Zn behavior and isotope fractionation in high altitude Andean lakes

Understanding the biogeochemical pathways of trace metals in wetland ecosystems enables the development of remediation strategies. In the Bolivian Andean Altiplano, Lake Titicaca is weakly affected by metal pollution compared to Lake Uru Uru, impacted by acid mine drainage and urban activities. Totora plants (Schoenoplectus californicus subsp. tatora) occupy extensive shallow areas of the lake shores. The behavior and isotope fractionation of zinc (Zn) in these aquatic ecosystems with contrasting levels of contamination were studied. Lake water, surface sediment, totora plant, and associated periphyton were characterized for Zn concentrations and stable isotope compositions (δ⁶⁶Zn). Larger variations in δ⁶⁶Zn were observed in filtered lake water (0-0.7‰) than in sediments (0.3-0.4‰) due to kinetic or equilibrium effects during Zn sorption on suspended particles. Zn tolerance of totora plants was achieved by exclusion, i.e., reduced sediment-to-shoot transfer (0.01 at high Zn level and 0.02 to 0.08 at low-medium Zn level). This reduced transfer was accompanied by greater sediment-to-shoot enrichment in light isotopes (-0.55‰) at high Zn compared to low-medium Zn (0.26 to 0.02‰), consistent with low-affinity uptake pathway and strong binding in the root apoplasm at high Zn. The periphyton represented a minor Zn storage compartment, and was enriched in heavy Zn isotopes compared to dissolved Zn (Δ⁶⁶Zn_{periphyton-dissolved} = 0.12 to 0.26‰). EDTA extractions showed that sorption onto inorganic particles and the cell wall was the main process of Zn fixation by the periphyton. High-resolution analysis of a sediment core and acid-volatile sulfides measurements of bulk sediments suggested Zn immobilization in the sediment and rhizosphere by co-precipitation with weakly crystallized (oxyhydr)oxides (presumably from root plaque) and in recalcitrant sulfide phases or minerals, depending on the sites. This study suggests that totoras, rather than periphyton, are key players in Zn fixation in the wetland through in planta and rhizospheric processes.