Lakes and Watercourses

Phytoplankton in lakes
Introduction
Planktonic algae (phytoplankton) are an essential part of lake food chains
as a producer of organic matter and oxygen, as food for grazing animals
(zooplankton, ciliates, benthic fauna, fish) and because they excrete
dissolved organic matter, which also serves as a source of energy and
nutrition for other microbes.
Algae respond rapidly to changes in water quality because of their
rapid reproduction rate. Changes in the physical and chemical status of
the water can be identified after only a week or so in the form of changes
in the balance of species and species abundance. Lasting water quality
changes can be discerned in the plankton community from one vegetative
period to another.
Some of the parameters used here to assess phytoplankton represent
fundamental characteristics of plankton assemblages and reflect impact
resulting from eutrophication. These include total volume of algae and
spring-developing diatoms. As well as being an indicator of early effects
of eutrophication, these latter organisms are also an important source of
food for the benthic fauna. Other parameters are associated with harmful
algae, which affect water use in various ways. These include waterblooming
and potentially toxin-producing cyanobacteria, as well as
Gonyostomum semen, a slime-producing flagellate.
Assessment parameters relate to different times of the year, depending
on the occurrence of algae and their impact on water use:
Parameter
Period
Total phytoplankton volume Seasonal mean during May –
October, and August
Chlorophyll-a Seasonal mean during May –
October, and August
Diatoms
May or April
Water-blooming cyanobacteria August
Potentially toxin-
August
producing cyanobacteria
Gonyostomum semen
August
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