Scientists study Cape lakes to predict climate change impacts

Both Princessvlei on the Cape flats and Eilandvlei on the Garden Route show ‘threshold type behaviour’, or tipping points, due to climate change. (Photo: Dr Jemma Finch, palaeoecologist)

By Yves Vanderhaeghen

26 Mar 2025

Scientists are zeroing in on two little lakes in the Cape to figure out how to save them from climate change, or if they can’t, then if they could recover.

When conservationists want to reboot a landscape, it helps to have a blueprint of what it looked like in its “untouched” state.

But is there even such a thing? Who do conservationists turn to when they need to peer back in time to know how to set about the future?

Jemma Finch is a palaeoecologist based at the University of KwaZulu-Natal, and her job is to work out which way a neighbourhood is going, ecologically, and to give a fantasy gardener a heads-up so they don’t plant forests where fynbos should reign. She will give insights into her field on a Tipping Points webinar, hosted by Oppenheimer Generations Research and Conservation on 27 March, titled: “Let’s get sedimental: conservation lessons where science meets silt”.

Dr Jemma Finch, palaeoecologist. (Photo: Supplied)

Finch, who describes herself as a “pollen nerd” and a “bogtrotter”, is part of an international team which has just started a three-year project researching two small wetlands in different lake systems in the Cape which face uncertain times. The one, Princessvlei, is on the Cape flats, and the other, Eilandvlei, is on the Garden Route near Wilderness. Each is being squeezed by water pressure, but in different ways. How will they cope, and how will the ecosystems and social arrangements to which they are central adapt?

Read more: Reviving Princess Vlei — how local communities are transforming Cape Town’s wetland ecosystem

The Western Cape, and Cape Town in particular, is acutely aware of how precarious its water supply can be, following its Day Zero crisis in 2018, when a drought, rapid urbanisation and unchecked water consumption pushed it to the brink. Add to that the long-term trends of a hotter climate and lower rainfall, and clues to how the region will cope are being scrutinised closely by researchers, policymakers and citizens.

Tipping points

“The reason why we zoomed in on these two areas,” says Finch, “is that they both exhibit what we call threshold type behaviour, or tipping points”. Tipping points are often unpredictable, can lead to rapid shifts, and have serious consequences once a system has crossed a threshold.

As “hothouse Earth” increasingly shapes and reshapes habitats and the possibilities of survival, environmentalists are monitoring where planetary boundaries, ranging from biodiversity loss and freshwater use to ocean acidification and climate change, are being exceeded. And when they are, how ecosystems are disrupted.

Princessvlei is both a cultural and a natural treasure. Folklore has it that a Khoisan princess was abducted by Portuguese sailors as she bathed there. Under apartheid, it was one of the few recreational areas available to residents of the Cape Flats, and its waters are used for immersion Baptism by church groups from Nyanga, Gugulethu and beyond. Over the last decade it has united communities to protect and restore its once-rich biodiversity, including fynbos varieties, and activists have kept destructive development at bay.

But its future is nevertheless in peril. As temperatures rise with global climate change, rainfall is expected to decrease in this already water-scarce region. As water dwindles and more people draw on it, the water quality of the lake is likely to degrade and tip over into a polluted, nutrient-rich (eutrophic) state in which stinky algal blooms squeeze out other life and reduce the recreational appeal to people.

The fate of Eilandvlei looks different. It falls within South Africa’s year-round rainfall zone, and it has been moulded by shifting dunes and the rise and fall of the sea level. The tourist charm and distinctive ecosystems we see now are the legacy of a dwindling marine influence which established an estuarine/coastal lake system. But “if we have significant sea level rise going forward in the next 50 to 100 years, all those vleis will be inundated with seawater again,” says Finch.

Palaeoecology

But these ecosystems also differ from their historical states because humans have been changing things for 300 years and more. And that’s where palaeoecology comes into its own, offering a pre-disturbance reference point, which gives conservationists a more accurate baseline for restoration goals than short-term ecological data.

“When you start to look deeper back in time, you’re obviously not looking at what you’re seeing in the present day,” says Finch. “You can see how this lake system fluctuated over 2,000 years or 8,000 years or however long the record is. And what we’ve seen in the case of these two sites is that you’ve got two different stable states in the biota and the system. Here we’re looking specifically at diatoms, single-celled algae that are great indicators of changes in water chemistry as indicators. Our collaborator Kelly Kirsten of Coventry University is the diatom expert behind these fantastic datasets, and can interpret the complex environmental signatures they yield.”

“If we look at those algal communities, we can see two very different types of communities existing at different points in time. So, for a few thousand years, we’ll see one type of community, and then we see this very abrupt changeover into a completely different type of community. That is what really interested us because you’ve got these tipping points evident in the record.

“But the drivers of change that are causing those tipping points are very different. The one system allows us to look at sea level whereas the other allows us to think about climate and land use.

princessvlei climate change Both Princessvlei on the Cape flats (above) and Eilandvlei on the Garden Route show ‘threshold type behaviour’, or tipping points, due to climate change. (Photo: Dr Jemma Finch, palaeoecologist)

“In the case of Princessvlei, there’s no marine connection. It’s inland. It’s sitting at about 30 metres above sea level, and changes are driven by rainfall, so moisture availability, and the surrounding land use.

“Whereas in the case of Eilandvlei, you’ve got a marine connection, and those changes are being driven by fluctuations in sea level over the last 8,000 years.

“For us, it’s interesting to then contrast the types of change we see, the types of response, and then start to think about future scenarios.

“Princessvlei, for example, is a eutrophic (nutrient-rich) system. And in the past, we see these states tend to coincide with wetter periods. But going forward, we’re obviously expecting a drying climate. We’re expecting warmer temperatures, more land use pressure.

“We expect to see a reduction in available moisture to the system, but it will probably continue to become more eutrophic just because of the land use going on in the catchment. Princessvlei itself may not be a system that is relied upon as a water reservoir. It’s more broadly about wetlands in that water-scarce environment, but the projected conditions going forward are not good for future water availability and water conservation.

“We’ll model different scenarios of what the change might look like based on what we can see in the past. Will we see history repeating itself? Is it likely that some of those processes that operated in the past are quite similar to what we might see going forward? Or are we going to be in a scenario where we have a no analogue situation, where there’s no modern equivalent or there’s no historical equivalent to what we see now and what we see going forward.

“If we’ve got long records and a good historical understanding of how the system has changed in the past, if we look at where we are now and how that contrasts with where we’ve come from, we can start to make much more informed decisions about whether it is actually viable to try and take it back to what it was, say, pre-European colonisation? Do we need to maybe think about setting a more novel Anthropocene baseline and a target that is more pragmatic? It might be that because we’ve come so far away from where we were, that it’s just not feasible to go back, or it might be that it’s just financially not feasible.”

The research team will be drawing together a vast range of information to draw a map of how past, present and future might come together.

For Princessvlei, says Finch, “there’s a lovely set of data available, generated by Kelly Kirsten and colleagues”. “We look at physical aspects of the lake sediment, its organic composition, colour, grain size, and then the chemical indicators, such as carbon isotopes which tell the proportions of different plants in the system, or other isotopes which you can use as proxies for moisture availability or temperature in the environment. They give you nice independent data.”

Micro-fossil clues

To get a picture of what the surroundings might have looked like, there are clues in plant and animal remains at a micro-fossil level.

Finch uses fossil pollen to put the plant puzzle together, “a sample of what the surrounding vegetation looked like at one particular time. Putting together a whole time series of samples shows the variability of the ecosystem”.

Other pieces of the puzzle are produced by diatoms. “These unicellular algae live in aquatic environments, and they are beautiful indicators of water chemistry and water conditions. They can tell us about salinity and pH, whether it’s marine, freshwater, brackish, all sorts of things including pollution levels. You can even look at the deformations in the remains and they can tell you about heavy metal pollution.”

But the trump card in sifting through the meaning the sediments may hold is environmental DNA. “You take your sediment and test it and spit out a profile of all the organisms whose remains are sitting there and you get a sample of the broader environment, everything that ended up preserved in that sediment.”

Human activity

The picture would be incomplete without tracking the impact of humans on the environment, and two giveaways of their activity are dung and charcoal.

“When you see dung, you often see fungi growing out of it, and those fungal spores are often specific to particular types of dung. So, it turns out that the fungal spores are really great indicators of whether you’ve got domestic livestock or just livestock in general in the area. That’s a nice indicator of human activity. So are charcoal particles. Charcoal is great as a proxy for fire, and we can then try to reconstruct how much burning was going on in a landscape,” which is important in assessing whether grasslands or forests were dominant.

Other useful human indicators are exotic pollen or introduced pollen, which point to exotic trees, agricultural crops such as maize, weeds.

“All those things are telling us about people coming into the landscape.”

Then “we can model tipping points, where we tip over into another state, and we can start to think about how resilient those diatom communities are to change.” Long-term palaeo datasets underscore the need to incorporate tipping point thinking into current restoration and management strategies. At the moment, much of restoration is often still based on a linear approach, but recognising and planning for tipping points could make interventions more effective and resilient.

Once the science is in, the management challenges start for organisations like SANParks or Cape Nature.

But Finch’s colleague, Professor Lindsey Gillson, notes how palaeoecology can help steer them along the right path. The work of Gillson and others has increased understanding of fire and herbivory in the Kruger National Park by reconstructing past vegetation patterns, climate variations, and fire, herbivory and nutrient regimes. By reconstructing interactions between fire, climate and other factors, research helps to identify when savanna systems might reach a tipping point and become either a forested or grassland ecosystem. DM

Gillson and other members of the research team into Princessvlei and Eilandvlei will talk about palaeoecology and planetary boundaries on the Tipping Points webinar, “Let’s Get Sedimental”, hosted by Oppenheimer Generations Research and Conservation, on March 27. Gillson is Professor of Anthropocene Biodiversity, Department of Biology at York University. The other members of the panel are Jemma Finch, (Associate Professor, University of KwaZulu-Natal, Professor Ke Zhang (Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences), and Estelle Razanatsoa, postdoctoral fellow at UCT.

Yves Vanderhaeghen writes for Jive Media Africa, science communication partner of Oppenheimer Generations Research and Conservation.

https://www.youtube.com/watch?v=REeWvTRUpMk

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