Cape Town’s reporting on algae getting better, but still needs work to be accurate

1 April 2012

The City of Cape Town recently reported on a bloom of a cylindrical, filamentous diatom in their Molteno Reservoir [Source: Water Rhapsody].  They correctly indicate that there are no known health problems associated with this benthic (bottom-dwelling) alga – which, en masse, can cause considerable problems in the flocculation systems of water treatment works.  

However, I am not sure if they are referring to Melosira varians, or the more likely to be the problem, Aulacoseira granulata.  The genus for the latter used to be Melosira, (i.e. Melosira granulata) but the name was changed some years back.

Aulacoseira granulata (Source: South African Diatom Assessment Protocol)

There are distinct differences between the two species, but both indicate elevated levels of nutrient enrichment.  M. varians, however, is typically found in waters that are slightly brackish.  Accordingly, it is important to distinguish which one is the problem and, irrespective, check why it has appeared in larger numbers than normal, given the indication that the eutrophication level may have increased!

Melosira varians (Source: South African Diatom Assessment Protocol)

Diatoms provide THE MOST POWERFUL bio-indicator of water quality conditions available.  South Africa is one of very few countries possessing a developed system for using these algae to determine water quality issues.  As the simple example between two similar species above illustrates, closely-related forms tell different water quality stories.  Important, therefore, to get the identifications correct!

(Bill Harding is the originator and co-author of the South African Diatom Assessment Protocol).


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