Investigating the physicochemical parameters that drive geosmin production in benthic cyanobacterial systems

This study investigated the impact of physicochemical parameters on geosmin production in Hope Valley reservoir during periods of absent pelagic cyanobacteria. Over 42 months, extracellular geosmin was consistently detected, with concentrations ranging from less than 4 ng/L to 37 ng/L during benthic cyanobacterial periods and increasing to 128 ng/L when pelagic cyanobacteria were active. Notably, four significant geosmin peaks occurred, predominantly between the October and December periods. A 20-year historical analysis corroborated these findings, revealing instances of geosmin in the absence of pelagic blooms. Factors such as phycocyanin, fluorescent dissolved organic matter, true colour, and alkalinity were identified as having a weak linear relationship with geosmin concentration, aligning with a principal component analysis. The evidence of sporadic detection of benthic cyanobacteria in the water column suggest that benthic cyanobacteria are likely to be the source of the geosmin. This infers that the physicochemical variables, while influential, are only part of a more complex ecological narrative. The total phosphorus concentration was ideal for benthic cyanobacterial growth (<0.02 mg/L) as higher total phosphorus concentration resulted in pelagic cyanobacterial blooms. The findings suggest that benthic and pelagic cyanobacteria will remain in competition for dominance each spring and summer. Consequently, further research into the shifts in benthic cyanobacterial diversity is needed to understand the geosmin production mechanisms in this environment.