Shifting waters: how habitat alteration affects host-parasite dynamics in freshwater ecosystems.

Human activities have profoundly altered freshwater ecosystems, reshaping host-parasite dynamics with sometimes far-reaching ecological and public health consequences. Based on the examples of stream degradation and restoration, dam construction, agriculture, and urbanization, this review synthesizes how habitat alteration influences parasite communities in freshwater systems. Restoration efforts, such as those in Germany's Emscher River, demonstrate partial recovery of parasite diversity, though persistent stressors often favour generalist taxa with simple life cycles, reflecting incomplete ecosystem recovery. Dams and reservoirs, by contrast, frequently amplify parasite transmission by creating lentic habitats that aggregate hosts and vectors, increasing risks for diseases such as schistosomiasis and malaria. Agricultural land use further complicates these dynamics: while eutrophication and irrigation can boost trematode transmission, agrochemicals and habitat fragmentation may depauperate parasite communities or disrupt complex life cycles. Additionally, urbanization-related stressors, such as pollutants and artificial light, can suppress sensitive parasites while benefiting those adapted to disturbed environments. Our review demonstrates that, under habitat alteration, freshwater parasites fulfil multiple roles: they constitute a component of the impacted biodiversity, serve as indicators of ecosystem health, and influence disease risk, with their responses governed by life cycle complexity, host specificity, and environmental resilience. These findings underscore the need for integrated management strategies that account for parasites in conservation and public health planning.