Estuaries are the meeting places of the rivers and the sea, and are characterised by the interaction between the two. Conditions in an estuary are always changing, and this instability or variability is one of the most important features of estuaries.

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.

WHY ARE ESTUARIES IMPORTANT?

Conditions in estuaries are very different from those in the sea. Estuaries are usually calm, sheltered and shallow, and vary greatly in temperature, salinity and turbidity (murkiness). As a result they are specialised environments.

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 AND PEOPLE

Estuaries are favourite sites for human settlement, urban development and recreation (boating, fishing etc.). Many cities and towns along the coast depend on estuaries for harbour facilities, tourism and recreation, e.g. Durban, Richard's Bay and Knysna.
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.

THREATS TO ESTUARIES

Anything that happens to a river in its catchment can have an impact on the estuary. A river flowing through farmlands can become polluted by pesticides, herbicides and nutrients from fertiliser. Soil eroded from badly farmed or overgrazed lands will also be washed into estuaries after heavy rains. This excessive silt load has the effect of filling up the estuary and in some cases resulting in the estuary mouth closing. Silt smothers animals and reduces light penetration so that plants are unable to grow except in very shallow water.

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.

WHAT YOU CAN DO

* Get to know the estuaries in your area by walking along their shoreline or canoeing throughout their length. Report signs of damage to the local authority.

* 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."

DID YOU KNOW?

* Bad catchment management is the major cause of estuarine damage in southern Africa.

* 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.

FURTHER READING

SOUTH AFRICAN ESTUARIES AND THEIR IMPORTANCE TO FISHES. Port Elizabeth Museum.

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.

USEFUL ADDRESSES

JLB Smith Institute of Ichthyology. Rhodes University, PO Box 94, Grahamstown 6140. Tel. 0461-22023.

Oceanographic Research Institute. P O Box 736, Durban 4000. Tel. 031-373536.

Universities: Cape Town, Port Elizabeth, Rhodes, Natal and Zululand (Zoology Departments).