Life depends, among other things, on the proportion of nitrogen (N) and phosphorus (P) that is available in the environment. There is normally much more nitrogen than phosphorus, and species have evolved to live in these conditions. But, if this ratio is altered, organisms reduce their ability to grow and maintain vital functions.
A study published in the journalEcology Letters, in which Josep Peñuelas, CSIC researcher at the Center for Ecological Research and Forestry Applications (CREAF) and Jordi Sardans, also from CREAF, have analyzed the proportion of nitrogen and phosphorus in different freshwater ecosystems around the world. Researchers have found that this proportion is changing in river basins where human activity is most intense and where there is more population.
"If a river or lake is in good condition, the most common is that we find much more nitrogen than phosphorus, so that, if algae and aquatic plants do not grow more, it is mainly due to lack of phosphorus," he explains Peñuelas.
But, with the continuous discharge of detergents, pesticides, fertilizers and urban and industrial wastewater in many river basins of the world we are adding exorbitant amounts of phosphorus, which is accumulating much faster than nitrogen (and other elements, such as potassium ). Thus, phosphorus has gone from being a desirable nutrient to being a pollutant in the most densely populated urban regions and in areas with intensive agriculture.
Algae naturally have seven times more nitrogen in their tissues than phosphorus, and in terrestrial plants this proportion increases up to fifteen times more. "Most soils and rocks contain low amounts of phosphorus, an element that is also very poorly soluble in water. For this reason, inland water species have adapted to accumulate phosphorus and recycle it efficiently", explains Sardans.
The problem is that now, with the pollution of rivers and lakes, these species are finding enormous amounts of phosphorus within their reach and accumulating much more than they would need. According to the analysis of the data available from the last 30 to 40 years, this situation worsens the more human activity and more density of population there are near the river basins.
More nitrogen and more phosphorus means less oxygen
Excessive nitrogen and phosphorus inputs favor a process known as 'eutrophication' of water. It consists of a fertilization that causes excessive growth of some species of algae on the surface that end up preventing the passage of light, so that the algae at the bottom cannot carry out photosynthesis and the water ends up losing almost all the oxygen. In addition, this nitrogen and phosphorus pollution also affects aquatic plants that live submerged or floating in water.
"We think that these algae and aquatic plants play a fundamental role in maintaining water quality and biodiversity in freshwater ecosystems, since they are the food base of a large network of organisms", says Josep Peñuelas.
"If we continue with this situation, the problems can pass to other nearby ecosystems," says Peñuelas.
Jordi Sardans also warns that, if we continue with this situation, the problems could pass to other nearby ecosystems and affect areas of vital importance for humans, in addition to reducing the availability of water for our use. "Therefore, we must continue to improve the global management of these nutrients," he concludes.
China should greatly improve wastewater management
The study points out the need to limit the contributions of phosphorus and nitrogen in aquatic ecosystems. Beginning in 1990, Europe and the United States established more restrictive environmental policies, and also relocated many of their industries. The result has been that nitrogen and phosphorus pollution has since been reduced.
On the other hand, China has experienced the opposite process: it has taken in a large part of the world's industrial production, it does not have such effective policies from an environmental point of view and it continues to use fertilizers and pesticides rich in phosphorus. "In China, three times as much nitrogen and phosphorus are applied in the form of fertilizers than in the United States, and more than twice as much as in Europe," laments Peñuelas. Fifteen times more untreated wastewater reaches river basins in China than in Europe.
The solution to the conflict, however, is a difficult challenge to achieve while the consumption of goods from China continues to grow both in Europe and in the US. "We cannot forget that a small or very important part of China's pollution is produced to manufacture goods that we consume here," recalls Sardans.
Yan, Z., Peñuelas, J., Sardans, J.,et al. (2016). "Phosphorus accumulates fester than nitrogen globally in freshwater ecosystems under anthropogenic impacts."Ecology Letters. 19: 1237-1246. DOI: 10.1111 / ele.12658