Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 34

accurate means of classification and identification,
undisturbed sediment layers and absolute dating
of the layers - for example by the lead isotope
Pb 210 . Techniques for the extraction and counting
of chrysophyte remains from sediment cores, with
determination of concentrations using microspheres,
are described by Laird et al. (2013).
Two main types of chrysophyte remains are useful
in freshwater sediments - silica scales (from vege-
tative planktonic cells) and stomatocysts (resistant
spores). Stomatocysts have the advantage of bet-
ter preservation compared to scales, but the disad-
vantage that (with a few exceptions) they cannot
be referred to particular species. Although stom-
atocysts cannot be identified in terms of species,
they can be recorded as distinct morphological types
(morphotypes). These can be correlated with other
microfossils (e.g. diatoms) and with pollen to obtain
environmental indicator values. Quantitative assess-
ment of past aquatic environments in relation to fossil
chrysophytes (scales, stomatocysts) parallels that of
diatoms (see Section 3.2.2) and can be obtained in
relation to species ratios (environmental indices),
transfer functions (Facher and Schmidt, 1996), group
analysisandtotalplanktonicchrysophytepopulations
(flux).

environmental data (pH, conductivity, phosphorus
contents and maximal depth of the lake).
The transfer functions have been derived by multi-
variate analysis of sediment calibration sets, obtained
from a series of North American lakes in which
both environmental data and stomatocyst assem-
blages were available (Charles and Smol, 1994).

Group analysis The various environmental
factors which influence correlations between mor-
photypes can be identified by statistical analysis -
particularly principle component analysis (PCA).
StudiesbyDuffandSmol(1995)on181morphotypes
obtained from 71 different lakes in the United States
showed that differences in pH and lake morphometry
had the greatest influence in determining statistical
associations. PCA separated morphotypes into three
main groups relating primarily to acid environments,
neutral/alkaline environments and lake morphometry
(e.g. lake depth).

TotalChrysophytepopulations Assessmentof
overall changes in planktonic chrysophytes can be
determined from sediments by multiplying the con-
centration of chrysophyte scales by the annual
sediment rate. Laird et al . (2013) used this
approach in their analysis of limnological changes
of Saskatchewan (USA) lakes, showing that an
increased flux of chrysophyte scales was consistent
with recent climatic changes deduced from changes
in diatom assemblages.

Species ratios The ratio of chrysophytes to other
algal groups may provide a useful index to assess
environmental change. Studies by Smol (1985) on the
surface sediments of Sunfish Lake (Canada) demon-
strated a marked increase in the ratio of diatoms to
stomatocysts, coincident with the arrival of settlers
to the lake catchment area. The diatom/stomatocyst
ratio is a trophic index, signalling an increase in
eutrophic status of the lake at the time of human set-
tlement. This ratio change resulted from a population
decrease in the chrysophyte Mallomonas (indicating
oligotrophic conditions) and an increase in eutrophic
diatoms. The increase in trophic ratio parallels an
increase in the pollen count of ragweed, an indicator
of forest clearance and farming activity.

1.10 Diatoms

Diatoms (Bacillariophyta) are a very distinct group
of algae, identifiable under the light microscope by
their yellow-brown coloration (Fig. 1.13) and by
the presence of a typically thick silica cell wall.
This normally appears highly refractive under the
light microscope, giving the cell a well-defined
shape. Removal of surface organic material by
chemical oxidation (see Section 2.5.2) often reveals
a complex cell-wall ornamentation - as illustrated
in the light microscope images comparing living and
digested cells of Stephanodiscus (Fig. 4.58). Cell-
wall ornamentation can be seen even more clearly

Transfer functions The environmental proper-
ties of stomatocyst morphotypes can be assigned as
numerical descriptors or 'transfer functions'. These
define each morphotype in relation to a range of