Didymosphenia geminata blooms are not exclusively driven by low phosphorus under experimental conditions

Despite two decades of research, the cause of blooms of the diatom Didymosphenia geminata remains uncertain. Blooms have been associated with low nutrient, oligotrophic streams. In this study, we used available data from across the globe and conducted experiments to determine how D. geminata responds to soluble reactive phosphorus (SRP) concentrations. Globally, D. geminata blooms have been found in streams with SRP below 11 mu g P l(-1). In North America, blooms only occurred at high equivalent latitudes when SRP was very low, whereas at lower latitudes blooms were observed under higher SRP concentrations. Using an in situ experiment, we found that following physical removal of D. geminata from stones, regrowth did not occur despite low SRP concentrations. In a second experiment, we found that there were no differences in D. geminata growth between a treatment which depleted SRP and a treatment which maintained elevated springtime SRP levels. These findings indicate that D. geminata blooms do not always form when SRP is low, even when cells are present. Bloom formation that is not exclusively related to low SRP suggests additional chemical or biotic factors, specific physical conditions, a seasonal timing requirement, or some combination of these with low P that are necessary to produce blooms.