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Lake Okeechobee, Florida's largest freshwater lake, plays a vital role in the state's ecosystem and water management. Spanning 730 square miles with an average depth of just 9 feet, it serves as a crucial water source for agriculture and flood control. Connected to the Everglades through canals, it's also a popular destination for fishing, boating and birdwatching.

However, the lake faces increasing harmful cyanobacteria blooms, particularly from the toxin-producing species Microcystis aeruginosa. These microscopic algae thrive in warm, nutrient-rich waters and can form harmful algal blooms. Known for their diel (daily) vertical migration, cyanobacteria move up and down the water column daily to access light and nutrients, making them more resilient in turbid waters like Lake Okeechobee.

While diel vertical migration is well-documented, its impact on bloom development and water quality remains unclear. Understanding this movement is crucial for managing the risks associated with harmful algal blooms.

Using a new physical-biogeochemical model that combines water movement and biological activity, researchers from Florida Atlantic University's Harbor Branch Oceanographic Institute, in collaboration with the University of South Florida College of Marine Science, studied the daily vertical movement of cyanobacteria in Lake Okeechobee during the summer. While previous models of the lake have explored both physical and biological processes, none have specifically addressed this daily vertical migration of cyanobacteria.

For the study, the team focused on how the daily movement of cyanobacteria interacts with lake currents and vertical mixing—processes that cause water to move up and down in lakes and oceans. This mixing is crucial for distributing nutrients and oxygen between the surface and deeper waters, influencing where plants and organisms, like cyanobacteria, can thrive. Factors such as wind and temperature differences drive this mixing.

Results from this new model, published in March in the journal , and in April, , reveal how cyanobacteria in the center of Lake Okeechobee move toward the surface in the morning to access more sunlight, which boosts their growth and increases their densities. At the same time, winds—mainly from the south or southeast—push these surface-dwelling cells toward the northern and northwestern parts of the lake.

As night falls, cooler temperatures and wind-driven mixing stir the water, redistributing the cells more evenly throughout the water column. While wind influences the movement of blooms around the lake, the daily vertical migration, surface growth, and vertical mixing have a more significant overall impact.

As a result, surface concentrations of cyanobacteria tend to peak late in the morning to midday, then quickly decline in the afternoon. However, wind-driven movement occurs almost daily during late spring and summer, often resulting in visible algae streaks—narrow bands less than 2 kilometers wide—along the northern shores.

The study also found that the seasonal pattern of blooms is largely driven by temperature and wind changes, where higher temperatures lead to stronger blooms and stronger winds tend to reduce blooms. This improved understanding of bloom behavior could enhance monitoring, forecasting and management of these harmful events.

"Our study shows that the daily rise and fall of cyanobacteria, driven by vertical mixing and migration, along with their rapid growth near the surface, are the main forces behind bloom formation in Lake Okeechobee's central basin," said Mingshun Jiang, Ph.D., senior author and an associate research professor, FAU Harbor Branch.

"High temperature and calm winds favor algae growth, but strong winds can mix the cells below the surface, limiting their access to light. While horizontal movement does play a role over time, it's the vertical processes that set the stage each day. Understanding this helps us better predict when and where blooms will intensify."

To validate the cyanobacteria's vertical movement, researchers gathered data through multiple methods. They collected water samples from both the surface and bottom at various locations, used a sensor to monitor cyanobacteria levels throughout the day in the central part of the lake, and analyzed satellite images taken several hours apart. These combined observations confirmed the daily migration patterns of cyanobacteria.

"Our model results were in good agreement with field data, including cyanobacteria biovolume and radiometer measurements taken in the lake," said Jiang.

"Both the model and satellite imagery revealed two primary bloom zones around midday: a widespread bloom across the central basin and narrow intense bands along the northern and northwestern shores. Temperature and wind were found to be key drivers of when and where blooms form and intensify."

Although cyanobacteria are one of the most studied groups of phytoplankton, this modeling effort offers new insights into how physical and biological processes interact to shape blooms in Lake Okeechobee.

"Further research is necessary to better understand key biological factors, such as colony formation and senescence of Microcystis, a key cyanobacteria species," said Jiang. "Additional field data will be essential to confirm vertical migration patterns and to refine details such as migration speed, timing and colony characteristics."

Lake Okeechobee's watershed receives inflows from the Kissimmee River and surrounding areas. Water then flows out through various discharge points, including the Everglades to the south. During periods of excess water, particularly during the wet season, the lake also discharges water into the St. Lucie and Caloosahatchee Rivers.

Therefore, blooms in the lake may significantly affect water quality and phytoplankton blooms in these estuaries. This complex water system makes Lake Okeechobee a key component of the region's hydrological balance.