Ecological impacts of treated effluent on multitrophic biodiversity and their interactions

The reuse of water, particularly treated effluent from wastewater treatment plants (WWTPs), is a crucial and sustainable strategy for mitigating water scarcity, especially in megacities with high water demand and limited resources. However, the ecological risks associated with effluent discharge into receiving waterbodies have gained significant global attention. Understanding the dynamic effects of WWTP effluent on multi-trophic groups and their interactions is essential for assessing ecological impacts in aquatic ecosystems and informing management strategies. In this study, we examined five taxonomic groups representing different trophic levels of the freshwater food web - bacteria (decomposers), algae (primary producers), zooplankton (primary consumers), and benthic macroinvertebrates and fish (predators) - across two rivers to elucidate ecological responses to WWTP effluent from a multi-trophic perspective. Our results revealed significant but variable biological responses among these groups, depending on river conditions and trophic level. In the nutrient-rich river, primary consumers (zooplankton) were most affected, whereas in the nutrient-poor river, primary producers (algae) exhibited the strongest responses primarily derived from environmental disturbances. Notably, interactions between environmental variables and taxa were highly diverse, with trophic dynamics influenced by both bottom-up and top-down processes in the nutrient-rich river, whereas bottom-up effects dominated in the nutrient-poor river. Furthermore, niche overlap in algae-zooplankton networks was higher in the nutrient-rich river than in the nutrient-poor river. This study underscores the importance of integrating multi-trophic biodiversity profiling and trophic interaction analyses to comprehensively assess the ecological effects of WWTP effluent in receiving aquatic ecosystems with contrasting environmental contexts. Our findings highlight the importance of conservation and sustainable management practices, especially in urban aquatic ecosystems located in (semi-)arid regions that experience prolonged periods of low precipitation.