Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 2

and clogging filters/water courses - affecting the pro-
duction of drinking water and recreational activities.

topic focuses on aquatic algae present within con-
tinental boundaries, where water is typically fresh
(non-saline), and where water bodies are of two main
types:

1.1.2 Algae as primary producers

Standing (lentic) waters - particularly lakes and
wetlands.

As fixers of carbon and generators of biomass, algae
are one of three major groups of photosynthetic
organism within the freshwater environment. They
are distinguished from higher plants (macrophytes) in
terms of size and taxonomy and from photosynthetic
bacteria in terms of their biochemistry. Unlike algae
(eukaryotic algae and cyanophyta), photosynthetic
bacteria are strict anaerobes and do not evolve oxygen
as part of the photosynthetic process (Sigee, 2004).
The level of primary production by algae in fresh-
water bodies can be measured as fixed carbon per unit
area with time (mg C m −3 h −1 ) and varies greatly
from one environment to another. This is seen, for
example, in different lakes - where primary produc-
tion varies with trophic status and with depth in the
water column (Fig. 1.1). Eutrophic lakes, contain-
ing high levels of available nitrogen and phospho-
rus, have very high levels of productivity in surface
waters, decreasing rapidly with depth due to light
absorption by algal biomass. In contrast, mesotrophic
and oligotrophic lakes have lower overall productiv-
ity - but this extends deep into the water column due
to greater light penetration.
Although algae are fundamentally autotrophic
(photosynthetic), some species have become sec-
ondarily heterotrophic - obtaining complex organic
compounds by absorption over their outer surface or
by active ingestion of particulate material. Although
such organisms often superficially resemble proto-
zoa in terms of their lack of chlorophyll, vigorous
motility and active ingestion of organic material,
they may still be regarded as algae due to their
phylogenetic affinities.

Running (lotic) waters - including streams and
rivers.

The distinction between lentic and lotic systems
is not absolute, since many 'standing waters' such
as lakes have a small but continuous flow-through of
water, and many large rivers have a relatively low
rate of flow at certain times of year. Although the
difference between standing and running waters is
not absolute, it is an important distinction in relation
to the algae present, since lentic systems are typically
dominated by planktonic algae and lotic systems by
benthic organisms.
Although this volume deals primarily with algae
present within 'conventional freshwater systems'
suchaslakesandrivers,italsoconsidersalgaepresent
within more extreme freshwater environments such
as hot springs, algae present in semi-saline (brackish)
and saline conditions (e.g. estuaries and saline lakes)
and algae present within snow (where the water is
in a frozen state for most of the year).

1.1.4 Planktonic and benthic algae

Within freshwater ecosystems, algae occur as either
free-floating (planktonic) or substrate-associated
(largely benthic) organisms. Planktonic algae drift
freely within the main body of water (with some
species able to regulate their position within the
water column), while substrate-associated organisms
are either fixed in position (attached) or have lim-
ited movement in relation to their substrate. These
substrate-associated algae are in dynamic equilib-
rium with planktonic organisms (Fig. 2.1), with the
balance depending on two main factors - the depth
of water and the rate of water flow. Build-up of phy-
toplankton populations requires a low rate of low

1.1.3 Freshwater environments

Aquatic biology can be divided into two major dis-
ciplines - limnology (water bodies within conti-
nental boundaries) and oceanography (dealing with
oceans and seas, occurring between continents). This