In situ fluorometry reveals a persistent, perennial hypolimnetic cyanobacterial bloom in a seasonally anoxic reservoir

AbstractCyanobacterial blooms are increasing in waterbodies worldwide because of anthropogenic forcing. Most blooms occur at the water’s surface, but some cyanobacterial taxa, such as Planktothrix, are able to modify their buoyancy to access more favorable growing conditions in deeper waters. Here, we used in situ fluorometry to examine the vertical distribution and biomass of Planktothrix in a seasonally anoxic reservoir for 3 consecutive summers. We also collected depth profiles of photosynthetically active radiation, temperature, and nutrients to evaluate which environmental drivers were most important for predicting Planktothrix biomass. In all 3 summers, Planktothrix dominated the phytoplankton community, exhibiting a large (concentrations ~100 μg/L), persistent (lasting ~100 d) bloom below the thermocline. The bloom consistently exhibited maximum biomass at or below the depth reached by 1% of surface light. Light availability probably was the most important factor driving the vertical distribution o...