Grant: Alliance Grants

Risks from algal blooms to health and economy remain widespread even after decades of work to improve water quality. Public awareness and more frequent sediment oxygen depletion (anoxia) may be why reports of cyanobacterial harmful algal blooms (cHABs) are increasing even in low-nutrient and northern lakes where cHAB risk used to be low. Nutrient management targets for phosphorus (P) based on past lake responses no longer offer sufficient protection against cHABs. This is likely because climate change increases the risk of anoxia in sediments, which alters iron (Fe) and nitrogen (N) loads from sediments. Cyanobacteria have much higher Fe requirements than algae and are almost entirely restricted to using Fe(II) (unlike eukaryotic algae). Hence access to Fe(II) released from anoxic sediments provides cyanobacteria with an essential micronutrient. Fe(II) release can lead to cHABs onset if other pre-conditions are met (e.g., nutrients, temperatures, low wind, light). This concept provides an integrative framework explaining how and why cyanobacteria can dominate in small inland and large transboundary lakes, soft and hard water lakes, unproductive and productive lakes, tropical to north temperate lakes, those with and without dreissenid mussels, freshwater and saline lakes, lakes limited by P and those limited by N. Importantly, it tells us what conditions are necessary to prevent bloom formation, cyanobacterial dominance, and thus can lead to major improvements in management tools. Our whole-lake experiments are a 'proof of concept' test of the hypothesis that low sediment redox leads to Fe(II) release from sediments that leads to cHABs. Low sediment redox is caused by depletion of dissolved oxygen and nitrate, which are both oxidizing agents and prevent Fe(II) release. This would lead to nutrient objectives that target sediment redox in order to prevent cHABs.