This study determined changes in the ecological status of a lowland river under the impact of an urban agglomeration after modernizing the wastewater management. For 174 years the Ner River was loaded with pollution from expanding industry. Construction of the Group Sewage Treatment Plant (GOŚ) in 1994 and its subsequent modernization should have improved the water quality. After 20 years of proper sewage treatment the ecological status of the river hasn't improved.

Study sites were established on the river above and below sewage disposal from GOŚ, and the ecological status was assessed on the basis of benthic diatoms. Diatom indices: IO, GDI, IPS were calculated in order to determine the ecological status and water quality. Hierarchical cluster analysis and multivariate analysis of diatom assemblages were used to reveal differences of the study sites. It was revealed that the river is divided into two sections: the first is above the sewage disposal and the second is below it. In the first section, domination of species sensitive to organic pollution Meridion circulare and Ulnaria ulna was noted, while in the second section species resistant and tolerant to organic pollution, Hippodonta capitata and Navicula gregaria, were noted. The ecological status at the first section was moderate to poor, while at the second it was poor to very poor.

The division of the urban river into two sections with different ecological status stems from the history of water management. The present location of the GOŚ outflow served in the past as a dump of untreated sewage from the industry of the Łódź Agglomeration. Development of the city led to the degradation of the ecosystem. Modernization of wastewater management did not reverse the degradation of the river. The persistence of accumulated industrial pollution in sediments caused by the long-term impact of urban agglomeration almost completely destroyed the ecosystem of the river.

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