Life in the rivers is changing. The rate of decomposition of the organic matter that reaches them is being disrupted by the increase in temperature and the greater availability of nutrients. Using canvas (the kind used by painters), hundreds of scientists have measured the rate at which plant debris degrades in more than 500 waterways on six continents. In addition to achieving a standard method valid for the entire planet, the authors of this enormous research have detected the global patterns by which the carbon present in leaves and other plant residues is released into the atmosphere in the form of CO₂ or is trapped in the bottom of lakes and seas in which rivers die. The first path accelerates climate change, the second would help stop it.
If the seas are the arteries of the planet’s circulatory system, the rivers are its capillaries. Enormous amounts of organic matter reach them from terrestrial ecosystems. It is estimated that about 720 million tons per year. This plant debris has several destinations on its way to the sea. Much of it is incorporated in the microorganisms that degrade it, in the microbes that feed on the remains of plants and form the base of the food chain, of the cycle of life. In this process of degradation of plant compounds into their essential components, called catabolism, a good part is released into the atmosphere as carbon dioxide or as methane, a greenhouse gas much worse than the first.
A third of these millions of tons end up trapped in the terminals of the rivers, such as flood zones, lakes and, especially, the oceans for decades, centuries or millennia. The distribution depends on the rate of decomposition; the faster it is, the lower the percentage that is trapped and mineralized. But measuring the rate of decomposition and doing so in a universal and comparable way seemed impossible. It involves dozens of factors that are highly dependent on local conditions, from the acidity of the soil to the temperature, including the characteristics of the leaf to be degraded or the existing microorganisms. Now, more than 800 experiments in hundreds of waterways have found, first, a model to predict decomposition and then, with it, the global patterns that govern it. AND They have released the model for use by the rest of the scientists in their field.
“Globally, the increase in temperature is expected to favor microbial decomposition”
Luz Boyero, researcher and co-leader of the River Ecology Group of the University of the Basque Country
Of the more than a hundred variables that they measured at work, published in Science, verified that temperature and nutrient availability are among those that most critically affect the rate of decomposition. “Temperature has a direct effect on microbial decomposition, more or less, as predicted by the metabolic theory of ecology,” recalls Luz Boyero, from the Department of Plant Biology and Ecology at the University of the Basque Country and co-author of the research. The thermal variable could explain the main global pattern they have observed: the rate of organic decomposition increases as latitude decreases. Hence, the highest rates of degradation have been found in Central America, Western Africa (through which the gigantic Congo River flows) or Southeast Asia. “But the relationship with total decomposition is not so direct,” adds Boyero. What they have observed is that while the average air temperature does not seem to change the rate of degradation, the water temperature does.
Another critical variable is the presence of nutrients. “Cellulose is basically carbon, but in order to degrade it, microorganisms need other elements not present in plants, such as nitrogen or phosphorus,” explains Antonio Camacho, professor of ecology at the University of Valencia, whose group research team has participated in the study, providing data from Iberian rivers in the Mediterranean basin and (the only ones who have done so) from Antarctic water courses. Much of the green revolution of the last century and the continuous increase in agricultural production is due to the use of fertilizers. But many of them end up in rivers or lakes, doping their microscopic ecosystems in a process known as water eutrophication, which has become a global threat. Camacho’s team went to the headwaters of the rivers to isolate the natural presence of nutrients from the anthropogenic one. “Thus we have been able to determine that the availability of elements such as nitrogen or phosphorus is critical for the decomposition rate,” concludes the professor.
Although many other elements are involved, the human impact via fertilizers could explain some results of the work. The area of the great lakes of North America and the rivers of central Europe, being in mid-latitudes, degrade organic matter at almost the same rate as the Congo River or the Ganges, considered one of the most degraded on the planet. Meanwhile, the large Amazonian bodies of water, such as the Orinoco or the Amazon, have comparatively lower ratios. What do the Danube and the Brahmaputra have in common? They run through densely populated areas maintained by agriculture that is very demanding of fertilizers. The geographical pattern is completed with the higher latitudes. The rivers of Canada, the Nordic countries and, to a lesser extent, those of Siberia, degrade organic matter at a very slow rate, only surpassed by that observed by Camacho’s team in a watercourse on the island where one of the Spanish Antarctic bases.
“We have been able to determine that the availability of elements such as nitrogen or phosphorus is critical for the decomposition rate.”
Antonio Camacho, professor of ecology at the University of Valencia
The study was carried out by hundreds of scientists using canvas. “It is a standardized material, with its cellulose percentage and fabric tension determined,” Camacho highlights. The canvas is made with cotton fibers, rich in cellulose, the vegetable polymer most present in plants. Using it, scientists were looking for a standard method valid for the entire planet and independent of local variables. “We determine the decomposition rate with the loss of tension in the strips, an indication that the cellulose is degrading,” adds Camacho. The main product of this degradation is carbon. The repetition of these experiments with leaves of 35 plant genera (coupled with previous data from local studies) has allowed them to validate this cellulose-based method to predict the decomposition rate of almost any river.
The director of the Catalan Institute for Water Research (ICRA, for its acronym in Catalan), Vicenç Acuña remembers that “trees are a CO₂ sink, their wood retains carbon for centuries, but there are also the leaves.” Whether because they are deciduous or due to their natural renewal, a good part of the leaf litter ends up in the rivers. “It was believed that most ended up in other carbon sinks, such as the bottom of lakes and oceans,” he adds. “But now we know that it decomposes in rivers and the carbon reaches the atmosphere, feeding back climate change,” he completes. For Acuña, finding a model like cellulose to predict the pace of this process in practically all rivers is the great contribution of this work.
In line with Acuña and from the United States, another of the authors details the consequences of these changes. David Costello of Kent State University says that “faster decomposition in rivers means that more CO₂ returns to the atmosphere instead of moving downstream into lakes, estuaries and oceans, where it could potentially be buried and stored for a long time.” term”.
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