A green, slimy substance can once again be seen covering the surface of the Vaal River and the first word that comes to mind, is ‘pollution’.

But according to a summary put together by AJH  Pieterse & Sanet Janse van Vuuren of the Department of Plant and Soil Sciences, PU for CHE, Potchefstroom it is phytoplankton and there is no great cause for concern, although it hampers angling in the river and does give off a smell. 

Phytoplankton consists of photosynthesizing microscopic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water. They are agents for the ‘primary production’ in the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web. 

Phytoplankton obtains energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake or other body of water. It accounts for half of all photosynthetic activity on Earth, thus it is responsible for much of the oxygen present in the Earth’s atmosphere  – half of the total amount produced by all plant life. 

According to the summary, when compared with similar studies on rivers in other regions of the world, it is clear that the Vaal River, although unique in many ways, also has characteristics in common with other rivers.  The Vaal River is the most important and the most regulated river in South Africa and has a high chlorophyll-a level, plus inorganic nitrogen and phosphorus concentrations.  Massive developments of phytoplankton are experienced in certain sections, resulting in aesthetic problems, health hazards, interferences with treatment processes and problems in water distribution systems.

Consequently, the causes and consequences of the wax and wane of phytoplankton assemblages in the Vaal River are continually investigated.  Studies show that a variety of phytoplankton species (at least 124 species and varieties), occur in the Vaal River.  Green algae shows the greatest species diversity.

During the summer periods, diatoms tend to dominate in the Vaal River from January to August of each year, while green algae is dominant from  September to December.  Dominance of diatoms during January to April and December, can be ascribed to blooms of Melosira granulata.

During the study period, indications of pollution were reflected in high chlorophyll-a concentrations in the Vaal River in comparison with other river systems.  Periods of maximum chlorophyll-a concentration in the Vaal River occurred from January to March (summer) and again from July to November (winter-spring) of each year.

Increased nutrient concentration in the Vaal River apparently increased the capacity of the water to support high production rates and to maintain large standing crops of phytoplankton. 

Visit www.wrc.org.za for more information on the subject.   KF