A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 141

4

A Key to the More Frequently
Occurring Freshwater Algae

4.1 Introduction to the key

1. Initial observations with the light microscope .
Before using the key the user should carefully
observe the specimen noting size, shape, cell fea-
tures, both external and internal, and anything else
of interest. It is important to observe a number
of examples of the alga to be identified to get a
good overall picture of its characteristics. This is
especially true of cleaned diatoms when views of
different aspects of a cell are often needed. Note
that it is always best to observe fresh live mate-
rial as well as preserved, and that (in the case of
diatoms) cleaned and mounted material may also
be needed to see certain features such as markings
on the silica wall surface.

This key allows the user to identify the more fre-
quently encountered algae to genus level with some
mention of the more common species within that
genus and is constructed on the basis of features that
can be observed using a reasonable light microscope.
It is not a taxonomic key, but one in which algae
are separated on the basis of readily observable mor-
phological features such as shape, motility, cell wall
structure and colonial form. Only occasional and very
simple laboratory preparations might be required as it
is generally assumed that observations will be made
onfreshspecimens.Themainexceptiontothisiswith
some diatoms, where cleaning is required to remove
organic cell material and reveal surface markings on
their silica walls (see Section 2.5.2).
Before using the key the reader should familiarise
themselves with the main characteristics of each of
the major groups (phyla) of algae as described in
Chapter 1. The characteristics described there will
help understand the points of identification in the
specimen being observed.

2. Using the key . As with all binary keys, two sets of
alternative features are presented at each stage and
the user must consider which best fits the spec-
imen being observed. Each alternative leads on
to a further (numbered) binary choice, with the
sequence of choices ultimately reaching the name
ofthegenustowhichthespecimenisprovisionally
assigned.

3. Confirmation of genus can be made with refer-
ence to the fuller description given and the pho-
tographs plus line drawings provided. In some
cases,detailsandillustrationsofindividualspecies
are also given. Algal photographs are invariably of
fresh specimens unless stated otherwise. In many

4.1.1 Using the key

Using the key to identify a particular alga involves a
number of stages: