Heavy metal pollution and phytotoxicity of small urban stream sediments

The aim of this study was to assess how the urban stream syndrome appears in a small stream Mārupīte, Riga, Latvia. The stream is characterized by high heterogeneity of sediment composition, physico-chemical parameters and heavy metal concentrations. The concentrations of metals differ between sampling sites at least by order or by two orders of magnitude, and the general trend is an increase in concentrations in the river flow from upstream to downstream. In stretches with higher content of water and organic matter is observed positive correlations with concentrations of all of studied metals Cu, Pb, Fe, Zn, Ni, Cr, Mn, Co and Cd. The concentrations of heavy metals near major streets in the stream lower reaches mostly exceed the good environmental quality limits specified in various regulatory enactments. Toxicity of these metals in the sediments of the stream watercourse is approved by phytotoxicity tests using early-stage of turions of Sprirodela polyrhiza. Negative effect on morphological changes of S. polyrhiza have all of analyzed metals but particularly strong negative relationship was established between the morphological changes of turions and the concentrations of Fe, Ni, Cr, Co and Cd in the first 7 days of turions development. The results of the study confirm that the stream is characterized by urban stream syndrome, both according to the increased concentrations of heavy metals in sediments and the results of the Spirodela phytotoxicity test. The results are important for environmental health assessment and urban planning. The study demonstrates large spatial variability in sediment composition, physico-chemical properties and heavy metals concentration in a small urban stream. The concentrations of heavy metals differ between sampling sites at least by order or by two orders. The general trend is an increase in concentrations near major traffic streets in the river flow from upstream to downstream. The concentrations of heavy metals in sediments mostly exceed the good environmental quality limits specified in various regulatory enactments of different countries. These limits are different and the need to create Sediment Environmental Quality Standards is clearly necessary. The phytotoxicity of heavy metals in sediments is distinctly demonstrated by the changes in morphological abnormalities and developmental disorders of duckweed S. polyrhiza (L.) Schleiden turions.