Freshwater Plankton Biodiversity Under Urban Pressure: Contrasting Responses of Phytoplankton and Zooplankton in Urban Ponds

Urban ponds are increasingly recognised as important refuges for freshwater biodiversity within human-dominated landscapes. However, phytoplankton and zooplankton are often studied separately, leaving unresolved whether different plankton components provide a consistent picture of biodiversity responses to urban pressure. We investigated both plankton groups in 15 small urban ponds in Poznań (western Poland) differing in the degree of human impact. We compared community structure, species richness, abundance, indicator taxa, and relationships with trophic state. A total of 339 phytoplankton and 148 zooplankton taxa were recorded, including 99 rotifers, 35 cladocerans, and 14 copepods. Zooplankton responded strongly to human impact, with significantly lower species richness in more strongly impacted ponds than in less disturbed ponds. This difference was also reflected in cumulative richness, which reached 128 taxa in only seven less disturbed ponds compared with 91 taxa in eight human-impacted ponds. In contrast, phytoplankton richness did not differ between pond categories, although its abundance was significantly higher in more strongly impacted ponds. Increasing trophic state was associated with lower zooplankton richness but higher abundance of both zooplankton and phytoplankton. Notably, corresponding biodiversity metrics of phytoplankton and zooplankton were not significantly related, indicating that the two groups conveyed largely independent ecological information. Community composition analyses further revealed contrasting responses: the degree of human impact significantly explained zooplankton composition, whereas none of the tested environmental variables significantly explained phytoplankton composition. Several eutrophic rotifer taxa, including Brachionus angularis, Anuraeopsis fissa, and Keratella tecta, were associated with anthropogenically influenced ponds, while fourteen indicator species characterised less disturbed ponds. Our results show that the ecological picture of an urban pond depends partly on which plankton component is examined. Despite sharing the same ponds and environmental setting, phytoplankton and zooplankton conveyed contrasting and largely independent biodiversity signals. Zooplankton provided a clearer signal of local anthropogenic disturbance, whereas phytoplankton responses were expressed mainly through changes in abundance and trophic-state-related patterns. Thus, relying on a single plankton group may provide only a partial assessment of biodiversity responses to urban pressure. Integrating phytoplankton and zooplankton can provide a more complete ecological assessment, while the high taxonomic richness recorded particularly in less disturbed ponds reinforces the biodiversity value of small urban waters.