Exposure pathways modulate antidepressant effects on leaf litter decomposition through microbial-detritivore interactions

Antidepressants can disrupt freshwater ecosystem processes, yet how different exposure pathways influence these effects remains unclear. This study used the freshwater snail Cipangopaludina cathayensis as a representative detritivore to examine the effects of fluoxetine and amitriptyline on leaf litter decomposition under waterborne, dietary, and combined exposure pathways in freshwater microcosms. Antidepressant exposure significantly affected microbial biomass, extracellular enzyme activities, litter quality, detritivore physiology, feeding behavior, and decomposition processes. Microbial biomass increased by 35.18%-99.41% during early exposure but declined by 4.23%-65.34% at later stages under waterborne exposure. Enzymes involved in carbon and nutrient acquisition showed distinct responses among exposure pathways, indicating shifts in microbial resource-acquisition strategies. For detritivore, antidepressant exposure induced pathway-dependent oxidative stress responses and suppressed feeding activity of C. cathayensis. Dietary fluoxetine and amitriptyline exposure reduced detritivore-mediated decomposition by 20.34% and 25.83%, respectively, whereas microbially mediated decomposition increased by 145.74% and 68.19%. Despite this microbial compensation, total decomposition rate declined significantly under dietary exposure. These findings show that exposure pathways strongly modulate antidepressants effects on detritus processing and emphasize the need to incorporate dietary and combined exposure scenarios in ecological risk assessments of freshwater ecosystems.