Not all toxic algae float…

23 May 2011


Typical blue-green algal scum in a farm dam (Photo: Bill Harding)

The commonest form of blue-green algal blooms in lakes and ponds are of the floating, scum-forming variety.  These are easy to spot as the water is very green, often paint-like, scums form at the edges, either in the water or on the rocks.

Dense algal bloom in large lake (Photo: Bill Harding)

Algal scum washed onto lake edge (Photo: Bill Harding)

Wind-harvested scum (Photo: Bill Harding)

Some scums may only be apparent for short, calm periods, disappearing completely as the wind picks up and mixes the algal cells into the water.

Windrows of toxic algae in large dam (Photo: Bill Harding)

There are, however, many cases where the toxic algae are not readily visible and no scum or floating algae are evident.  These situations can create a false sense of security or animal death or illness may occur, with no obvious reason.  In many cases, some species of blue-green algae form dense benthic mats from which toxins are released into the overlying water.

Algal mats formed on lake sediments, no algae in overlying water (Photo: Bill Harding)

Dense bloom on bed of artificial lake (Photo: Bill Harding)

The algae adopt benthic habitats for various conditions, often to avoid unfavourable water chemistry or to take up nutrients available in the sediments and not in the water.  A classic case occurred in South Africa during the mid-90s when toxins developed in a small livestock-watering reservoir (see photo following), enough to kill 400 pregnant cattle.

Toxins form in small, concrete tank at right of photo, cattle died close to where they drank (Photo: Bill Harding)

In this case, in order to avoid low pH in the water, the algae grew in the walls of the tank, releasing toxins into the water.

Algal mat in tank wall (Microscopy by Bill Harding)

Toxic cyanobacteria growing on tank wall (Microscopy by Bill Harding)

Cyanobacteria grow in all sorts of environments, not just in water.  The photo below shows a toxic growth in soil irrigated with wastewater effluent.

Toxic algae growing in nutrient-rich sand (scrape shows normal sand colour) (Photo: Bill Harding)

The water in this dam (see photo following) did not look as though it had much algal biomass.  The photo following shows the algae that emerged from the sediment, stirred up in a basin!

Algal development in nutrient-poor dam (Photo: Bill Harding)

Algal found in the sediments from the same dam (Photo: Bill Harding)

Probably the most amazing algal bloom I have seen was in the Hardap Dam (Namibia), the water source for the Mariental irrigation scheme.  The entire dam was covered with a scum so thick that it resembled a layer of oil.  The water was extremely toxic and I measured 17 (yes, seventeen) variants of the hepatotoxin, microcystin, in the samples I collected.

Hardap Dam (Namibia). Toxic algal bloom over entire lake surface (Photo: Bill Harding)

Hardap Dam toxic algal bloom (Photo: Bill Harding)



Leave a Reply

Your email address will not be published. Required fields are marked *