Threshold responses of multi-trophic freshwater communities to browning and eutrophication
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Browning and eutrophication strongly influence aquatic ecosystems by altering nutrient dynamics, light availability, and food web structure. To investigate their combined effects on aquatic communities, we conducted a nine-week mesocosm experiment in a clear-water north-temperate lake, crossing dissolved organic carbon (DOC) and total phosphorus plus total nitrogen (TP+TN) enrichment treatments. Multi-trophic plankton communities (bacterioplankton, phytoplankton, and zooplankton) were monitored over time using environmental DNA (eDNA) marker gene amplicon sequencing. Beta-diversity analyses highlighted temporal and treatment-driven community restructuring, while PERMANOVA and Principal Response Curve analyses identified the treatments and taxa driving these changes. Our results show that elevated DOC favoured taxa associated with the microbial loop, while nutrient enrichment and lower DOC promoted the green pathway. Threshold responses across trophic levels were observed at 5–7 mg L⁻¹ DOC and 30–70 μg L⁻¹ TP, marking the levels at which compositional shifts propagated through the food web. Overall, this study demonstrated how aquatic communities respond dynamically to browning and nutrient enrichment, offering insight into the mechanisms shaping multi-trophic interactions under a multiple stressor scenario.
Highlights
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Browning and nutrient pulses drove coordinated shifts across bacterioplankton, phytoplankton, and zooplankton.
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Temporal community succession and treatment effects were captured through beta-diversity and multivariate ordination analyses.
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Threshold responses propagated through the food web at 5–7 mg L⁻¹ DOC and 30–70 µg L⁻¹ TP.
Scientific Significance Statement Topic
This study provides insights into the interactive effects of browning and eutrophication on community composition shifts in freshwater ecosystems. Using a mesocosm experiment, we identified thresholds for dissolved organic carbon and total phosphorus + nitrogen concentrations that drove compositional changes across bacterial, phytoplankton, and zooplankton communities, as determined by environmental DNA amplicon sequencing data.
