Data collected for more than 20 years in the Paraná River basin of Brazil has produced evidence of a drastic decline in the number of aquatic insects in the region. The research suggests that this decline is linked to the construction of over 180 dams on the Paraná basin and its tributaries.
“Our study analyzed data collected on a seasonal basis over a 20-year period. We detected a decline from thousands to tens of individuals per square meter,” said Professor Gustavo Romero.
The project was supported by the FAPESP Research Program, and the fieldwork was conducted by researchers with the State University of Maringá’s Center for Research in Limnology, Ichthyology and Aquaculture (NUPELIA-UEM).
The team studied a floodplain with an area of 40 square kilometers containing rivers, shallow lakes, channels, and backwaters.
According to Professor Romero, the drastic decline in insect populations is a global phenomenon and studies have shown its correlation with human activities.
“A sharp decline was observed not only in more susceptible species but in all aquatic insect orders and families that live in the area. These insects inhabit freshwater environments until they reach adulthood when they migrate to terrestrial environments. This includes dragonflies and water beetles, to mention only the most well-known,” explained Professor Romero.
Considering that some insects transmit diseases, many people assume that all insects are harmful to humans, but Professor Romero pointed out that the insects that are being wiped out in the Paraná River basin are extremely useful.
“They provide many ecosystem services, including pollination, biological control of crop pests and disease-transmitting insects, decomposition of organic matter, and nutrient cycling.”
Professor Romero explained that dams have impacted aquatic insects in three ways. First, the dams make the water much clearer. Without the camouflage of murky water, insects are more vulnerable to predators.
In addition, the exotic fish species introduced into dam reservoirs eat insects as well as native fish. Furthermore, the researchers detected a chemical imbalance of the nutrients in the water, changing the proportions of nitrogen and phosphorus.
“The algae that proliferate in dam reservoirs fix nitrogen from the atmosphere and transfer it to the water. Part of the phosphorus is deposited on the reservoir bed. The water that flows through the dam spillway is poor in phosphorus and proportionally richer in nitrogen as a result. This changes its nutritional quality, affecting the animals that depend on a balanced quantity of these nutrients,” explained Professor Romero.
Changes in the ecosystems of the Paraná River sub-basin are significant for all of South America. This is because the river basin touches seven Brazilian states and is part of one of South America’s three main basins.
The study is published in the journal Biology Letters.