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

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.