Causes of eutrophication in small water reservoirs in urban areas

The aim of the study was to determine the causes of eutrophication in small urban water reservoirs located in the UMCS Botanical Garden in Lublin, supplied via surface and groundwater. The research (hydrological, hydrobiological, and hydrochemical), which included both field and laboratory work, was conducted during the growing season in the years 2022–2023. These ponds are fed by waters from the Czechówka River and, to a lesser extent, by groundwater (seepage). Both river and groundwater are characterised by high concentrations of mineral forms of nitrogen and phosphorus, ranging from 1.49 to 12.0 mg N∙dm–3 and 0.07 to 0.21 mg P∙dm–3, respectively. This contributes to the intensive development of phytoplankton, especially during the summer period, with diatoms dominating the phytoplankton structure. The trophic state of the ponds ranges from eutrophic to hypertrophic. The study showed that the high degree of eutrophication was due to the load of nutrients delivered by the waters of the Czechówka River. Despite having several times higher concentrations of mineral and total nitrogen than in the river water, the spring water feeding the ponds had a negligible impact on the quality of the pond water due to low flow rate (<0.5 dm3∙s−1). Therefore, the construction of urban ponds as part of green-blue infrastructure should consider the possibility of reducing nutrients through biogeochemical barriers and suspended sedimentation. It is also advisable to partially shade the water surface by planting trees in the shoreline zone to limit water heating and phytoplankton development.