The salinity of estuarine water varies depending on the tide and the strength of the inflowing river. In addition, a river also brings silt and nutrients to the estuary in varying quantities, depending on conditions in the catchment (drainage basin) of the river concerned.
A nursery for marine species: Over 100 species of fishes, prawns and crabs in South African off-shore waters use estuaries as nurseries and/or feeding grounds. The life cycle of most of these species involves egg production at sea, often close inshore and near an estuary mouth. Eggs and larvae develop at sea, but the larvae and juveniles migrate to estuaries in great numbers. In fish, this migration takes place mainly during late winter, spring and early summer when millions of juveniles swim into estuaries.
Estuaries are good nurseries because they offer protection from most marine predators, and their high temperatures and rich food supplies favour rapid growth of the juveniles. The source of this food supply is estuarine plants growing in the water, as well as the plants of the neighbouring wetlands, e.g. mangroves and reeds. These plants supply most of the detritus (fragmented remains of dead plants and animals) which, together with bacteria responsible for decomposing detritus, forms the basis of the estuarine food web.
Most juvenile fish migrate back to sea at an age of about one year. These sub-adults tend to live close to the shore, where they join adult spawning populations once they become mature.
Estuaries are particularly popular with anglers when adult fish enter seasonally to feed. At these times fish are easier to catch and are important as a source of both food and recreation. An example of this is the famous spotted grunter "run" into KwaZulu/Natal and Cape estuaries. Of the 81 fish species which depend on estuaries in South Africa, 29 are sport angling species and an additional 21 species are used for human food.
Damming of rivers and the use of water for irrigation or industry can lead to freshwater starvation of an estuary. This upsets the ratio of freshwater to seawater in the estuary which in turn affects the plants and animals living there.
* List the plants and animals living in these estuaries. Photograph the upper, middle and lower reaches of the estuary from fixed vantage points and monitor change between seasons and from year to year.
* Find out what the estuary is used for, which local authority is responsible for its management and what strategies have been prepared to control development alongside it.
* Read "The Biology and Conservation of South Africa's Vanishing Waters" (see below) which has a very useful chapter entitled "What you can do."
* The numbers of many important commercial (e.g. prawns) and angling species (e.g. kob, grunter, perch) which rely upon estuaries are dwindling as a result of disturbed estuarine environments.
THE LIVING SHORES OF SOUTHERN AFRICA. Margo and George Branch. Struik, Cape Town, 1981.
THE BIOLOGY AND CONSERVATION OF SOUTH AFRICA'S VANISHING WATERS. B.R. Davies and J.A. Day. CEMS, University of Cape Town and the Wildlife Society of Southern Africa, 1986.
ESTUARIES OF THE CAPE (PARTS 1&2). CSIR.
ESTUARIES OF NATAL (PARTS 1&2). Natal Town and Regional Planning Commission, Pietermaritzburg.
ESTUARINE ECOLOGY WITH SPECIAL REFERENCE TO SOUTHERN AFRICA. J. Day, Balkema, 1981.
Oceanographic Research Institute. P O Box 736, Durban 4000. Tel. 031-373536.
Universities: Cape Town, Port Elizabeth, Rhodes, Natal and Zululand (Zoology Departments).