Trace metal extraction and biomass production by spontaneous vegetation in temporary Mediterranean stormwater highway retention ponds: Freshwater macroalgae (Chara spp.) vs. cattails (Typha spp.)

A field study on the capacity of spontaneous vegetation of four motorway ponds to extract four trace metals (Cd, Cu, Zn and Pb) was carried out. High biomasses (ca. 400 g m−2 DW) of freshwater macroalgae Chara spp. were found in all ponds. However, higher biomasses (ca. 900 and 1600 g m−2 DW for aerial and root parts, respectively) were found for broadleaf cattails Typha spp. with perennial structures. Biomasses of Chara were mainly controlled by seasonal variations and water level of the ponds. Metal contents in Chara vulgaris beds were higher than those in Chara globularis and, with less than half of the biomass corresponded to the same range than those found in Typha. For the latter, the highest contents of metals were found in the root system. Sediments of the ponds were moderately contaminated and the concentrations of the four metals were very low in the water (often under detection level). Translocation factors of the contaminants from sediment to plants were very low (‰) for all species and phytoextraction of metals from the sediment seemed not to be the main effect of the spontaneous vegetation. However, this latter prevent pollutant transfers provided that the dry and crumbly Chara are collected during summer before being blown outside the pond by strong winds.