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Consistent diversity loss but divergent functional responses of freshwater microbial communities to salinity and nutrient enrichment across climatic regions

Fuggle, Rose1; Mayer-Pinto, Mariana2; Matias, Miguel3; Marzinelli, Ezequiel1

Published Jul 17, 2026; Updated Aug 13, 2026 on Dryad. https://doi.org/10.5061/dryad.xsj3tx9v8

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Jul 17, 2026 version files 3.31 GB
Aug 13, 2026 version files 3.35 GB

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Abstract

Freshwater salinisation and nutrient enrichment are intensifying globally. Despite their widespread co-occurrence the extent to which their combined ecological effects are generalisable across distinct climatic regions remains unclear, preventing appropriate scales for policy implementation to be identified. Using outdoor 1,000 L open freshwater ponds established > 10 years ago, replicated across inland Mediterranean, coastal Mediterranean and mountain regions, we examined the independent and interactive effects of salinity and nutrient enrichment across regions using benthic biofilms as a model system. We tested the effects of elevated salinity (0, +5, +15 g NaCl L -1) and nutrient enrichment (10:1 N:P additions) on freshwater benthic bacterial community structure (richness, diversity, taxonomic structure) and whole-community biofilm functions after 30 and 60 days. Across all regions, salinity was the dominant driver of bacterial community structure and function with effects strengthening through time. Nutrient enrichment had no significant main or interactive effects, possibly due to the strong nutrient limitations in the systems. After 30 days, the structure of high and ambient salinity bacterial communities differed significantly at all regions, and by day 60 all salinity levels differed significantly from each other. Bacterial richness and diversity (Shannon’s index) also declined significantly with increasing salinity at all regions. Functional responses to salinity, however, varied by region. In the inland Mediterranean region, biofilm biomass, gross primary productivity (GPP) and net primary productivity (NPP) were significantly higher under high salinity compared to moderate and ambient salinity; in the coastal Mediterranean region, GPP and NPP were lower under high salinity compared to moderate and ambient salinity but biomass did not differ; while in the mountain region salinity had no effect on the measured functional variables. Photosynthetic efficiency was unaffected by treatment in any region. Policy implications: In sum, salinisation consistently decreased freshwater bacterial diversity in biofilms across three distinct climatic regions, whereas functional outcomes were regionally contingent. This suggests management policymakers can set cross-regional salinity thresholds to maintain bacterial diversity, but functional responses to salinisation must be assessed regionally to predict and manage the impacts of freshwater salinisation under accelerating global change.