Traditionally, it was believed that phosphorus was the sole factor impairing Lake Okeechobee, leading to a focus on minimizing agricultural runoff. Recent, in-depth sampling efforts throughout the Lake Okeechobee Waterway and its adjoining estuaries have revealed that rising nitrogen levels from human waste and urban runoff are also significant contributors to toxic algal blooms. The increasing nitrogen content, particularly during heavy rainfall, greatly exacerbates the intensity of these blooms. This information highlights the necessity for a comprehensive approach to nutrient management along with improved wastewater treatment to safeguard the lake and its surrounding ecosystems.
Lake Okeechobee stands as Florida’s largest lake and the second largest in the Southeastern United States. Over the last twenty years, blue-green algae (Microcystis) blooms have proliferated in the lake and have been swept into nearby urban estuaries, posing serious environmental and public health challenges.
Nutrient overloads from agriculture, industries, and urbanization—especially nitrogen and phosphorus—are recognized global causes of harmful algal blooms. Historically, concerns for Lake Okeechobee have primarily focused on phosphorus, prompting targeted measures to reduce agricultural phosphorus runoff in the watershed.
Recent research marks a significant advancement in understanding and safeguarding this vital ecosystem. Experts from Florida Atlantic University’s Harbor Branch Oceanographic Institute have carried out the initial thorough sampling across the Lake Okeechobee Waterway, which stretches from the St. Lucie Estuary on the east to the Caloosahatchee River Estuary on the west.
To investigate the causes of recent cyanobacterial blooms, researchers undertook two research cruises along the Lake Okeechobee Waterway and conducted three sampling activities targeting these blooms. They analyzed nitrogen isotopes in phytoplankton to assess the roles of human waste and fertilizers.
The study’s findings, published in the journal Harmful Algae, indicate that tackling harmful algal blooms in Lake Okeechobee demands the management of both phosphorus and nitrogen, with human waste also contributing to the swings in Microcystis populations. The research additionally emphasizes the significant impact of rainfall and extreme precipitation on the occurrence of large blooms.
Inorganic nitrogen levels have surged in urban estuaries and the Kissimmee River—flowing from the Kissimmee Chain of Lakes to the greater Orlando region—before entering Lake Okeechobee. Furthermore, rising urbanization in Orlando has been recognized as a contributing factor to the increasing frequency of these blooms in the lake.
“Based on our study results, strategies focused solely on phosphorus reduction are inadequate,” said Brian Lapointe, Ph.D., the lead author and research professor at FAU Harbor Branch. “To combat toxic cyanobacterial blooms, we must reduce both nitrogen and phosphorus. Managing these two nutrients is essential due to their combined influence, which often results in more severe and persistent harmful algal blooms. In urban areas such as the St. Lucie and Caloosahatchee River watersheds, human waste is a significant nutrient source.”
The findings reveal notable changes in Lake Okeechobee’s algal conditions over the years. The nitrogen-to-phosphorus ratios, microcystin levels, and Microcystis cell counts in the lake are now higher than seen in past decades. Phosphorus levels rose from 50 micrograms per liter in the mid-1970s to above 100 micrograms per liter by the late 1990s, facilitating the dominance of harmful cyanobacteria like Dolichospermum over other algae.
Sampling was conducted between 2018 and 2021, with cruises taking place during 2019 and 2020. This methodology provided a comprehensive overview of conditions across the entire waterway during a specific timeframe, rather than relying on intermittent or isolated sampling.
After the hurricanes of 2004-05, which caused extensive losses in aquatic flora and fauna, toxic Microcystis became more prevalent in the lake throughout the warm, wet months from May to October. Major blooms returned in 2013, 2016, and 2018. As the research cruises were conducted in 2019 and 2020—relatively dry years—no significant Microcystis blooms were detected in the St. Lucie or Caloosahatchee estuaries.
“It is during heavy rainfall events that significant nitrogen loading happens, which we recognized occurred in 2013, 2016, and 2018,” remarked Lapointe. “Large blooms emerged in the lake and its estuaries following these extreme rain incidents.”
The research also highlighted that higher nutrient levels in the urbanized Caloosahatchee River Estuary and St. Lucie Estuary intensified bloom formations in those areas. The nutrients—ammonium, nitrate, and phosphate—are highly reactive, resulting in the thick “scum” observed on the water surface in these estuaries.
“When water containing Microcystis from Lake Okeechobee is released into the estuaries, the growth response is akin to ‘Miracle Grow,'” Lapointe explained. “We detected the highest concentrations of ammonium, nitrate, and phosphate in these estuaries. These nutrients are typically linked to human waste, as evidenced by the elevated nitrogen isotope levels, confirming human waste as a notable nitrogen source.”
Significant variations in water quality across different locations were identified, influencing the spread and intensity of algal blooms. The most substantial blooms were recorded at Pahokee Marina and Cape Coral, where human waste had a pronounced impact.
“Evidence from the Florida Keys exhibits similar nitrogen enrichment trends, and our research hints that these changes could be occurring throughout the entire watershed,” stated Lapointe.
Previously, water in the Lake Okeechobee watershed flowed naturally southward through the Everglades. However, adjustments to the water management system resulted in the creation of the Lake Okeechobee Waterway, which periodically redirects lake water. This water is channeled eastward into the St. Lucie Estuary via the C-44 canal and westward into the Caloosahatchee River and Estuary through the C-43 canal.
“Microcystis blooms and their toxicity in Lake Okeechobee and the St. Lucie Estuary are influenced by nitrogen levels, and our study indicates that nitrogen enrichment is on the rise. Minimizing nitrogen levels is vital for controlling harmful algal blooms along the lake’s waterways and in the downstream estuaries,” said Rachel Brewton, Ph.D., co-author and assistant research professor at FAU Harbor Branch. “Considering the high nitrogen load from the Upper Kissimmee River, reducing these contributions should be prioritized. Although Lake Okeechobee currently lacks a Total Maximum Daily Load established for nitrogen, as it does for phosphorus, the St. Lucie and Caloosahatchee estuaries do have one, and recent strategies for the lake’s tributaries aim to target nitrogen reductions.”
The impact of blooms varies according to the watershed sizes and the specific characteristics of the receiving waters, including their hydrology and chemistry.
“In the lab, we assessed chlorophyll a, microcystins, and both Microcystis and other phytoplankton using cell concentrations and pigment analysis in water samples,” noted Malcolm McFarland, Ph.D., co-author and associate research professor at FAU Harbor Branch. “We employed flow cytometry on water samples aboard the research vessels to analyze live algal cells and prevent losses from preservation and storage methods.”
Large lakes, rivers, estuaries, and coastal regions are increasingly subject to these blooms, raising concerns about the potential impact of more frequent harmful algal blooms and their toxins on drinking water quality, recreational activities, tourism, and fisheries in the future.
“These blooms not only affect Florida’s essential water resources but also pose national concerns regarding public health, environmental quality, and economic stability,” emphasized Lapointe. “Understanding these developments is crucial for tackling the challenges involved in protecting our water resources to ensure clean and safe water for communities nationwide.”
This research underlines the need for enhanced wastewater treatment processes and improved water storage solutions to the north of Lake Okeechobee, including the Lake Okeechobee Watershed Restoration Project and the Lake Okeechobee Component A Reservoir project spearheaded by the South Florida Water Management District.
Nicole Stockley, a former research engineer at FAU Harbor Branch, is a co-author of this study.
This research received funding from the U.S. National Aeronautics and Space Administration (NASA) Water Resources Program (80NSSC19K1200). Further support was provided through the Florida Center for Coastal and Human Health, which was developed and funded by the Harbor Branch Oceanographic Institute Foundation.
