Salt marshes

Salt marshes of the Knysna Estuary

What’s so important about salt marshes?

Salt marshes are the foundation upon which much of the estuarine food chain is built. Preserving them is vital to the sustainability of the marine environment - not least because many fishes of the open ocean rely on estuaries during at least a part of their life cycles. The existence of the salt marshes is one reason why the Estuary has been declared The Knysna Hope Spot by Mission Blue (which defines Hope Spots as special conservation areas that are critical to the health of our oceans - which forms Earth’s blue heart).

Salt marshes - rare and threatened

Salt marshes only occur in temperate climates, which means that in South Africa they are found only on the Cape Coast (the tropical equivalent of the salt marsh is the mangrove swamp such as are found in some estuaries in KwaZulu Natal).

Although they occur in about 70 of the Cape's 155 estuaries, three quarters of South Africa's total salt marsh area is concentrated in just four places: the Langebaan Estuary (5,700 ha), the Knysna Estuary (1,800 ha), the Olifants River Estuary (200 ha) and the Swartkops Estuary (170 ha).

Salt marshes are under constant threat because estuaries are so often associated with coastal development. Of particular concern is the amount of salt marsh which has been lost due to the construction of harbours and marinas.

Siltation of salt marshes can occur as a result of soil erosion in river catchments, while salinities may be affected when the fresh water supply increases as a result of flooding or diminishes when the river is dammed, when too much water is the withdrawn for irrigation or other purposes or when the catchments are invaded by water-hungry alien plants (such as black wattle - Acacia melanoxylon).

Pollutants which can affect salt marshes include heavy metals, sewerage, effluent and oil, and mechanical damage is caused when humans and livestock trample the marsh plants.

Knowing and understanding the importance of salt marshes - and their vulnerability - will help to protect them and to ensure their sustainability.

The environment of salt marshes

The plants which grow in salt marshes have to be specially adapted to withstand one of the world's harshest environments.

Salt marshes of the Knysna Estuary are flooded by tides twice a day. When the water rises, the temperature surrounding the plants drops sharply and the oxygen supply and light intensity are reduced dramatically. Although the actual level to which the water rises is fairly predictable (tides work on a cycle of fourteen days according to the phases of the moon) the water can rise higher than usual during winter and summer solstices or as a result of abnormal weather. Wind can cause waves in the estuary which can damage the plants in the marsh, and the amount of fresh water flowing into the estuary can dramatically change the salinity of the marshes.

Nevertheless, the plants cope extremely well, and it’s been estimated that the average salt marsh can produce more food per hectare than any of humankind’s most advanced agricultural systems.

Plants of the salt marshes

The plants found in salt marshes are always arranged in zones according to height above the low water mark. If the marsh is island-shaped, these zones may appear as concentric circles.

Eel grass - Zostera capensis - grows in the lowest zone. Not a true grass, this flat-bladed plant has stems which grow underground and long, narrow leaves which lie flat when the plants are exposed at low tide. They're easily identified - hold a leaf up to the light and you'll see the ladder-like veins.

Rice grass - Spartina maritima - occurs in the second zone. It grows to between 10 cm (on higher ground) and 80 cm (in deeper areas) and may appear as single stems or tufts.

In the third zone you'll find the succulent-like Sarcocornia perrenis which grows to a maximum height of about 15 cm. Its leaves look like the jointed stems of a spineless cactus and may be pale green or orange to reddish in colour (when these plants are stressed, their green-coloured chlorophyll pigments break down, allowing the orange carotenoid pigments to dominate). Also in this zone you may see arrow grass - Triglochin bulbosa - which is actually a bulbous plant. It has narrow, grooved leaves. The young flowers are tightly-packed in arrow-like formations.

Both Sarcocornia and arrow grass grow on hard-packed soils in the salt marshes and both are palatable to wild animals and livestock - which makes this zone particularly vulnerable to damage and overgrazing. Damaged Sarcocornia and arrow grass plants do not re-grow easily, and bare patches in this zone are easily and quickly eroded.

In the upper reaches of the marshes - which are seldom flooded - you may occasionally find the purple sea lavender - Limonium depauperatum, which has flat-growing leaves and fern-like flower heads with tiny mauve-coloured flowers.

Salt marshes & the food chain

Salt marshes form a beautifully organised and delicately balanced system which produces an extraordinary amount of food every day and provides much of the organic matter on which the estuarine food chain depends. For example - when parts break off the eel grass they become colonised by fungi and bacteria. These fungi and bacteria help with the decomposition process whilst at the same time adding nutritional value to the chain. They in turn are eaten by invertebrates such as crabs, snails, mussels, mud prawns and blood-worms as well as by fish such as mullet.

Many juvenile fishes migrate into estuaries and spend considerable time there - which is why estuaries are known as the 'nurseries of the sea.' Thus, species such as juvenile Cape stumpnose will eat the living leaves of the eel grass - and deliver much of the material back to the system though their faeces, which provide nourishment for yet other organisms.

With so many fishes, invertebrates and palatable plants on the menu, salt marshes also attract predators such as birds (flamingos, terns and waders) and even small mammals (otters) and grazers (such as livestock).

Nurseries of the sea? They should call them the pantries of the sea!

Protecting the Knysna River Estuary

The Knysna River Estuary is protected as part of the Garden Route National Park and is subject to various bodies of legislation.

Please contact SANParks for any queries about the Estuary.

Garden Route National Park, Knysna Lakes Section
Thesen Harbour Town, Knysna
Telephone 0027(0)44 302 5600

Text: Martin Hatchuel www.thistourismweek.co.za