Faecal Bacteria Are Governed by Distinct Assembly Processes Within the Bacterioplankton of an Urban Stream Network

The ecological fate of human faecal bacteria entering hydrological networks remains poorly understood. We investigated bacterioplankton assembly across an urbanized stream network in the Buenos Aires Metropolitan Area (Argentina), distinguishing faecal from non-faecal taxa by cross-referencing 192 human gut microbiome samples from three South American countries; independent source tracking confirmed a predominantly human origin. Faecal taxa occurred throughout the watershed, but their abundance was not significantly associated with Escherichia coli (Spearman's ρ = 0.25; p > 0.05), regardless of the assembly process governing them. Homogeneous selection was the most frequent assembly outcome in both faecal and non-faecal fractions, yet dispersal limitation separated them sharply: 38.2% of non-faecal site pairs versus 1.8% of faecal ones. Accordingly, compositional similarity declined with hydrological distance in the non-faecal fraction but not in the faecal one, where selection was strongest between sites of similar non-vegetated surface cover. Nestedness contributed far more to faecal than non-faecal dissimilarity (42.7% vs. 4.3%), consistent with an external inoculum progressively filtered along the network. Faecal and non-faecal bacterioplankton are therefore shaped by different ecological processes, which single-indicator monitoring cannot capture. Reducing sanitary inputs, while necessary, should be complemented by strategies enhancing the environmental filtering capacity of urban stream habitats.