Anchored in Cameroon’s Wouri Estuary, where rivers, tides, and ships converge, Claris Sunjo leans over the side of her boat. She’s twenty-four hours into collecting water from a tidal creek, a narrow channel connected to the Wouri Estuary system, asking a question scientists have largely ignored in tropical mangrove estuarine systems: What is the role of these narrow channels in carbon cycling and transport to the open water of tropical mangrove estuaries?
Sunjo is a Ph.D. Candidate in the Department of Earth and Planetary Sciences at UC Davis, working under the supervision of Professor Eliot Atekwana in the Atekwana Research Lab Group. Sunjo's research focuses on investigating carbon cycling in tropical mangrove estuaries, including tidal creeks, among the most important yet historically understudied components of tropical mangrove estuaries. She is examining how carbon is sourced, processed, and moved from these channels through the estuary. This is a unique cycle that could reshape understanding of tidal creeks’ role in the global carbon budget, which tracks how much carbon is stored, released, or transported around the planet.
“Most research on carbon cycling has been conducted in temperate environments,” Sunjo said.
Mangrove estuaries, such as the Wouri, are among the planet’s most productive and biodiverse ecosystems, hosting migratory, marine, and freshwater species. The Wouri Estuary serves as a nursery for fish, crustaceans, and mollusks that depend on the fragile balance of nutrients, shaped by natural and anthropogenic factors. Moreover, this system is a vital carbon sink, absorbing and releasing carbon as water moves through channels from surrounding lands.
“We have very limited data on tropical mangrove estuaries in general, and a paucity of carbon data, especially across the wide Atlantic coastal region of Sub-Saharan Africa, which means a large part of the global estuarine carbon budget has been missing,” she said.
Dissolved inorganic carbon (DIC) cycling describes how carbon dioxide (CO2) moves and is processed among local reservoirs in the estuary. One of the simplest ways to explain DIC cycling is that atmospheric CO2 dissolves into the estuary water column and is taken up by aquatic plants during photosynthesis. Alternatively, atmospheric CO2 undergoes physical and chemical processes that store carbon in the water column, which is released to the atmosphere or transferred to the coastal ocean.
“That is a basic way to visualize DIC cycling.” Sunjo said.
A hidden source of estuary pollution
Sunjo conducted fieldwork in 2023 and 2024 in the Wouri Estuary, examining areas with elevated nitrate pollution and carbon dioxide concentrations, both of which are important for understanding the water’s acidification, a hazard to aquatic life. That research challenged assumptions about where local pollution originates and how it alters water chemistry.
“In my first research paper, I examined how we can trace nitrate pollution that is brought into the open water estuaries and how that affects DIC cycling. The three major rivers draining into the Wouri Estuary were my main focus for nitrate pollution sources,” Sunjo explained.
Contrary to researchers’ expectations, rivers were not the primary source of nitrate pollution. Elevated nitrate concentrations were observed in the lower to mid-estuary areas, which are heavily influenced by sewage release from ships. The Wouri Estuary serves as a major seaport for Cameroon and several landlocked countries, with ships usually waiting in the lower Wouri Estuary to load and unload cargo.
“I identified hotspots of nitrate pollution in the open estuary water column. The nitrate concentrations are anomalously high, but not from the river, they’re coming from shipping activities,” she said.
The overlooked role of tidal creeks
Sunjo’s second research paper focuses on the role of tidal creeks in carbon cycling. These are narrow channels that connect mangrove wetlands to the larger open water estuary. Very little is known about how or when tidal creeks process carbon, and whether that carbon is released into the estuary or retained in the tidal creeks.
“People view estuaries as basins where rivers and the ocean mix. They consider carbon cycling in this basin to be controlled only by the mixing of the two water masses,” Sunjo said. “But they overlook the role of tidal creeks, which contribute significantly to DIC cycling. I argue that previous studies miss the major contribution of tidal creeks to the estuarine carbon budgets.”
Sunjo tracked changes in DIC, salinity, and other water chemistry properties within a tidal creek using high-frequency sampling, taking measurements every five to ten minutes over 24 hours spanning two semidiurnal tidal cycles. The approach required sitting in an anchored boat for more than 24 hours to capture subtle yet critical changes that researchers often miss. Sunjo paid close attention to how the water's properties responded to tides and to the day and night.
“What I found were relatively high partial pressures of carbon dioxide (pCO2) concentrations that were more than five times greater than atmospheric in the water column, but they were not related to the tidal movement of water in and out of the tidal creek,” she said. “It was also counterintuitive to observe a reversed salinity behavior where water in the tidal creek got fresher with rising tide and became saline with falling tides.”
Instead, the dominant driver of high DIC input to the tidal creek was submarine groundwater discharge. This water flows through soils along the tidal creek bank and through sediments beneath the creek bed and discharges directly into the tidal creek channel. While groundwater has long been recognized as a source of water solutes and nutrients in estuaries, its influence on carbon cycling in tidal creeks has been widely underestimated.
“People did not think submarine groundwater discharge could alter the carbon cycling significantly,” Sunjo said.
The findings challenge assumptions that changes in carbon dioxide levels are primarily driven by tides or day-and-night driven aquatic processes. Sunjo’s results suggest that tidal creeks account for a substantial portion of carbon processed within the tropical mangrove estuaries. Tidal creeks alone account for about 46% of the carbon, making them impossible to ignore in future estuarine carbon budgets.
Why tropical mangrove estuaries data matters
Sub-Saharan Africa has the second-largest mangrove area globally, yet carbon data from these systems remain scarce. The implications of Sunjo’s research extend far beyond a single estuary.
“We are producing some of the first results on carbon cycling and pCO2 concentrations in sub-Saharan Africa’s mangrove estuaries. This information is critical for the estimates of global estuarine carbon budget,” Sunjo said. Understanding where carbon is released, stored, or transported affects climate models, local water quality, and ecosystem health.
It’s also providing insights into how we view carbon source-sink relationships in tropical mangrove estuarine systems. Elevated carbon dioxide and acidification can threaten fish populations that rely on estuaries as nursery grounds. Estuaries can also emit a tremendous amount of CO2 to the atmosphere and can transfer it to the ocean. This is not accounted for in regional or global carbon cycling models.
“My results are applicable to all tropical mangrove estuaries,” Sunjo said. “The Wouri Estuary is just a laboratory where I can test my ideas.”
Beyond peer-reviewed journal publications, Sunjo has shared her scientific insights in leading scientific communities. Last year, she chaired a session on estuarine dynamics and water chemistry at the Geological Society of America Conference 2025 and was invited by the American Geophysical Union (AGU) to serve as a panelist at the Blue Carbon Coastal Science Town Hall during AGU 2025.
Sunjo has already conducted follow-up fieldwork to examine nine tidal creeks across the Wouri Estuary and determined how DIC cycling vary across space and time, providing insights into how to quantify estuarine carbon budgets.
For Sunjo, the broader impact of the research is clear. Mangrove estuaries are not only critical to global carbon cycling but also living systems that support communities, fisheries, and biodiversity. Funded in part by the Earth and Planetary Sciences department at UC Davis, the US National Science Foundation (NSF), the Schlumberger Foundation Faculty for the Future Doctoral Degree Fellowship, and the American Association of University Women (AAUW) International Doctoral Degree Fellowship, her research helps design programs to sustainably use and manage goods and services in tropical mangrove estuaries and inform estuarine policy.
“It’s a very valuable ecosystem that we need to protect,” Sunjo said.
This article originally appeared on the UC Davis Department of Earth and Planetary Sciences' website.
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