Ironing out the question: what is limiting cyanobacteria in freshwater lakes in the Prairie Pothole Region?

Abstract

The Canadian Prairie Pothole Region is a notable hotspot for cyanobacteria-dominated lakes. This study found minor variations in cyanobacterial genera across these lakes yet observed significant differences in standing biomass, as the lakes ranged from oligotrophic to hypereutrophic classifications. A correlational analysis of nutrients, specifically total phosphorus (TP) and total nitrogen (TN) revealed that the limiting nutrients varied considerably across the region. Of the lakes studied, cyanobacterial biomass was P-limited in 21 lakes, N-limited in 3 lakes, and co-limited by both P and N in 23 lakes. Surprisingly, in 32 lakes, the biomass was limited by neither P nor N. In these lakes, iron (Fe) emerged as the most likely limiting nutrient, given a relatively narrow range of free ferric Fe (pFe) between 18 and 26. Cyanobacteria can create biomass under Fe stress by producing Fe-scavenging siderophores that target pFe. However, in neither P- nor N-limited lakes, there was a lack of correlation between siderophore concentrations and cyanobacterial biomass (r = 0.05), indicating that the siderophores were unable to scavenge Fe and thereby utilize the available P and N to produce further cyanobacterial biomass. Our findings suggest that these Fe-starved eutrophic lakes exhibited a paradox of slow-growing yet high cyanobacterial biomass, challenging the notion that only oligotrophic lakes embody slow-growing metabolisms. Overall, our study highlights the importance of considering nutrient limitations on cyanobacterial growth and incorporating macro- (P and N) and micro- (Fe) nutrient limitation considerations into existing nutrient management strategies to mitigate cyanobacterial dominance effectively.