Tropical lakes are carbon super sinks, even more than forests, study shows

Amazonian lakes sequester carbon dioxide at a rate 39% higher than the rainforest itself. This is according to a 2022 study by researchers from Brazil, Australia, the U.S., France, Portugal, the U.K. and Sweden. They found that these bodies of water trap 113.5 grams of carbon per square meter per year in their sediment, while previous studies have shown that the forest absorbs an average of 81.72 grams per square meter per year.

The research also revealed that lakes located in tropical regions with preserved forests sequester 10% of the carbon in these locations. In addition, it showed that tropical lakes absorb three times more carbon than temperate ones, and 10 times more than lakes located in subpolar regions.

The researchers analyzed the sediments deposited at the bottom of the lakes, which are rich in organic matter.

“This carbon is in the form of biomass,” Leonardo Amora-Nogueira, who led the study as part of his Ph.D. in geography at Fluminense Federal University (UFF), told Mongabay by phone. “Forests sequester carbon, aggregate it in biomass for growth, and when they die, it is carried to the lake and accumulated. This accumulation in the sediment prevents degradation from occurring and this carbon from being emitted into the atmosphere in the form of gas. The material at the bottom of the lakes acts as a carbon reservoir.”

Fellow researcher Rodrigo Abuchacra, from Rio de Janeiro State University (UERJ), said aquatic environments are the main destination of sediment production, surface and underground runoff, as well as terrestrial biomass and water in the surroundings and in the lake itself.

“Consequently, much of this production ends up deposited at the bottom of lake water bodies, and sediments are natural records of the evolution of these environments,” Abuchacra told Mongabay by phone.

By investigating each sediment layer in high resolution, he said, it’s possible to trace the trajectory of human interventions in the surroundings.

“It is possible to identify, over time, the different levels of environmental impacts on each of the lakes and compare with other biomes locally and abroad,” he said.

The first step of the research was to compile 43 earlier studies on tropical rainforest lakes from around the world. According to the researchers, this approach doubled the representation of these warmer water bodies in tropical forests when compared to what had been previously reported in the literature. This made it possible to have a better representation of the global distribution of lake carbon burial rates.

Based on these data, the researchers found that the ability of lakes to accumulate carbon was closely related to the higher productivity of tropical forests compared to those of other biomes.

“The Amazon [rainforest] is one of the most productive in the world,” Amora-Nogueira said, referring to the sheer mass of vegetation that it generates. “Therefore, the amount of organic matter that ends up being available to be accumulated in the lakes is much higher in this region. This explains the three to 10 times higher values that we find in these tropical rainforest areas, compared to temperate and subpolar regions.”

Dive into time

The next step was fieldwork in the Brazilian Amazon. They studied 13 lakes scattered across a total area of 580,000 square kilometers (224,000 square miles) in two clusters 1,170 km (730 mi) apart. This increased the number of rainforests represented by about 75, or 130%, compared to previously published studies. They analyzed the material deposited at the bottom of the lakes over a period of the last 50 to 100 years.

“We studied a succession of layers, from the oldest at the bottom to the top of the sediments, which are the most recent,” Amora-Nogueira said.

To collect the material, the researchers drilled vertical holes in the sediment at the bottom of the lakes at the point of greatest depth, extracting core samples up to 90 centimeters (35 inches) long.

“Thus, we collected and preserved a sequence of layers, starting from what was deposited before and what was then stored there over time until reaching the present day. Then we took these tubes to the laboratory and cut them into 2-centimeter [nearly 1-inch] slices, and then we analyzed them. The most important thing is that they told us exactly how old each one was,” Amora-Nogueira said.

To accurately date the samples, the team used lead-210, an isotope that occurs naturally and decays at a known and constant rate of about 22 years, turning into bismuth. Based on this known decay rate, the researchers were able to assess the age of each layer and carried out organic carbon analyses to verify the effective amount in each sample.

This way, it was possible to verify all the changes that occurred in the surroundings of the lakes, whether natural or human-driven, that led to an increase or decrease in the accumulation of organic matter into the body of water over the years. In other words, the story of these changes are recorded in the core samples. Thus it was possible to verify that deforestation, detected from satellite imagery, was linked to reduced rates of carbon sequestration.

“Through geoprocessing, we were able to know the size of the area that was deforested and the period when the forest was cleared,” Amora-Nogueira said. “And from the evidence at the bottom of the lake, we were able to capture this decrease or increase, which coincides exactly with these dates that were verified in the satellite images.”

Their superior carbon sequestration capacity notwithstanding, the world’s natural lakes are tiny compared to its forests. They cover approximately 2.67 million km2 (1.03 mi2), or 1.8% of the Earth’s surface, in almost all climatic zones. In the Amazon, there are about 15,000 lakes occupying an area of approximately 20,000 km2 (7,700 mi2), or 1% of the total rainforest area, and sequester 2.7 million metric tons of carbon per year.

This volume may be an underestimate, however, because there are lakes beneath the tree canopy that don’t show up in satellite imagery and therefore aren’t studied. In addition, sediment that accumulates in river floodplains and swampy areas haven’t been assessed. In any case, the study’s findings may also serve to determine priority areas for conservation, which should include both terrestrial and wetland ecosystems.

The importance of lakes within river basins comes from the fact that these are environments of intensive organic material deposition, with recent carbon burial rates generally high, playing an essential role in the global CO2 cycle, which makes lakes’ capacity disproportionate to their relatively small area.

“Our study was able to provide evidence that, despite being small in area, these lakes have a very large capacity to absorb carbon,” Amora-Nogueira said.

Humberto Marotta, Amora-Nogueira’s Ph.D. adviser at UFF, told Mongabay by phone that the lakes may be small in area, but they receive contributions from vast areas of the drainage basin: rivers flow into them, surface runoff reaches them over the ground, and water absorbed into the soil also finds its way to them through the water table.

“At the same time, lakes are ecosystems conducive to the deposition and accumulation of inorganic and organic materials,” he added.

Marotta said that when considering the importance of the forest, it’s necessary to preserve not only the trees, but also the lakes and other bodies of water as a whole.

“We have to put on the conservation radar not only water and food security and the preservation of biodiversity wealth,” he said. “All that is fundamental, but we have to think in terms of carbon credit, the preservation of lakes to accumulate biomass and avoid the release of greenhouse gases into the atmosphere. And to emphasize our finding that deforested areas have less biomass accumulated in the lakes.”

According to Amora-Nogueira, the study represents an initial effort to understand carbon sequestration by tropical rainforest lakes.

“Given all the diversity found in these environments, much research is still needed to understand this relationship between organic matter produced by the forest and what is accumulated in the lakes,” he said. “But on the other hand, it was a fundamental research, which ended up inserting data from different regions. We showed that these lakes in these humid tropical regions have a capacity to accumulate carbon that is much higher than what we found for other biomes.”

He added that ecosystem conservation shouldn’t be focused solely on carbon accumulation capacity. There’s also the rich biodiversity and ecosystem services that are lost when these deforestation events occur.

“When we think about the discussion about the carbon market, if it is not associated with the protection of ecosystems, it becomes empty,” Amora-Nogueira said. “What we also draw attention to in the work is that it is necessary to look at biodiversity sustained by this environment. And also for the people who live in those places and depend on those natural functions, which are offered by that ecosystem.”

Further research into this issue can help address other aspects that were left out. The researchers say higher annual temperatures may actively stimulate the degradation of considerable amounts of carbon trapped at the bottom of these tropical lakes, increasing uncertainty about the role of temperature variation in determining sediment burial rates. This degradation may also eventually release large amounts of carbon.

 
Banner image: Amazonian lakes like Puruzinho, in Brazil’s Amazonas state, capture 39% more carbon per unit area than the rainforest itself. Image courtesy of Humberto Marotta.

This story was reported by Mongabay’s Brazil team and first published here on our Brazil site on July 26, 2023.

 
Citation:

Amora-Nogueira, L., Sanders, C. J., Enrich-Prast, A., Sanders, L. S., Abuchacra, R. C., Moreira-Turcq, P. F., … Marotta, H. (2022). Tropical forests as drivers of lake carbon burial. Nature Communications, 13(1). doi:10.1038/s41467-022-31258-8