Wastewater Treatment Plants (WWTPs) play a fundamental role in managing and treating municipal and industrial wastewater to protect public health and the environment. However, a major environmental concern related to WWTPs is their role in the release of microplastics (MPs) into aquatic ecosystems. While WWTPs are designed to remove pollutants, their effectiveness in capturing MPs varies. The treatment process typically involves several stages, from the physical separation of large particles, through biological processes to break down organic matter, to advanced treatment methods, such as filtration and disinfection, aimed at removing finer particles and pathogens. Despite these treatments, a considerable portion of MPs can still pass through and be released into receiving water bodies. Consequently, WWTP effluents containing these emerging elements at various concentrations are discharged into aquatic environments, affecting ecosystem biodiversity and health.

On the other hand, microalgae are single-celled photosynthetic organisms found in various aquatic environments, including freshwater systems. They play a crucial role in aquatic ecosystems and are often used as indicators of water quality due to their sensitivity to environmental changes. Monitoring microalgae communities provides crucial information for assessing the health of aquatic ecosystems, detecting pollution, and guiding water management practices.

The objective of this work is to analyse two target rivers in the Principality of Asturias, northern Spain, which are subjected to different anthropogenic pressures: the Sella and Nalón rivers. The rivers were sampled from their sources to their mouths, quantifying and analysing the composition of MPs using the FTIR technique, as well as assessing the microalgae community using eDNA tools. These results will be related to the various anthropogenic pressures in order to develop practical guides for improved water management and to maintain ecosystem health.