Factors influencing the success of pelagic cyanobacteria

Abstract: Cyanobacteria are present in most aquatic systems, and pelagic bloom-forming species often flourish in lakes. Since blooms of cyanobacteria are undesirable for several reasons, e.g. they can result in foul odours and tastes as well as toxicity, these organisms have been subject to an extensive amount of research around the globe. A number of hypotheses have been put forward to explain the strong competitive ability of cyanobacteria in phytoplankton communities. In this thesis results are presented from field and laboratory experiments on the influence of inorganic nitrogen and iron nutrition on cyanobacterial population dynamics. The results are discussed in the light of previously published theories.It was hypothesised that cyanobacteria are favoured over eukaryotic phytoplankton when inorganic nitrogen is present in the form of ammonium rather than nitrate, and vice versa. This hypothesis was supported by three field-enclosure experiments performed in the moderately eutrophic Lake Erken in which it was found that cyanobacteria responded positively to additions of ammonium and phosphate, but did not respond to additions of nitrate and phosphate.Laboratory competition experiments with mixed cultures of a cyanobacterium and a green alga showed that the former was favoured by highly frequent, small pulses of inorganic nitrogen either as ammonium or nitrate, whereas the green alga was favoured by larger pulses applied less frequently. Based on these results and the fact that, regardless of the inorganic nitrogen source, it was largely eukaryotic phytoplankton that responded to the nutrient additions in the field-enclosure experiments in Lake Erken, the hypothesis regarding inorganic nitrogen and cyanobacterial success was reformulated. It is suggested that in water bodies in which eukaryotic phytoplankton are disfavoured owing to nitrogen limitation, cyanobacteria are likely to be strong competitors unless limited by other elements.The importance of iron availability as a factor limiting cyanobacterial growth was studied in three field-enclosure experiment in Lake Erken. Despite additions of phosphate and inorganic nitrogen every second day, cyanobacterial biomass decreased. However, in cases where dissolved iron was also supplied, extensive outgrowths of cyanobacteria occurred. This result has implications for the management of wastewater treatment plants in which iron is used to precipitate phosphate.