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Understanding Cyanobacteria Dynamics in Lake Okeechobee

Lake Okeechobee, the largest freshwater lake in Florida, is essential for the state’s ecological health and water management strategies. Covering an area of 730 square miles and averaging a depth of just 9 feet, this lake is a significant reservoir for agriculture and plays a critical role in flood control. It also acts as a vital link to the Everglades through a network of canals, making it a favored spot for recreational activities such as fishing, boating, and birdwatching.

Unfortunately, the lake is increasingly troubled by harmful cyanobacteria blooms, predominantly from the toxin-producing Microcystis aeruginosa. These small, algae-like organisms thrive in warm, nutrient-rich environments and can generate toxic algal blooms. Notably, Microcystis species exhibit a phenomenon called diel vertical migration, where they shift up and down in the water column daily to optimize exposure to light and nutrients, enhancing their resilience in murky waters like those found in Lake Okeechobee.

Despite the extensive research on diel vertical migration, its influence on bloom development and water quality remains inadequately understood. Gaining insight into this phenomenon is crucial for managing the environmental risks associated with harmful algal blooms.

To delve deeper into these dynamics, researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute and the University of South Florida College of Marine Science utilized an innovative physical-biogeochemical model that integrates water movement with biological activity. This study focused on the summer behavioral patterns of cyanobacteria in Lake Okeechobee, marking a significant advancement as previous models had not specifically examined the daily migration of these organisms.

The research team analyzed how the daily vertical movements of cyanobacteria interact with lake currents and vertical mixing—processes that facilitate the exchange of nutrients and oxygen between upper and deeper layers of water. Various factors, including wind and temperature variations, contribute to this mixing process.

The findings from this new model, published in March in the journal Ecological Modelling, demonstrated that cyanobacteria in the central region of Lake Okeechobee rise to the surface in the morning for increased sunlight exposure, which enhances their growth and elevates their population density. Concurrently, southern and southeastern winds tend to push these surface-dwelling organisms towards the lake’s northern and northwestern shorelines.

As evening approaches, a drop in temperature and wind-driven water mixing facilitates a more even distribution of these cells throughout the water column. Although the wind does impact the spatial distribution of algae blooms, factors such as vertical migration, surface growth, and the mixing of water layers play a more substantial role overall.

Consequently, the concentration of cyanobacteria is observed to peak during the late morning to midday, quickly tapering off in the afternoon. The seasonal patterns of algal blooms appear to be influenced predominantly by shifts in temperature and wind conditions, where warmer temperatures tend to promote bloom strength, while more vigorous winds can diminish them. Such insights into bloom behavior are vital for enhancing monitoring and management strategies against harmful algal blooms.

Mingshun Jiang, Ph.D., the senior author and an associate research professor at FAU Harbor Branch, noted, “Our study illustrates that the daily fluctuations of cyanobacteria, influenced by vertical mixing and migration along with their rapid surface growth, are primary factors driving bloom development in Lake Okeechobee’s central basin. Elevated temperatures and calm winds encourage algal proliferation, while strong winds can submerge the cells, limiting their light access. Although horizontal movements are significant, it is the vertical dynamics that predominantly establish the daily bloom conditions. This understanding will improve our predictive capabilities regarding the intensity and location of algal blooms.”

To validate their findings, the research team employed multiple data-gathering methods. They collected water samples from various depths and locations, monitored cyanobacteria levels with sensors throughout the day, and utilized satellite imagery taken at intervals. These comprehensive observations corroborated the daily migration patterns the model predicted.

“Our model results aligned well with field data, including measurements of cyanobacteria biovolume and radiometric readings in the lake,” Jiang reported. “Both the model and satellite imagery identified two main bloom areas around midday: a broad bloom in the central basin and narrow, intense bands along the northern and northwestern shores. Temperature and wind emerged as critical factors governing the formation and intensification of these blooms.”

While cyanobacteria have been extensively studied within the realm of phytoplankton, this research offers fresh perspectives on the interplay between physical and biological processes that influence bloom dynamics in Lake Okeechobee.

“Future research is crucial to unraveling additional biological factors, including colony formation and the lifecycle of Microcystis,” Jiang emphasized. “More field data is needed to validate vertical migration trends and refine aspects like migration speed, timing, and colony characteristics.”

Lake Okeechobee’s watershed captures inflows from the Kissimmee River and adjacent areas, with water then discharging through various outlets, including the Everglades to the south. In seasons of excess rainfall, especially during the wet months, the lake also releases water into the St. Lucie and Caloosahatchee Rivers. Therefore, blooms occurring in the lake can significantly impact water quality and phytoplankton dynamics in these estuarine systems. This interconnected hydrological network positions Lake Okeechobee as a crucial element in the regional water cycle.

The study’s co-authors include Ashley Brereton, a former postdoctoral researcher at FAU Harbor Branch; Jennifer Cannizzaro, a scientific researcher at the University of South Florida; Malcolm McFarland, Ph.D.; Zackary Wistort, Ph.D.; Brian Lapointe, Ph.D.; Jordan S. Beckler, Ph.D.; Timothy Moore, Ph.D.; Rachel Brewton, Ph.D.; and Chuanmin Hu, Ph.D., a professor of optical oceanography at the University of South Florida.

The research received funding from a Florida Department of Environmental Protection Technology Innovative grant awarded to Beckler, Jiang, Moore, and McFarland, alongside a NASA Water Resources Program grant awarded to Hu, Jiang, Lapointe, and McFarland, with additional support from an EPA South Florida Program award to Jiang and Beckler.

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