Ciliate-Dominated Periphyton Communities Along Urbanization Gradients in Two Streams in Zagreb, Croatia

Urban streams typically exhibit altered hydromorphology and large fluctuations in water quality variables, creating stressful conditions for biota. In this study, we investigated periphyton along two urban streams (Bliznec, B, and Veliki Potok, VP) in Zagreb (the Croatian capital) over one year. Both streams were sampled in an upstream pristine reach within Medvednica Nature Park, a middle reach influenced by either agriculture or low-density residential areas (houses with gardens) and affected by channelization, and a lower reach, also channelized, impacted by a mix of agricultural influence and more intensive residential development with higher population density. Nutrient concentration, conductivity, COD, and chlorophyll a showed an increasing trend from upper to lower sites, reflecting the influence of urbanization. The number of periphytic taxa and their abundance correlated positively with the increasing urbanization, probably due to increased food sources. Periphyton consisted mainly of ubiquitous taxa, with 55 phagotrophic protist and 10 micro-metazoan taxa. Ciliates dominated both in diversity (44 taxa) and abundance (over 90% of mean abundance), mainly comprising bacterivorous taxa. Periphyton exhibited pronounced seasonal dynamics, with occasional high similarity between the two urban streams studied and high turnover rates of assemblages between samplings. This pattern indicates that urban streams support highly dynamic periphytic communities, strongly shaped by environmental disturbance and that these assemblages have the capacity to withstand frequent environmental variability in urbanization-influenced reaches.