Water treatment causes changes in the flora and fauna of the rivers.

Fertilizers, phosphate detergents or industrial pollution caused the more industrialized economies to accumulate an excess of phosphorus in rivers and lakes and, therefore, to proliferate phytoplankton (floating algae that give the water a greenish and muddy tone) . The problem was solved a couple of decades ago with the help of water treatment plants. However, a group of scientists has discovered that this cleaning management has changed the ecosystem, causing variations in the flora and fauna of the rivers. In Spain a plague of black fly has appeared in the riverside municipalities that is disturbing the life of the residents of the area.

In the waters of the rivers there are now not enough phosphates for phytoplankton to grow, and they are more transparent. These algae consumed oxygen and created anoxic situations (without oxygen) such as putrefaction, and that is why processing plants were created that improved water quality. But this circumstance has created new conditions. "The light reaches the bottom of the river and results in the growth of large aquatic plants called macrophytes much better than before," explains Josep Peñuelas, an ecologist at the CSIC at the Center for Ecological Research and Forest Applications (CREAF) in Barcelona.

In nature, any small variation produces changes in ecosystems, and more when they are transformations made by humans. "As the trophic web has been altered, an insect that was not there previously has grown like a great plague: the black fly," says Peñuelas.

This insect makes the life of the inhabitants of the riverside municipalities unbearable. Its sting, or rather a bite because it cuts the skin with saw jaws, is very painful and causes thousands of visits to health centers.

The appearance of aquatic plants and, therefore, of the black fly is one of the consequences of the decrease of phosphates in the rivers. But not the only one. The increase in nitrogen oxides has helped create this new scenario. “The idea would also be to eliminate phosphates, reduce nitrogen. These nutrients must be proportionate so that the conditions of before occur, ”says Peñuelas.

Phosphorus and nitrogen are nutrients that plants need to grow along with the sun and water, but in their right measure. “In addition to the change in habitat conditions, both due to excess and default of nutrients (phosphorus and oxygen), if water quality worsens, there can be a loss of biological diversity,” warns Miguel Cañedo-Argüelles, aquatic ecologist and professor at the University of Barcelona (UB).

The excess nitrogen reaches the rivers because we use nitrogen fixers and nitrogen fertilizers. This translates into greater food production but also more nitrogen emissions. The other input of this gas is generated by the combustion of transport vehicles. “In addition to generating carbon dioxide, the heat of the engines produces oxides of nitrogen that are deposited in the medium. These three entries are huge. Much more than what the biosphere has done in your life naturally, ”says Peñuelas.

Organic crops to reduce nitrogen emission.

Some experts advocate reducing the use of fertilizers, as they are the products that emit more nitrogen into the waters. "You have to bet on greener crops that allow you to use a very fair dose of fertilizer that the plant absorbs because if there is an excess of application, nitrogen leaks," says Cañedo-Argüelles. "Nitrogen has been accumulated in the soil and in the underground layer for a long time, therefore it is best to keep it from being retained," he adds.

In addition to the emission of carbon, the other great element that we are adding to the environment is nitrogen, which is not limited because humans produce it artificially. Current nitrogen removal methods are not as efficient and, therefore, scientists believe that we must take a step forward and develop new models. "The ecologists have analyzed the quality of the waters and the consequences it has for life, now we have to agree with the technologists to have greater efficiency," concludes Peñuelas.