Popular Science: Hidden and forgotten?
The river landscape can be considered to be the most dynamic one. Streams change their direction while, in the floodplains of rivers, water bodies are formed and disappear, including oxbow lakes. They are distinguished by the uniqueness of their ecosystem, they have stabilizing functions, including an increasing retention capacity along with flood protection, but they are also vulnerable to anthropogenic pollution. A considerable amount of transported material is deposited at their bottom as the water flow velocity decreases. If the material contains heavy metals and persistent organic compounds, sediments represent a potential threat in remobilization during floods.
The researchers focused on an oxbow lake in the middle part of the Elbe River, about 2 km from Poděbrady, because it was initially an artificially-separated meander of the Elbe River that still communicates intensively with the river. The lake is surrounded by a vast floodplain, so the only source of pollution is represented almost exclusively by river water. For the most detailed analysis of the anthropogenic impact, the authors used research for the 2004–2015 period, including measurements of lake basin morphometry, hydrological regime observations, water quality sampling, determination of chlorophyll-a concentrations, zooplankton species composition evaluation, and sediment quality analyses including contamination assessments.
The total volume of the elongated lake was almost 20 thousand m3 with a maximum depth of 2.3 m, the bottom of the lake was very steep with the lowest depth near the connection with the river, where intensive sedimentation occurs during higher water levels. The hydrological regime corresponded to the river regime because of the permanent connection with the river by the culvert. The highest water levels were recorded in spring and the lowest in late summer and early autumn. Flood events were recorded mainly in connection with snow melting accompanied by rainfall in spring and winter. Summer floods were of shorter duration and generally with a smaller volume of the flood wave.
From the chemical point of view, there was a decrease in pH and in the concentration of dissolved oxygen with depth, while the value of electrical conductivity (approximate rate of ionically dissolved substances in water) increased. At the same time, fluctuations occurred during the year, mainly in connection with the activity of phytoplankton. The water quality corresponded to the influence of the river water, which was mainly a source of nitrates and phosphorus. Compared to cut oxbow lakes with a very restricted hydrological connection, there were lower concentrations of organic matter.
The results of sediment contamination were alarming. Although the contamination was lower than in oxbow lakes near Pardubice, where the Synthesia a.s. chemical plant, the PARAMO refinery and the more distant Spolana a.s. chemical plant in Neratovice are nearby, the limits for cadmium, arsenic, mercury and lead were exceeded. These substances pose a serious risk to the aquatic ecosystem and the flood plain. Due to the same spatial distribution of the increased concentrations of contaminants, the source and time of contamination could be considered to be the same.
A comprehensive analysis of the oxbow lake near Poděbrady contributed to the general knowledge of oxbow lakes in Central Europe and beyond. However, due to the increased levels of nutrients and sediment load in the lake, contaminants represent a major threat to the aquatic and alluvial ecosystem, and it is essential to continue not only in the research but also remediation measures to prevent environmental disaster.
Havlíková, P., Chalupová, D., Chuman, T., Šobr, M., Janský, B. (2019): Long‑term water and sediment quality of the Elbe River’s oxbow lake near the town of Poděbrady, the Czech Republic. Environmental Earth Sciences. 78:559. https://doi.org/10.1007/s12665-019-8566-8.