Persistence of Cyanobacterial Blooms Under Low-power Ultrasonication in a Deep, Mesotrophic, Stratifying Lake

Cyanobacteria are a natural component of aquatic ecosystems. However, when proliferating excessively, they can degrade water quality and restrict recreational use by forming harmful algal blooms (HABs). Consequently, there is growing interest in developing rapid HAB management technologies. Among these, ultrasonication has gained attention as a potential control method, with laboratory studies suggesting cyanobacterial bloom prevention. Despite this promise, significant uncertainties remain regarding its effectiveness in natural aquatic systems, and its large-scale application is debated due to potential risks to non-target organisms. To address these knowledge gaps, we investigated the effects of ultrasonication on plankton communities in deep, stratifying Lake Ahvenlampi, Finland, during a 3-month ultrasonication pilot. We found that the applied low-frequency (24–57 kHz), low-intensity (3.76 × 10−⁵ W cm−²), and low-power (17 W) ultrasonication neither prevented the development of cyanobacterial blooms, caused no visible morphological changes in planktonic organisms nor affected phyto- or zooplankton communities. Instead, their seasonal succession closely resembled that observed in nearby Lake Hiidenlampi, which served as a control site. Nevertheless, we emphasize that stronger ultrasonic treatments – particularly those involving higher intensities and power – may produce substantial impacts on non-target plankton communities. Such effects could propagate through higher trophic levels, potentially impairing overall ecosystem functioning.