Monthly cyanobacteria absolute abundance (cell mL⁻¹) of the species which bloomed throughout the study period (vertical lines indicate standard deviation). For the nitrogen fixing cyanobacteria Dolichospermum circinale and Anabaenopsis arnoldii the secondary axis shows heterocyte abundance (het mL⁻¹, dotted line). The horizontal line denotes the threshold of >50 000 cell mL⁻¹ used in the study to consider a cyanobacteria bloom. M = Mono-specific bloom, P = poly-specific bloom. * A. arnoldii surpassed the threshold of 50 000 cell mL⁻¹, yet it contributed with less than 15% of total bloom abundance in September.