Recruitment from resting stages among bloom-forming cyanobacteria

Cyanobacteria are known to form heavy blooms in eutrophicated freshwaters and also form resting stages, associated with the sediments, during harsh growth conditions. These resting stages can serve as inoculum for pelagic populations and function as a potential source for bloom formations. However, the factors involved in initiating recruitment from resting stage to the pelagic phase are not fully understood. In my thesis, I have been studying several potential triggers. I found that elevated temperatures and increased light availability were important. In addition, sediment mixing, e.g. from bioturbating animals, also played a significant role in increasing recruitment rates of cyanobacteria from the sediments. Hence, the littoral-zone, where these factors have substantial impact, should be the major recruitment site, which was also confirmed in my studies. Cyanobacteria has a low internal N:P ratio and in eutrophic freshwaters, cyanobacterial dominance is often correlated with low N:P ratios. Accordingly, I found that recruitment rates of cyanobacterial resting stages were increased at declining N:P ratios. A plausible explanation to my result is that a declining N:P ratio signals improving nutrient conditions for cyanobacteria and is therefore a suitable trigger for recruitment to the water column. This implies that a declining N:P ratio could be an important mechanism behind, and an early warning of cyanobacterial blooms.