15 C
Mossel Bay
12th Aug 2022
Nature & Nurture

The Great Brak Estuary and the Big Stink

Whilst the flamingos quietly feed in the shallow waters of the estuary they seem blissfully unaware of the putrid smell being emitted from the black and green slime that surrounds them. Their contentment is, however, not shared by other two legged residents of the estuary and the island with some even worried about loss of income due to the stench and its presence. What exactly is this intrusive slime and what is the big stink actually about?

Some may be quick to accuse the current state of the estuary on leaking septic tanks, nutrient runoff from farms or the golf course. Fingers are also quickly pointed to the mouth hardly being manually opened as well as the dam not releasing water on a more regular basis. If we put the blame game aside and apply our minds to science we will find the cause for this assault on the senses to be both biological and botanical.

The “slime” is in fact a filamous freshwater  macroalgae known as Cladophora Glomerata. Cladophora Glomerata traditionally follows a seasonal cycle during the closed mouth state whereby growth occurs during the autumn and winter months and die back occurs during the spring and summer months. This die back and general decomposing can be seen with the changing of the matted algae from green to black. With the colour change so comes the sulphur like smell.

Large masses of decomposing Cladophora Glomerata may smell, but being a primary producer its importance is profound within the food chain. Both as fresh feed and as detritus, it supports primary, secondary and tertiary consumers. On the other hand one cannot over look that extremely large rotting masses in the river and estuary system can facilitate the reduction of dissolved oxygen within the water. Very much a case of a double edged sword.

Certainly we can just wash it out to sea? Even if there was sufficient water within the Wolwedans Dam to entertain the idea of a manual flush of the estuary and mouth, would this even help?  In a study of Great Brak estuary undertaken in 2011 on the “Responses in a temporarily open/closed estuary to natural and artificial mouth breaching” it would suggest that artificially breaching the mouth actually exacerbates the prolific growth of Cladophora Glomerata. 

 Research found that in March 2011 C. glomerata attained its highest recorded coverage area directly after the summer die-off period and out of its normal seasonal growth cycle. The growth response coincided with the artificial release of freshwater from the Wolwedans dam. The flow from the dam was not sufficiently strong to flush water, sediments, and the algae out of the estuary and resulted in the mouth closing after only a week.

As water levels rose, the algae was deposited onto the marsh areas. After breaching occurred, water drained out of the estuary, leaving the algae stranded on the marshes. And as the flood tide entered, the macroalgae was once again redistributed. The algae was then able to utilise the available nutrients in the water column and double its original size directly after its initial die-off period.

Manual and artificial breaching simply does not have the ability, let alone water capacity and significant scouring force, required to mimic the after effects of an estuary cleared by natural flood. If anything the man made solution of the artificial breach, timed with freshwater releases into the Great Brak River and Estuary, may be having negative consequences.

This phenomenon is by no means an isolated incident but in fact a global one.  

Our estuary simply highlights the direction that many small microtidal estuaries around South Africa and the world are heading. Given the increase in population and ever increasing pressure on the Wolwedans dam, available freshwater inflow to our estuary will become an even scarcer resource leading to an open/closed estuary system like that of Great Brak to close more frequently and for longer periods of time.

If we do not rethink the effects of our actions regarding the management of water and our impact of estuary systems down stream we may just find ourselves having to sit through the stink that comes with Cladophora Glomerata for the foreseeable future.

Caption: Some of the Greater Flamingo feeding amongst the Cladophora Glomerata on the Great Brak River estuary.

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