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

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.

Keywords:

• blackwater
• estuary
• peat
• humic acid
• tannins

ACKNOWLEDGEMENTS

This project was funded by the Water Research Commission Project K5/2259. The project was awarded to Professor B.E. Kelbe (Hydrologist, University of Zululand), Dr R.H. Taylor (Ecologist, University of KwaZulu-Natal) and G.C. Bate (botanist, Nelson Mandela University), who each studied different components of the greater project. Mr S. Naidu (UKZN Microscopy Department) ran the scanning electron microscope analysis of particulate matter from the different water bodies. Ms Anneke Botha of the Department of Finance, Nelson Mandela University is thanked for her assistance in arranging transport and accommodation during the three years that the project ran and for keeping track of the finances. Ms Marina Ward of the Nelson Mandela University Library was extremely helpful in obtaining much of the literature cited. Ms Sue van Rensburg and Ms Monique Nunes (SAEON) provided considerable support during field collections. Ms Mary Pieters (KZN Wildlife) is thanked for the many occasions that she provided logistical support during the time that we worked on the estuary and in other areas of the Park. iSimangaliso Wetland Park is acknowledged for permission to undertake various studies within the park.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

Notes

1 The mm amounts here refer to mm per annum.

Additional information

Funding

This project was funded by the Water Research Commission Project K5/2259. The project was awarded to Professor B.E. Kelbe (Hydrologist, University of Zululand), Dr R.H. Taylor (Ecologist, University of KwaZulu-Natal) and G.C. Bate (botanist, Nelson Mandela University), who each studied different components of the greater project.