Long-term salinity trends in Zandvlei estuary and implications for dominant macroalgae

Bachelor Thesis


Permanent link to this Item
Journal Title
Link to Journal
Journal ISSN
Volume Title

University of Cape Town

Zandvlei is a system that has been highly modified by the urban environment of Cape Town. Salinity is a major driver determining the physical environment of estuaries. The macroalgal community of estuaries forms an important part of the ecology of these systems as primary producers. The salinity of this environment is an important parameter determining the composition, abundance and diversity of these communities. Patterns of salinity fluctuations in the long term (1978-2003) and annually are described in order to establish how communities may vary. Zandvlei is in a Mediterranean climate and salinity was found to vary seasonally and monthly with fluctuations in rainfall. Historical records of macroalgae identified in the estuary were summarized and no record of the Polysiphonia sp., now dominant in the estuary, was found. Dominant macroalgae in the estuary were identified and grown under a range of salinities (0, 1, 5, 10, 20, 29 ppt). Dominant macroalgae included Cladophora sp., Polysiphonia sp. and Enteromorpha prolifera. None of these algae survived at O ppt. In 1 ppt E. prolifera survived but growth was retarded, Polysiphonia sp. survived but did not grow and Cladophora sp. grew successfully at this salinity. All three species have different optimum ranges for growth with E. prolifera preferring higher salinities, Cladophora sp. grew similarly across the range from 1 to 29 ppt and Polysiphonia sp. grew most rapidly at 5 ppt. Depending on the salinity range in the estuary different macroalgae will be dominant. This makes the macroalgae community quite resilient to fluctuating salinities. There should therefore always be estuarine macroalgae present in the estuary provided salinity does not drop below 1 ppt for an extended period. If salinities did drop below 1 ppt there could be an increase in freshwater algae.