Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 47

(Fig. 2.3c) are important aspects of lake ecology
since all areas of plankton biology ultimately occur
at the microlevel. The spatial scale at which sampling
should be carried out relates particularly to organism
size - including both single organisms/colonies and
aggregates (DeAngelis, 2004). In the case of phy-
toplankton, the size factor ranges from population
aggregates (aspects of horizontal and vertical patchi-
ness, over distances of 0.1-5 m) down to single cells
and colonies, with analysis of nanoscale heterogene-
ity at the microscopic level.

2004). Localised phytoplankton distribution is also
affectedbywind-inducedturbulenceandmixing,plus
directionaldriftinsurfacewaters.Insomecases,local
plankton populations may be limited to a particular
microenvironment - such as photosynthetic bacte-
rial restriction in some mesotrophic and oligotrophic
lakes at the bottom of the photic zone, where there
is adequate light under anaerobic conditions (Sigee,
2004).

Heterogeneity at the microlevel Lookingatthe
nanoscalepatchinessofalgaeandassociatedmicroor-
ganisms within lake water (over distances of 10-100
μm) is potentially useful for studying intercellular
dynamics. Defined associations between algae and
other organisms are important in the air-water sur-
face biofilm, and in the algal phycosphere (Sigee,
2004), and other associations may also be important.
These can only be analysed via discrete samples if
the scale of collection matches the scale of aggre-
gation. Another approach proposed by Krembs et al .
(1998) is to collect small volumes of water containing
algae and other particulate material (without destroy-
ing their relative spatial distribution), rapidly freez-
ing the sample, then analysing the distribution in two
dimensions as a projection on a microscope slide.
Variation within species populations is also an
important aspect of algal microecology. The micro-
scopical techniques outlined in Section 2.6.2 involve
observation and analysis of phytoplankton samples
directly isolated from the lake and processed within
a short period of time.

Patchiness of phytoplankton distribution In
both marine and freshwater systems, observations
of patchiness have stimulated new approaches to
phytoplankton sampling and recording. Conventional
sampling procedures, using phytoplankton nets or
volume samplers (see Section 2.2), provide little
information at the microlevel, since quite large vol-
umes are trawled or collected, and are assumed to be
homogeneous for the purpose of measuring aquatic
parameters. Studies by Wolk et al . (2004), for exam-
ple, taking 20-ml subsamples of lake water from a 5-l
Niskin volume sampler, showed considerable hetero-
geneity of algal biomass (chlorophyll- a concentra-
tion) within the collected sample. This information
would normally be lost, since the water samples were
normally split up into different aliquots for bulk anal-
ysis of inorganic nutrients, dissolved organic matter,
phytoplankton (biomass), phytoplankton production
and particulate organic matter.
The development of new in situ instrumentation
(see also Section 2.3.3) has been particularly useful
in resolving small-scale phytoplankton distribution in
the lake environment. Wolk et al . (2004) used a high-
resolution bio-optical sensor (fluorescence and tur-
bidity) to monitor chlorophyll- a distribution in Lake
Biwa (Japan), revealing vertical patterns of phyto-
plankton distribution over spatial scales of

2.1.2 River phytoplankton

The phytoplankton biomass in rivers is typically (but
not always) lower than standing waters since the
transport of suspended algae by water current (poten-
tially short residence time) limits the development of
established populations. The phytoplankton commu-
nity of rivers is also dominated by a smaller number
of taxa compared to lakes, with fast-growing algae
such as centric diatoms being particularly important.
In shallow rivers, light penetration to the sediments
means that sampling of benthic algae may also be
important in assessing total algal population (see Sec-
tions 2.8-2.10).

1cm.
Cowles (2004) has emphasised the importance of
small-scale plankton distribution in terms of 'pattern
and process' - particularly in relation to rates of bio-
logical production and transfer of material through
the food web. The importance of local phytoplankton
distribution in relation to population growth includes
aspects such as nutrient patchiness, local competition
with other microorganisms and critical algal density
for fungal infection and zooplankton grazing (Sigee,

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