Writes Jonathan Britton, Marine Ranger, Wilderness: Garden Route National Park

During recent weeks residents in Sedgefield have raised concerns about a distinct “rotten egg” smell that occasionally emanates from the adjacent Swartvlei Estuary. The smell is often mistakenly attributed to the smell of sewage. The odour is actually that of the hydrogen sulphide (H2S) gas produced by bacteria during the decomposition of mostly plant organic matter.

The decomposition of organic matter with proteins containing sulphur in deoxygenated waters results in the formation of H2S. The conditions for H2S production occur most frequently when mats of the floating filamentous alga, which grow primarily during winter, start to die and rot during spring. Localised areas of deoxygenated water may temporarily be caused by the respiration of bacteria, which decompose the alga. This is usually restricted to the sides of the channel and the bottom waters of deeper portions of the estuary.

The production of H2S by bacterial decomposition in Swartvlei is a natural process and records dating back several decades indicate the occurrence of occasional sulphurous odours. It is difficult to ascertain if the smell has gotten worse or not although it is possible as there is an increase in the biomass of aquatic plants.

Aquatic plants are a natural part of the estuarine environment. They play a vital biological role by providing food, shelter and a substratum for aquatic organisms, as well as trapping and recycling nutrients. The development of filamentous algae mats (greenish mats upon the water’s surface also commonly known as ‘pond scum’ or ‘pond moss’ forms) in Swartvlei Estuary is a natural phenomenon and part of the biotic cycle associated with this estuary. The production of H2S when the alga decomposes is a natural process and is a part of life in Sedgefield.