Andrea Abbott hops aboard a boat with a pair of scientists and is dazzled by iSimangaliso’s Kosi Bay estuary
Ratty in The Wind in the Willows was correct. There really is nothing like messing about in boats. And it’s even better if the boat is cruising around the aquatic magnificence of Kosi Bay, and two of your companions are estuarine experts on every nook and cranny of not only Kosi’s multiple-lake system but the many other estuaries in KwaZulu-Natal.
Biologists Nicolette and Professor Anthony Forbes have investigated to varying degrees all of KZN’s 75 estuaries, their findings underpinning important restoration projects. Today at Kosi, in the far north of iSimangaliso Wetland Park World Heritage Site, they’re checking aspects of the water chemistry – conditions such as salinity, dissolved oxygen, temperature and turbidity, whose levels provide insight into the state of the estuarine ecosystem. Apart from the privilege of puttering around in a boat on one of the most dazzling bodies of water anywhere, it’s fascinating to watch these scientists at work and learn what estuaries are, why they matter, and why we need to respect their boundaries.
The Call of Kosi Bay
For here’s the thing. An estuary is much more than an idyllic picnic spot and playground. Estuarine systems, I’m to discover that day, have long been misunderstood and yet, depending on their type, they perform a host of essential ecological functions such as protecting the land from floods and storm surges; filtering waste and pollutants discharged into rivers; providing critical habitat and refuge for fish and aquatic birds; and supplying sand to beaches.
“Different estuaries do different things,” the professor explains, as he guides the boat through the uMthando Channel between Lake Nhlange and Lake Mpungwini. “The word estuary derives from the Latin, aestuarium, meaning tidal inlet. The current definition is wider though, and denotes the transitional zone where a river meets the sea and there’s a mix of fresh and saltwater.”
But this mixing doesn’t always have to happen. Some estuaries will be closed at times and the water largely fresh, while at other times they’ll be open and the water saltier. In other words, it’s okay for estuaries sometimes to be closed and sometimes open, in line with natural cycles of rainfall and sea state. This dynamism impacts on the animals in an estuary and, to a certain degree, the plants. “That’s why there are unique assemblages of animals and plants in estuaries,” Nicolette says.
More Below the Surface
In the glinting water around us, shoals of fish of various size scud about. “Fish in estuaries are classified into different groups according to their dependence and the ways in which they use estuaries,” she explains. We’ve reached the next lake – Makhawulani – and are navigating past the fish traps that have long been a feature of Kosi. Cormorants and a Fish Eagle perch on the pole structures, sharp eyes scanning the aquatic pantry below.
“Some fish are resident in estuaries and have adapted to the fluctuating conditions. For many others, especially the recreationally important angling species, estuaries are critical nursery grounds.” Contrary to common belief, estuaries are not important fish- and crab-breeding grounds.
Species like spotted grunter, Zambezi sharks, tropical stumpnose, and prawns are dependent on estuaries to complete their life cycles. They’re born at sea where conditions are largely stable, but where high predation takes a toll on numbers.
The survivors, once they’ve developed a tolerance for estuary dynamics, move into estuaries where they find the conditions they need to grow up in before returning to the ocean to breed.
You can understand, then, why artificial breaching of a naturally closed estuary is detrimental to the functioning of that system. Juvenile fish, crabs and prawns might be flushed out to sea before they’re ready to return. This interruption of their life cycle can result in marine-fish populations being deprived of young recruits.
At a pre-selected site located by GPS, Anthony stops the boat for the team to lower a high-tech sampling device into the water. On the boat we can see the continuous measurements of the conditions in the water and the depth at which the readings are being taken.
The instrument records the data that are later transferred by WiFi to the computer in the laboratory, and then analysed. That done, we head for the next test site, passing a pod of hippos which keeps a wary eye on us, and us on it. It’s a glaring reminder of why swimming there is not a good idea.
The Different Estuaries
En route, Anthony explains that most of the world’s large estuaries are the same age, having been formed during the most recent rise in sea level, which stabilised about 6 000 years ago. In South Africa, while each has its own unique features – for example, some have mangroves, others don’t – estuaries can be grouped broadly into five categories – Estuarine Lakes, Estuarine Bays, Intermittently Open Estuaries, Permanently Open Estuaries, and River Mouths.
To experience the different types, you don’t have to travel far, for they’re all represented along KZN’s approximately 600-kilometre coastline, which has more estuaries per linear kilometre than any other province in the country (the Eastern Cape has a greater number but it also has a much longer coastline).
You also might like: Top 10 Estuaries in KZN to Explore During Your Next Trip
If you’re familiar with the KZN coast, you’ll know there are fewer estuaries to the north of Durban than to the south. You might also know that the northern ones are generally bigger, because of the flatter land of the Mozambique Coastal Plain that starts in the Mtunzini area.
That region is also associated with large estuaries like Kosi Bay, which falls into the Estuarine Lake category. So do iSimangaliso’s two other large, marine-linked water bodies – Lake St Lucia, and the lesser-known Lake Mgobozeleni that sprawls behind the swamp forest at Sodwana, then narrows into a channel through the forest to link with the ocean. These three estuaries are among the biggest in South Africa, with Lake St Lucia the biggest example in the country, comprising 60 per cent of the national estuary area.
That famous hippo- and croc-filled lake is also the subject of the country’s largest rehabilitation project. It saw the removal of dredge spoil between the uMfolozi River and Lake St Lucia, to reunite river with estuary for the first time since 1952, when they were artificially separated.
Nicolette, who project-managed the estuary-restoration programme, says the two bodies were rejoined initially in 2012 and, since the widening of this link, and with improved rainfall that began in 2016, the estuary water levels have significantly improved and stabilised.
The next history-making event will be when the river breaches the mouth naturally (who knows, as you read this, that already might have happened). “We’re monitoring conditions to see how the estuary responds to being back to its natural configuration,” Nicolette says. “But after six decades of separating one of the estuary’s largest rivers, which deprived it of water and also impacted mouth function, no one can say how long it will be before the estuary is fully restored.”
It’s lunchtime so we make for an area of the Kosi system that few visitors experience – the uSiyadla River that, with the small iNswamanzi River, is the only source of surface freshwater to the estuary, the main source being groundwater.
We glide past extensive patches of water lilies and impressive stands of raffia palms to arrive at a blissful stretch reminiscent of parts of the Okavango Delta. Anchored in the generous shade of a fig tree, we tuck into our picnic while watching an exquisite pair of African Pygmy-Geese which, my bird book explains, are absent from much of Southern Africa, but do occur on water bodies with water lilies.
Our day ends with sundowners on the wooden jetty jutting into the Lake Nhlange section of the estuary. Bathed in tranquillity, we watch the sinking sun cast a champagne-pink glow across the water. The first stars appear, water slaps gently against the jetty, a hippo belches nearby and a wood owl calls from the trees on shore. It’s a sunset that memories are made of, one rivalled only by the sunrise we basked in that morning when the lake resembled liquid gold. What a wonder-filled day messing about in a boat and discovering more of the magic of Nature. It doesn’t get much better than that.
The Lake St Lucia Estuary Restoration
For all updates and to find out more about Anthony and Nicolette’s research and restoration work, follow Marine and Estuarine Research on Facebook.
The Ins and Outs of Estuaries
The 300 or so estuaries in South Africa can be categorised into the following groups:
These are large estuaries that can be either permanently or temporarily linked to the sea. Their salinity thus varies and freshwater, marine and estuarine organisms might be present at different times. Other estuarine lakes are Wilderness, Verlorenvlei and Bot (Western Cape).
These are large, natural bays permanently linked to the sea and dominated by marine influences. Durban Bay, Richards Bay, and Knysna are the only three such systems in South Africa.
Intermittently Open Estuaries
The vast majority i.e. about 62 out of 75 of KZN’s estuaries fall into this category. Seasonal low river flows allow wave-driven sand movement along the coast to deposit sand across the mouths and temporarily close the connection with the sea. Other natural processes, e.g. seasonal flooding, breach the mouth. Examples of this category on the KZN Coast are oHlanga (at Umhlanga), aMatikulu and uMgababa. Elsewhere in South Africa are Kleinemonde (Eastern Cape), Krom (Western Cape).
Permanently Open Estuaries
In these, the mouth is open most of the time and the system is influenced by tides. Animals are predominantly marine and estuarine. Examples along the KZN Coast are uMngeni, uMzimkulu and uMtamvuna. Elsewhere in SA are Nahoon, Great Kei, uMngazi (Eastern Cape), and Breede (Western Cape).
These are systems with big catchment areas and where freshwater is the major driver. Major floods can impact on sea salinity and sea temperature over large areas. Strong outflows also transport sediment to beaches. Examples along the KZN coast are uMkhomazi, uMngeni, Tugela and uMvoti, and elsewhere in SA, Bloukrans and Storms (Eastern Cape) and Orange (Western Cape).
Specialist Estuarine and Coastal Tours
Nicolette and Anthony operate EcoInfo Africa, a company that specialises in Ecological Information for Africa and offers bespoke holiday, educational, and team-building tours along the KZN coastline. In addition to being estuarine scientists, they are top birders.
Words Andrea Abbott
Photography Nicolette Forbes; Andrea Abbott