What happens when salinization meets eutrophication? A test using stream microcosms

Nutrient and salt pollution often co-occur in rivers and streams due to human activities (e.g., agriculture, urbanization). Thus, understanding the interactive effects of nutrients and salinity on freshwater ecosystems is critical for environmental management. We experimentally assessed the interactive effects of nutrient and salt pollution on stream microcosms using biofilm and macroinvertebrates as model systems. Six treatments were performed in triplicate: control (C: N-NH₄+ = 0.05; P- PO₄^3- = 0.037; Cl- = 33.5 mg L^-1), intermediate nutrient (IN: N-NH₄+ = 0.4; P- PO₄^3- = 0.271; Cl- = 33. 5 mg L^-1), high nutrient (HN: N-NH₄+ = 0.84; P- PO₄^3- = 0.80; Cl- = 33.5 mg L^-1), salt (S: N-NH₄+ = 0.05; P- PO₄^3- = 0.037; Cl- = 3000 mg L^-1), salt with intermediate nutrient (SIN: N-NH₄+ = 0.4; P- PO₄^3- = 0.27; Cl- = 3000 mg L^-1) and salt with high nutrient (SHN: N-NH₄+ = 0.84; P- PO₄^3- = 0.80; Cl- = 3000 mg L^-1). After 14 days of exposure, biofilm chlorophyll-a increased across all treatments, with cyanobacteria replacing diatoms and green algae. Treatments with no added nutrients (C and S) had more P uptake capacity than the rest. The indicator species analysis showed 8 significant taxa, with Orthocladius (Orthocladius) gr. Wetterensis and Virganytarsus significantly associated with the salinity treatment. Overall, salt pollution led to a very strong decline in macroinvertebrate richness and diversity. However, salt toxicity seemed to be ameliorated by nutrient addition. Finally, both structural equation models and biotic-abiotic interaction networks showed that complex biological interactions could be modulating the response of the biological communities to our treatments. Thus, our study calls for species-level assessments of salt and nutrient effects on river ecosystems and advocates for better management of co-occurring pollutants.