Predicting when blooms will take off

Posted on July 20, 2015

Despite an abundance of research on cyanobacterial populations in lakes, attempts to manage problematic cyanobacterial blooms continue to be met with mixed success. Although a lot of work has been done on the ways that nutrients, temperature and light affect bloom development, it’s still not very easy to predict when blooms will start. A recent Canadian paper suggests that bloom prediction can be improved by integrating several strands of research into a more holistic framework, and it stresses the key role of ferrous iron (Fe2+) in allowing cyanobacteria to compete with plant plankton. Although ferrous iron is very abundant in oxygen-depleted sediments, from where it diffuses upwards through the water column, it’s rapidly oxidised to ferric iron (Fe3+) in the aerated surface layers. When sulphate levels are high, the iron is tied up in the form of iron sulphide (FeS), and when this occurs cyanobacteria are outcompeted by algal plankton. In contrast, when sulphate is low and ferrous iron gets close enough to the surface to be available to cyanobacteria, the reverse situation holds, and cyanobacteria outcompete plant plankton. If this explanation for the triggering of cyanobacterial blooms is correct, lake managers may achieve better success by modifying their bloom control strategies. For example, they could reconsider the use of iron-based coagulants to sink bloom organisms, if those coagulants increase the amount of reducible iron in sediments. They could also examine the possibility of using artificial aeration systems to increase sediment oxygen and/or disrupt the downward migration of cyanobacteria from the surface waters.

Reference: Molot, L.A. et al. 2014. A novel model for cyanobacteria bloom formation: the critical role of anoxia and ferrous iron. Freshwater Biology 59, 1323–1340.