Control of cyanobacterial blooms with iron addition can favor stress-tolerant toxic species

•Fe is used to mitigate eutrophication but is a nutrient for cyanobacterial growth.•FeCl3 tested in enclosures controlled a bloom dominated by P. agardhii.•Invasive R. raciborskii started recovering and tolerated high Fe levels and low P.•R. raciborskii grew in a wide range of Fe when N2 is not the only source of N.•Nitrogen control is needed to restrain R. raciborskii when restoring lakes with Fe. The control of internal phosphorus (P) load by in-lake measures has been the subject of decades of research. Although iron (Fe) is effective in precipitating P, it has been less tested due to its redox sensitivity. The effectiveness of Fe in controlling P availability and sinking cyanobacterial blooms contrasts to its function as a nutrient for phytoplankton growth. Both roles of Fe were tested in enclosures placed for 36 days in a shallow lake with a perennial cyanobacterial bloom, and in laboratory experiments with Fe-deficient Raphidiopsis raciborskii. Based on total P (TP) of lake water, we applied two doses of FeCl3, corresponding to 30:1 (16 mg Fe L-1) and 90:1 (47 mg Fe L-1) (Fe:P molar), to cause P precipitation, and flocculation and sinking of cyanobacterial populations. Three enclosures per treatment and three without FeCl3 additions (control) were used. The 90:1 treatment sank the main cyanobacterial biomass (50-fold) dominated by Planktothrix agardhii, with a significant decrease in turbidity, chlorophyll a and TP, without lasting decrease in pH, and achieved mesotrophic-like conditions. However, signs of recovery of R. raciborskii were detected between days 3 and 14. In laboratory experiments, Fe-deficient R. raciborskii MVCC19 grown under nitrate availability (+N) and N2-fixation (–N) were exposed to five FeCl3 concentrations from 0 to 17.9 (10:1) mg Fe L-1 (Fe:P molar). A remarkable tolerance to high Fe was found at concentrations 7-fold higher than culture medium and decreased under –N. Also R. raciborskii stood low Fe levels in +N and exhibited higher Fe requirements under N2-fixation. The increase in trichome length suggests resistance to the stressor, with shorter trichomes in –N. Therefore, effective management of R. raciborskii requires additional control of N in lakes. Our results point out the double role of Fe applications in which stress-tolerant species may become dominant under the reoligotrophication scenario. [Display omitted]