Manual - International Environmental Technology Centre

3.H. FRESHWATER BIOTA
PHYTOPLANKTON
Freshwater phytoplankton is the algal component
of plankton, which are free-living organisms within
aquatic environments. Phytoplankton is represented
by prokaryotic cyanobacteria and several
groups of eukaryotic algae.
Phytoplanktonic organisms are autotrophs, i.e.,
they fix solar energy by photosynthesis using carbon
dioxide, nutrients and trace metals. They comprise
the major portion of primary producers in
most fresh waters. Like plants on land, they provide
basic food for higher trophic levels such as
zooplankton and fish.
Nutrients are necessary for algal development, however,
their surplus (especially phosphorus)
due to catchment degradation, for example, may
lead to formation of blooms that degrade water
quality.
Cyanobacteria
filaments or round 3-4 µm diameter prokaryotic
cells that can build large dense colonies,
100 to 500 µm in diameter (Fig. 3.2);
they often form seasonal blooms in late summer
in temperate lakes, reservoirs, and seas.
produce toxic compounds that pose a health
hazard to people and animals;
colonies are not easily ingested by aquatic
fauna due to their large size;
destabilization of a reservoir’s hydrological
characteristics may reduce cyanobacterial
growth.
Fig. 3.2
Microcystis
wesenbergi
(photo: P. Znachor)
Fig. 3.3
Fragilaria
crotonensis
(photo: P. Znachor)
Diatoms
the most morphologically varied group, including
single-cell and colonial species (Fig. 3.3);
the cells are covered with a cellular membrane
hidden inside box-shaped silicate shell;
unable to move actively, they may have a problem
keeping suspended in the water column
and that is why they prefer turbulent
mixing conditions;
The dominant group in spring-early summer
and in autumn.
Phytoflagellates
possess flagella, which enable migration through
a water column;
some taxa, especially dinoflagellates, cryptomonads
and euglenoids, can be temporarily
heterotrophic (Fig. 3.4);
can dominate throughout the year, especially
in winter (some dinoflagellates and cryptomonads),
or early summer (chrysophyceae);
may form blooms that sometimes can be
toxic, e.g., Peridinium sp.
Green Algae
characterized by their grassy green colour,
they are among the main sources of food for
filtering fauna (Fig. 3.5).
high diversity of cell size and cellular organization
(single cells, colonial and filamentous)
and may be both motile and non-motile.
in fresh water they are dominant, especially
during the second half of summer-autumn.
Sometimes they may reach bloom density.
Fig. 3.4
Ceratium
hirundinella
(photo: P. Znachor)
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Fig. 3.5
Scenedesmus quadricauda
(photo: M. Tarczynska)
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Introduction: Basic Concepts & Definitions