Efectes dels efluents de depuradora sobre els biofilms fluvials

Abstract

Freshwater ecosystems continuously receive chemical stressors contributed by Wastewater Treatment Plant (WWTP) effluents. These effluents can produce important changes in receiving stream biological communities, particularly affecting their composition and functioning. One major impact of WWTP effluents into aquatic ecosystems is through river biofilms, which have a key role in the uptake and retention of inorganic and organic nutrients. Algae are part of river biofilms and rapidly respond to changes occurring in the river environment, making them very good indicators of the ecological status of river ecosystems. This study evaluates the effects of a long-term exposure of epilithic biofilm communities to a realistic urban WWTP effluent gradient (ranging from pure WWTP effluent to clean water) on the structure and composition of algal communities. The experiment was performed using artificial streams in a replicated regressional design. The initial hypotheses were that algal communities would: change as a result of the multiple stressors with an inverse relationship between diversity and pollution (i); the algal communities would shift towards tolerant taxa as a result of WWTP effluent exposure (ii); that algal communities would show a taxa succession during exposure and recovery phases, driving the community towards tolerant taxa during the exposure, and returning to the most sensitive taxa after recovery (iii). Results indicate that several taxa within the algal community were negatively affected by the WWTP effluents, while some others were favored. Overall, the algal diversity and taxa richness decreased at the end of the exposure and did not recover. The scores of the two first principal components of a PCA performed with the taxa abundance of algal communities were the best descriptors of the distribution patterns of the biological communities against the WWTP effluent exposure. Besides, these two principal components were fairly related with nitrites and DIN concentration. The results of the experiment allow to conclude that the environmental risk of WWTP effluents on the algal communities is high and produces long lasting and persistent effects