About Knysna Hope Spot
“Situated at the interface between fresh- and marine waters, estuaries are among the most biologically productive ecosystems in the world and are of great ecological and economical importance. Estuaries provide the human society with a myriad of invaluable ecosystem goods and services, ranging from basic human needs to supporting multimillion dollar commercial operations. Yet these water bodies are becoming some of the world’s most threatened habitats.” - Tanja Kaselowski: Physico-chemical and microalgal characteristics of the Goukamma Estuary (NMMU Faculty of Science 2012)
The Knysna Hope Spot became the world’s first estuarine Hope Spot when Mission Blue’s Dr. Sylvia Earle launched it on 7 December, 2014.
It includes two separate estuaries: the Knysna Estuary, and the Goukamma Lagoon.
According to education.nationalgeographic.com, an estuary is “an area where a freshwater river or stream meets the ocean.”
South Africa has 250 estuaries: about 177 of them are characterised by berms or sandbars across their mouths that cut them off from the sea at various times, while the rest remain permanently open to the sea.
Although the scientific definition encompasses any meeting place between a river and the sea, in common usage the words ‘estuary’ and ‘lagoon’ have different meanings: ‘estuary’ describes a river mouth that’s permanently open to the sea, while a ‘lagoon’ is occasionally closed to the sea.
This is why we prefer to refer to the Knysna Estuary (the river mouth - The Heads - is permanently open to the sea) and the Goukamma Lagoon (the mouth is occasionally closed).
Why are estuaries important?
Estuaries are important as the nurseries of the sea. Since their waters are protected from the weather, are rich in nutrients, are generally warmer than in the ocean, and are usually free of large predators, they’re ideal, safe environments in which the larvae and juveniles of many marine species can develop.
About 100 species of prawns, crabs, and fishes that produce their eggs and larvae at sea off the coast of South Africa rely on our estuaries in this way, while another 400 or so migrate into and out of our estuaries at some time during their lives. (Most of the fish that rely on estuaries as nurseries migrate into the estuaries from late winter through to early summer).
The food web in estuaries
The presence of broken and decaying plant and animal material - collectively known as detritus - together with the bacteria which break down this detritus - form the basis of the food web in the estuary. And because estuaries form contact points between land and sea, their waters are particularly rich in this detritus - which comes from the ocean itself, as well as from neighbouring wetlands, salt flats, and so on.
Tides and the food web
The tides, which regulate mixing, flushing, and retention of the water in estuaries, are important to the estuarine food web because they carry both detritus and living things - bacteria and young fish, for example - in from the sea when they flood (or rise), and they carry pollutants out to sea when they ebb (or fall).
The level of salt in the water is significant, too. Salinity in estuaries is influenced by the inflow of salt water from the ocean, and the inflow of fresh water from the river catchment, as well as by the ability of the two to mix (which is not a given - it depends on factors like temperature and the level of oxygen in the water).
When an estuary is closed to the sea by a sandbar, the salinity of the water will rise due to evaporation, or fall as a result of the inflow of rainwater.
But humans affect the estuarine food chain, too:
“Nutrient over enrichment is considered one of the most serious human-induced impacts on estuaries ... Sources of nutrients mainly include untreated effluent from industries (e.g. waste water discharge), urban areas (e.g. sewage outfalls) and agricultural lands (e.g. seepage of fertilizers).” - Tanja Kaselowski
Although the estuarine environment is, by its nature, one of the harshest on earth - subject as it is to daily, even hourly changes in things like temperature and salinity - it is also one of the most delicately balanced, and we humans need to act carefully since we can so easily upset that balance.
Damaging actions include
- The damning of rivers - which reduces the inflow of fresh water to estuaries;
- The application of fertilisers and the irrigation of land in catchment areas - which changes the chemical nature of the fresh water that flows into the estuary;
- The construction of bridges, causeways, and canals - which affect the free flow of the tides, and can cause the water to drop its silt onto the floor of the estuary;
- Filling of wetlands and salt marshes to provide land for housing, roadways, etc. - which reduces the size of the estuarine ‘pantry,’ and thus reduces the amount of detritus in the estuarine food web; and
- The introduction of chemicals and other forms of pollution - which can literally choke the estuary either physically or chemically.
The Knysna Hope Spot: the world’s first estuarine Hope Spot
The Knysna Hope Spot became the world’s first Estuarine Hopespot when it was declared by Dr. Sylvia Earle on 7 December, 2014.
The committee of the Knysna Hope Spot is committed to maintaining the environmental integrity of the Knysna Estuary and the Goukamma Lagoon through education and research, the application of practical knowledge, and the involvement of the entire community of Knysna and Sedgefield (and our visitors too, of course).
Hope Spots are all about the relationship between people and the marine environment. Choose any of these topics to read more: