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

Environmental Engineering Reference

In-Depth Information

Benthic algae

actively photosynthetic growth phase in the plank-

tonic environment, but overwinters as a dormant

metabolically inactive phase. Light micrographs of

the distinctive overwintering phases of two major

bloom-forming algae ( Ceratium and Anabaena )are

shown in Fig. 2.7.

Benthic algae occur at the bottom of the water col-

umn in lakes and rivers and are directly associ-

ated with sediments - including rocks, mud and

organic debris. These algae (usually attached) may

form major growths on inorganic surfaces or on

organic debris, where they are frequently present

in mixed biofilms (bacteria, fungi and invertebrates

also present). Under high light conditions, the biofilm

may become dominated by extensive growths of fil-

amentous algae - forming a periphyton community

(Fig. 2.23). Attached algae may also be fixed to liv-

ing organisms as epiphytes - including higher plants

(Fig. 2.29), larger attached algae (Fig. 2.28) and large

planktonic colonial algae (Fig. 4.35). Some substrate-

associated algae are not attached, but are able to move

across substrate surfaces (e.g. pennate diatoms), are

loosely retained with gelatinous biofilms or are held

within the tangled filamentous threads of mature peri-

phyton biofilms. (Fig. 2.29).

Many algal species have both planktonic and

benthic stages in their life cycle. In some cases,

they develop as actively photosynthetic benthic

organisms, which subsequently detach and become

planktonic. In other cases, the alga spends most of its

1.1.5 Size and shape

Size range

The microscopic nature of freshwater algae tends

to give the impression that they all occur within a

broadly similar size range. This is not the case with

either free floating or attached algae.

In the planktonic environment (Table 1.1), algae

range from small prokaryotic unicells (diameter

<

1 μm) to large globular colonies of blue-green algae

such as Microcystis (diameter reaching 2000 μm) -

just visible to the naked eye. This enormous size

range represents four orders of magnitude on a linear

basis (×12 as volume) and is similar to that seen

for higher plants in terrestrial ecosystems such as

tropical rainforest.

Table 1.1 Size Range of Phytoplankton.

Linear Size (Cell or

Colony Diameter) (μm) Biovolume a (μm 3 )