Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 41

Attached diatoms: biofilm formation

and the wide range of indices that have been devised
is discussed in Chapter 3.

Diatoms are major components of biofilms, where
they are present early in the colonisation sequence
and also within the mature periphyton community.
See later for examples of diatoms in river biofilm
(Fig. 2.23), reed biofilm (Fig. 2.29) and as attached
epiphytes (Fig. 2.28).

Lake sediments

Diatoms, more than any other group of algae, are used
to monitor historical ecological conditions from sed-
iment analysis - as discussed in Section 3.2.2. The
major role of diatoms in this lies in the resistance of
the diatom frustule to biodegradation, coupled with
ease of identification from frustule morphology and
the wide range of species with clear ecological pref-
erences seen in freshwater environments.
The importance of diatoms in environmental
analyses is indicated by the European Diatom
Database Initiative (EDDI). This key web-based
site includes electronic images and data handling
software and is particularly useful for lake sediment
analysis (see Section 3.2.2). Other databases, such
as the stream benthic database of Gosselain et al .
(2005), are more applicable to contemporary envi-
ronmental analysis. Techniques for the collection of
sediment cores, preparation of diatom samples and
procedures for making counts are widely described
(e.g. Laird et al ., 2013). Methods for cleaning
diatoms in preparation for microscopy are given in
Section 2.5.2.

1.10.4 Diatoms as bioindicators

The major use of diatoms as bioindicators of water
quality (both lotic and lentic systems) is detailed in
Chapter 3. These algae can be isolated from existing
livepopulations(contemporaryanalysis)orfromsed-
iments (fossil diatoms), and quantitative information
on water quality from species counts can be obtained
from taxonomic indices, multivariate analysis, trans-
fer functions and species assemblage analysis.

Contemporary analysis

Many diatom species have distinct ecological prefer-
ences and tolerances, making them useful indicators
of contemporary ecological conditions. These prefer-
ences relate to degree of water turbulence, inorganic
nutrient concentrations, organic pollution, salinity,
pH and tolerance of heavy metals. Round (1993) also
lists a number of other advantages in using diatoms
as indicators - including easy field sampling, high
sensitivity to water quality but relative insensitivity
to physical parameters in the environment and easy
cell counts. Diatoms are also the most diverse group
of algae present in freshwaters, making them the
most ideal assemblage for calculation of bioindices
(Section 3.4.5).
Diatoms, once cleaned and mounted for identifi-
cation, make excellent permanent slides which can
form an important historical record for a location.
Although there are hundreds of diatom species that
may be present, only dominant species need to be
used in an assessment. Benthic diatoms have been
particularlyusefulinassessingwaterqualityofrivers,

1.11 Red algae

Red algae (Rhodophyta) are predominantly marine
in distribution, with only 3% of over 5000 species
worldwide occurring in true freshwater habitats
(Wehr and Sheath, 2003). Although termed red-
algae, the level of accessory pigments (phycoery-
thrin and phycocyanin) may not be sufficient to mask
the chlorophyll - resulting in an olive-green to blue
(rather than red) coloration. The relatively common
river alga Batrachospermum , for example, typically
appears bluish-green in colour (Fig. 4.4) - giving no
clue to its rhodophyte affinity.
Major cytological features of red algae (Table
1.3) include the absence of flagella, presence
of floridean starch as the major food reserve,