Long-term changes in a small, urban estuary

Bachelor Thesis


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University of Cape Town

The Diep River estuary is a small, urban estuary situated in the suburbs of Cape Town and has been subjected to long-term modifications and abuse that far exceeds that of most other estuaries around South Africa. Activities in the Diep River catchment, together with the intensive urban development in the areas adjacent to the estuary, have resulted in massive changes in biodiversity, altered flow and salinity regimes; causing a marked deterioration in water quality and a frightening increase in non-indigenous species introductions. Several studies have been undertaken at the Diep River estuary, however, they are out-dated and many changes have occurred within the last few decades. The aim of this study was to provide an up-to-date list of the fauna residing within the Diep River estuary, as well as to provide a synthesis of all the major physical, hydrological and faunistic changes that have occurred within and around the estuary, dating back to the late 1800s, whereas faunistic changes are described using information provided by several earlier surveys dating back to the early 1950s. Infauna, epifauna and salinity were taken at designated stations along Milnerton Lagoon. Prawn (Callichirus kraussi) counts were also made in order to determine current abundance and distribution. Results showed a substantial decline in sand prawn abundance with the estimated standing stock calculated at just over 12 million. Fauna collected were generally poor in abundance and were mainly limited to euryhaline, detritus feeders. Earlier surveys conducted in the 1950s recorded at least 49 infauna and epifauna species residing within Milnerton Lagoon, whereas in 1974, only 23 species were found. Similarly, 24 species were recorded in this study. The European shore hopper (Orchestiagammarella) was recorded as a new introduction within the system. Only five fish species were recorded in the summer period, including the highly invasive mosquito fish (Gambusiaaffinis), which was found in relatively high abundance within the lagoon. A regular monitoring of the infauna and epifauna populations for this system needs to be established, in order to obtain a clear picture of the faunistic distribution and changes occurring within this highly dynamic environment. Additionally, serious management protocols need to be established in order to prevent the further degradation of this important system.

Includes bibliographical references.