Vertical migration by filamentous cyanobacteria (Aphanizomenon spp.) and the implications for algal blooms in Lake 227, Experimental Lakes Area, Ontario, Canada

Background/Question/Methods

Large cyanobacteria are superior phosphorus (P) competitors, so they are expected to dominate low-nutrient (oligotrophic) freshwater systems, yet they usually only dominate under high-nutrient (eutrophic) conditions, indicating that something other than P limits their growth in oligotrophic waters. Mounting evidence points to limitation by ferrous iron (Fe⁺²), which is normally found in very low concentrations in the euphotic epilimnion but much higher concentrations in the anoxic hypolimnia of eutrophic waters because of diffusion from anoxic sediments (i.e., internal loading). Ferrous iron was measured as the difference between total reactive iron, TRFe, in the epilimnion and hypolimnion. To determine whether filamentous cyanobacteria in Lake 227 are able to migrate between the low Fe⁺² euphotic zone and the high Fe⁺² anoxic hypolimnion, funnel traps were set for 1 to 2 days at multiple depths on four different dates in the summer of 2011. Microprofiles of algal fluorescence, and extracted chlorophyll were also conducted on 3 dates to determine whether vertical migration of cyanobacteria occurs on shorter timescales.

Results/Conclusions

Preliminary results suggest that the dominant filamentous cyanobacteria during blooms in Lake 227 (Aphanizomenon schindleri and A. gracile) are able to migrate between the epilimnion and the aphotic, anoxic, high-ferrous hypolimnion. Algae traps captured upward-moving filaments at all depths tested, with the highest migrating concentrations at 6.5 m, which is 2 m below the bottom of the metalimnion, and where the concentration of TRFe was greater than 2500 ug/L. There was high variation between trap replicates, indicating that migration may be patchy in space and time. Lake profiles show that the depth at which the highest concentration of algal biomass occurs fluctuates throughout the day. This vertical movement by filamentous cyanobacteria may be the key to explaining how they acquire sufficient ferrous iron to outcompete other phytoplankton taxa in eutrophic waters, allowing them to form blooms.