur-
face when the snow starts melting. The motile cells then reproduce sexu-
ally and produce more resting spores.
Microbial communities have also been described from the slush and
snow on the surface of alpine lakes (Felip et al., 1995). These communi-
ties include bacteria and autotrophic and heterotrophic ciliates. The pro-
duction of these communities can be higher than planktonic production in
the ice-covered water below. Many of the species present in the slush are
either derived from the plankton or from the snow pack above. Apparently,
some of the species are adapted to the icy habitats because they are found
mainly in the slush and not in the lake or snow nearby.
TEMPORARY WATERS AND SMALL POOLS
Drying is probably the most extreme disturbance that can occur in an
aquatic community. However, organisms colonize temporary or ephemeral
habitats within days or weeks. These habitats include temporary pools
(Fig. 15.6), streams, lakes, and wetlands. For some organisms, these rep-
resent marginal habitats, and for others they are the only habitats that can
be exploited successfully.
Temporary pools can be categorized according to the permanency of their
water. One scheme (Williams, 1996) divides pools and streams into those