Marine ecology: The Convex Seascape Survey team on a recent expedition in Algoa Bay to map
the blue carbon in the seabed.
Photos: Convex Seascape Survey
team in Algoa Bay
Benjamin Harris looked like a miner panning for gold as he swirled his sieve on the deck of the RV Observer. But the marine ecologist from the University of Exeter was sifting for life — tiny creatures hauled up from the seabed of Algoa Bay.
These seafloor-dwelling marine organisms — including worms, urchins, starfish and crabs — are more than curiosities. Harris and his colleagues are probing how they could shape the ocean’s capacity to store carbon and help mitigate climate change.
Harris, a postdoctoral research fellow, is part of the Convex Seascape Survey, a five-year global research initiative aiming to unravel the mysteries of how, when, and where the ocean’s continental shelves and muddy seafloor store carbon.
The Convex Seascape Survey is a partnership led by the Blue Marine Foundation, University of Exeter, and Convex Insurance Ltd. Local partners include Nelson Mandela University and the South African Environmental Observation Network.
In recent weeks, the survey has expanded its research to Algoa Bay to explore how rivers, marine life, and human activity influence carbon storage in coastal sediments.
On board the South African Institute for Aquatic Biodiversity’s research vessel, PhD student Sabrina Sykes, a research associate at the University of Exeter, and Gail Fordham, the science programme manager, deftly and expertly deployed the sediment grab.
Bent over their sieves and armed with tweezers, Harris and his colleague Frederick Mokumo, a postdoctoral fellow at Nelson Mandela University, worked with delicate precision, teasing the tiny invertebrates that would help reveal the carbon secrets of the seafloor that Sykes and Fordham unearthed. It’s a tactile, painstaking process that requires patience and steady hands.
“We put that sediment grab into a big sieve to wash away the finest grains — our sieve is about 1mm mesh,” explained Harris. “We’re capturing all the animals one millimetre and above and draining away the sediment. We’re panning for life to bring those animals out so that we can see them, pick them out and put them in sample jars to assess later what they are and how many there are.”
Harris specialises in how life on the seabed shapes the ocean’s carbon balance. As part of the international project, he studies how worms, shellfish and other seafloor organisms mix sediments and move carbon, influencing how long it stays locked away.
The Algoa Bay team, led by Mokumo, is investigating how carbon is stored in the bay and the role of seabed creatures in this process. Understanding these interactions is crucial for protecting marine ecosystems and supporting climate mitigation efforts.
Algoa Bay is an ideal laboratory for the Convex Seascape Survey scientists. Fed by the Sundays, Swartkops and Coega rivers and influenced by shipping, aquaculture and port activity, the bay is a dynamic system where natural and human forces intersect.
During their expedition, the researchers collected samples from 25 stations across three transects. Transect A runs near shipping lanes and dredging zones, Transect B receives sediment from rivers, and Transect C, farther offshore, serves as a baseline — that’s where their focus was today.
“Our job is to map where organic carbon accumulates, identify hotspots and advise policymakers to protect these critical areas,” Mokumo said.
“If we disturb these sediments through dredging or trawling, we risk releasing that carbon back into the atmosphere. We’re trying to break that cycle — to kidnap that organic carbon and keep it safely locked away.”
Steady hands: Benjamin Harris, of the University of Exeter, collecting
the infauna – the tiny animals living within the sediment of Algoa Bay.
In the lab after the fieldwork, the scientists will analyse sediment samples for organic carbon content, composition and historical accumulation. Sediment cores will measure how much carbon is stored and how it is distributed with depth.
Environmental factors like water temperature, salinity, and oxygen levels will be recorded to provide a complete picture of conditions in the bay.
They will also examine how the seabed creatures influence carbon storage. Advanced statistics will help the team understand which factors — natural or human-made — have the most significant impact.
This research will provide the first detailed map of carbon storage in Algoa Bay, highlight areas where carbon is most at risk and show how marine life helps preserve it. The findings will support sustainable coastal management, guide local conservation efforts, and contribute to the global understanding of coastal carbon and climate change solutions.
Beneath the waves lies a dense, interconnected world of organisms, sediments and carbon-rich layers built over centuries. But trawling, dredging, and pollution threaten these delicate stores.
Now in its third year, the Convex Seascape Survey, funded to the tune of $15 million, probes one of the planet’s least understood carbon stores: the continental shelves. Fordham, who leads research on seabed carbon and sustainable ocean management, said the project fills a critical blind spot in blue-carbon science.
“We’re really hoping to understand a lot more about the carbon stored in the seabed. People often think of blue carbon as seagrass, mangroves or kelp forests,” Fordham said.
“But actually, if you think about the ocean space, those habitats take up a very small area just along the coasts. And, actually, the most vast habitat on earth is seabed mud, but this area has been really understudied,” she said.
Seabed mud sequesters carbon just as other blue-carbon habitats do. Per hectare, it may store less, but “because of the sheer scale of continental shelves, it’s one of the most important carbon stores on Earth,” Fordham said.
“We’re only beginning to understand how fragile and vulnerable they are to human disturbance. Protecting these stores and ensuring the processes involved in replenishing and continuing to sequester carbon are maintained will accelerate action against climate change, alongside reducing emissions.”
The project is divided into three work packages. The first focuses on mapping global carbon hotspots, using historical tides, currents and sediment movement to predict where mud — and carbon — accumulates.
The second examines human impacts, particularly trawling, which drags nets across the seabed, scooping up organisms indiscriminately and potentially disturbing stored carbon.
“Our project is working to come up with robust figures for this impact so we can then start to look at management strategies around areas that are particularly carbon-rich and perhaps concentrate our fisheries in areas that are less valuable for the climate.”
The third work package is focused on life and biodiversity. Here, the question centres on whether a healthy, thriving ocean filled with life is better at sequestering carbon than an empty ocean that’s depleted of life.
“Global expeditions have taken researchers to South Africa, New Zealand, Western Australia and the United Kingdom. More than 100 scientists from nearly 20 institutions contribute to the project. It’s really inspiring,” she said. “The kind of questions we’re answering are incredibly complex, so you need that breadth of expertise.”
Sediment cores reveal carbon stored deep in the seabed, while sediment grabs sample the top layers. Remotely operated vehicles film epifauna — corals, sponges, gorgonians — that trap sediment and enhance carbon burial. Baited video systems capture fish communities, and divers conduct targeted sampling.
“When we bring up these samples, we’re looking at a multitude of different things,” Harris said. “One focus is the infauna [the animals living in the sediment of the ocean floor] — the invertebrates that dig through the sediment and pull carbon down deeper, helping it be stored for longer.
“We’re also looking at the carbon itself. We take sub-samples and then go into the lab to measure how much organic carbon is in each one. This shows us how much carbon is being stored, how it varies across the bay and how it changes with human impacts like dredging, trawling or river inputs.”
They are trying to get a clear picture of how carbon is stored in the seabed and how both humans and animals influence it.
“Every time we send a grab down, we find worms and other animals. Extrapolated across the seascape, that’s trillions of organisms living, dying, burrowing and interacting all day, every day. Their burrowing helps lock carbon into the seabed for potentially hundreds, thousands or even millions of years.”
Sykes, who contributes seascape-carbon and marine-life research to the survey, said: “On the boat, it just looks like mud, but in the lab, there will be a lot of species identification.”
She will identify these animals, potentially down to subspecies level, meticulously recording their abundance, tube construction and species differences.
As night settled, the research vessel turned toward shore. Gulls wheeled overhead, calling above the thrum of engines. Buckets brimmed with sediment, and the scientists’ arms and backs ached. Fieldwork stretched long into the evening.
Beneath the waves, Algoa Bay’s living mud continued its ancient work — sifting, burrowing, storing — a hidden system whose full role in carbon storage and climate resilience is only beginning to be understood.