Evaluation of mixing mechanisms in the control of cyanobacterial blooms

As water temperatures rise and nutrient runoff continues unabated, the frequency, duration, and severity of Harmful Algal Blooms (HABs) are rising globally, threatening human and ecosystem health. The majority of HABs research has focused on their detection and far less is known about the effective control of blooms in situ. Of the few control strategies available for short term management, artificial mixing has the fewest unintended environmental impacts and represents the most promising solution in many waterways. However, the lack of detailed understanding on how hydraulics and cyanobacterial growth interact, and how those interactions are impacted by artificial mixing, has resulted in regular examples of mixing systems that fail to control HABs. Detailed understanding on how destratification and turbulence features influence cyanobacterial growth is needed, as is engineering guidance on how to produce the hydraulic conditions that make cyanobacteria less competitive. This project integrated field observations and a coupled 2D hydrodynamic, water quality, and algal growth models of the Ross Island Lagoon (RIL), Willamette River, Oregon (USA) to understand how wind, tide, aeration, and mechanical mixing impacted cyanobacterial and diatom growth. Wind, tide, and aeration mixing impacted various turbulent mixing depths for suppressing the diurnal vertical migration of cyanobacteria relative to the depth of the photic zone. Mechanical pumping suppressed stratification and decreased surface temperatures by bringing up cold hypolimnetic water. For the range of conditions examined, results indicate that natural (wind, tide) and aeration mixing alone were not enough to suppress the bloom at RIL. Tidal and wind mixing did not have enough energy to mix more than the surface of the lagoon, whereas the depths and extents of aeration needed suppress blooms (via achieving the vertical velocities needed to overcome cyanobacterial migration across the lagoon) were practically infeasible. Instead, aggressive mechanical mixing (~3000 gpm @ 30m of head) was needed to reduce productivity of all phytoplankton via colder surface temperatures. The results provide mechanistic insight on why aeration mixing has been ineffective at multiple sites globally, the relative importance of different hydraulic mechanisms in suppression of HABs, and the scale of engineered mixing needed to effectively control HABs in reservoirs and rivers.