HABs and HAB nots: Dynamics of phytoplankton blooms across similar oligotrophic reservoirs

Abstract

Increasing frequency and intensity of cyanobacterial Harmful Algal Blooms (HABs) threaten human and aquatic ecosystem health. Improving our understanding of HABs across a range of systems will be critical to understanding and potentially minimizing risk, especially where HABs are occurring in less productive and less studied waterbodies. Here, the characteristics and annual dynamics of phytoplankton communities were examined, focusing on the timing, magnitude, and predictability of cyanobacterial blooms in five multi-purpose flood-control reservoirs from the Willamette Basin, Oregon, USA. A high similarity in phytoplankton composition and consistency in the timing of cyanobacterial dominance was hypothesized to occur across these oligotrophic reservoirs. However, periods of dominance by potentially HABs producing genera were inconsistent both in their timing and abundances among reservoirs and across years within each reservoir. The lack of regional predictability indicates the importance of local drivers in the formation, intensity, and composition of phytoplankton blooms. These findings have important implications for reservoir management and safeguarding freshwater drinking sources, as not all reservoirs appear to experience cyanobacterial blooms at the same time, demonstrating non-concurrent risks of HABs.