Blackwater in South African estuaries with emphasis on the Mgobezeleni Estuary in northern KwaZulu-Natal

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Abstract
The blackwater Mgobezeleni Estuary is a small system below a peat swamp. From the air the estuary is completely black, but the water revealed only a light tea colouration with a Secchi reading of >1.7 m. The sediment is covered with black low-density fragments overlying light coloured sand. When the estuary is open the colouration of the sea is blacker than might be expected from the light tea-coloured estuary water. The results of X-ray spectroscopy showed the black fragments have a high carbon content that is most likely peat. The dissolved organic carbon content of the Mgobezeleni Estuary water is lower than that in Swartvlei in the South Western Cape, which has very dark water. The presence of very large deposits of peat at the head of the estuary together with spectroscopy data indicate that the colouration of the water emanates from leached leaves of swamp vegetation. The penetration of light through the water is not reflected from the light sand due to the peat covering, giving the appearance of very dark water. The literature suggests that the cause of blackwater is due to humic acids, peat and tannins leached from fallen leaves. The results of this study point to peat fragments not reflecting sunlight rather than peat extract and humic acids as the main cause. The slight colouration of the water is likely due to the small amount of fresh leaf litter in the catchment for the release of tannins that colour the Mgobezeleni Estuary water.
Funding Information
  • Water Research Commission (K5/2259)

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