Deceptive ecological status of urban streams and rivers—evidence from diatom indices

AbstractUrbanization directly affects the ecological status of streams and rivers: Their beds and drainage basins are often heavily transformed and their habitat conditions disturbed. The aim of the study was to evaluate the extent of human impact in streams in the city of Łódź, Central Poland (~700,000 inhabitants), by pattern recognition analysis of benthic diatom assemblage samples using a Kohonen artificial neural network, principal component analysis, and indicator species analysis. The study also determined the ecological status of the streams using five diatom indices: Multimetric Diatom Index, Specific Pollution Sensitivity Index, Generic Diatom Index, Biological Diatom Index, and Trophic Diatom Index. The samples demonstrated considerable variation in ecological status. Examples of poor and bad ecological status were observed, clearly resulting from strong human pressure; however, good status and moderate status were also recorded, both at natural sites and, unexpectedly, those which were severely transformed, whose beds were covered with concrete and which were located downstream from combined sewer overflows. Such sites demonstrated high water conductivity of up to 1440 μS/cm, but this was accompanied by other more favorable conditions: The water flowed from upstream stretches enclosed in downtown underground canals with a fast current, low water temperature, and high average concentration of dissolved oxygen (approximately 10 mg/dm3). These conditions had a mitigating effect, shaping the diatom assemblages to a specific form with the highest species richness and the highest number of indicator taxa observed in the study. In summary, the hydromorphological transformation of streambeds may result in seemingly good ecological status at heavily degraded sites. Therefore, conclusions based on diatom‐based biomonitoring of urban streams and rivers should be drawn with caution.