Phytoplankton Community Structure and Functional Traits Along a Trophic Gradient in Small Urban Wetlands of China

Urban small wetlands face eutrophication. Using 15 small wetlands in Jinan, China, we analyzed phytoplankton community and trait responses to eutrophication. Based on autumn TLI (2020–2022), wetlands were classified into oligotrophic (3), mesotrophic (6), and lightly eutrophic (6). We evaluated functional α/β-diversity, phytoplankton standing biomass (a proxy for productivity), and resource use efficiency (RUE, biomass/TP). Total nitrogen, total phosphorus, and Chl-a increased with eutrophication, reaching 6.83, 0.14, and 0.03 mg/L in lightly eutrophic sites. Species richness, Shannon, and Simpson indices were highest in lightly eutrophic sites (25, 2.75, and 0.71, respectively), whereas functional richness was highest in oligotrophic sites (0.14). Taxonomic β-diversity was dominated by turnover components (63.7–73.7%), while functional β-diversity was dominated by nestedness components (79.7–97.1%). Mean phytoplankton standing biomass and RUE increased with eutrophication, reaching 44.79 mg/Land 350.64 in lightly eutrophic conditions. Biodiversity–ecosystem function relationships were gradient-dependent. Species richness positively correlated with standing biomass in mesotrophic and lightly eutrophic sites (p < 0.05), while evenness negatively correlated with standing biomass and RUE across all trophic grades (p < 0.05). Overall, these results reveal complex responses of phytoplankton community structure and function to eutrophication in small urban wetlands. The nestedness-dominated functional β-diversity reflects differences that arise mainly through nestedness rather than species turnover. We recommend incorporating these small wetlands into routine monitoring frameworks and prioritizing functional diversity in ecological health assessments.