Kai Liu

I am studying the role of Fe as a controlling factor of cyanobacteria dominance in harmful algal blooms. Natural abundance Fe isotopes are useful tracers for biogeochemical processes involving Fe. Within boreal shield catchments, we found the range of iron isotopes spanned more than half of the entire range of Earth surface samples. We studied two lakes: eutrophic Lake 227 with experimental fertilization and oligotrophic Lake 442. Within the lakes, two layers with distinct isotope compositions of dissolved and particulate Fe were observed in the water columns. In the oxic surface layer, dissolved δ⁵⁶Fe is more positive than particulates, consistent with dominance by Fe-DOC complexes. In the anoxic lower layer, upward flux from sediments dominates the dissolved Fe pool and redox processes produces isotope fractionation between dissolved and particulate Fe. The observed natural large fractionation of Fe isotopes provides a potential tool to probe acquisition of Fe by cyanobacteria and algae. Furthermore, δ⁵⁶Fe of bulk sediments in L227 clearly records the 50-year experimental fertilization regimes and currently reflects values in POM in the epilimnion, suggesting sediments could record the onset of changes in Fe cycling due to nutrient addition.