A pioneering study led by Mote Marine Laboratory, in collaboration with the Florida Fish and Wildlife Conservation Commission-Fish and Wildlife Research Institute (FWC-FWRI) and the U.S. Geological Survey (USGS), has uncovered a potential critical link between harmful algal blooms (HABs) and acidification in Florida’s estuaries.
The study reveals that distinct acidification events occurred following red tide blooms, and the growth of Karenia brevis (commonly referred to as Florida red tide) may contribute to ocean acidification and significant changes in water chemistry. This finding underscores the need for continuous monitoring to better understand and manage the interaction between HABs and acidification in coastal ecosystems.
Red tides occur when certain algae grow rapidly and overwhelm their environment, sometimes releasing toxins that harm humans, fish, shellfish, marine animals, and birds. These events can disrupt marine ecosystems, degrade water quality, and negatively impact local communities.
Ocean acidification happens when atmospheric carbon dioxide (CO2) is absorbed into the ocean, forming carbonic acid and increasing acidity. Coastal acidification results from a mix of this process and local influences such as nutrient runoff, freshwater inflow and algal respiration.
Masses of nutrients—like nitrogen (e.g., ammonia, nitrate, nitrite, and amino acids) and phosphorus (e.g., phosphate)—entering the water can worsen acidification. These nutrients promote the growth of CO2-absorbing algae, which release CO2 back into the water as they decompose, lowering pH levels.
This cycle exacerbates acidification, which harms coral reefs and hinders the growth of calcifying organisms’ shells and skeletons. Organisms with calcium carbonate skeletons or shells include corals, oysters, clams and mussels. They are essential to maintaining the marine ecosystem and food chain.
By analyzing the growth and decomposition of algal cell communities during blooms, the study revealed that biological processes play a significant role in altering water chemistry. These processes can sometimes intensify water acidity, a phenomenon closely tied to harmful algal blooms.
“This study highlights the importance of understanding how elevated CO2 affects red tide growth in natural ecosystems,” said Dr. Emily Hall, Senior Scientist and Manager of Mote’s Ocean Acidification Research Program. “By doing so, we can better anticipate and mitigate the impacts of harmful algal blooms on coastal communities.”
Additional Key Findings:
The study, conducted from 2020 to 2023, analyzed three Florida estuaries: Tampa Bay, Charlotte Harbor, and the Caloosahatchee River. Researchers recorded water chemistry data (nutrient and carbonate chemistry concentrations), and K. brevis cell densities before, during, and after red tide blooms.
- In Tampa Bay, where pH levels have improved in recent decades, researchers observed stable nutrient levels during the study. However, harmful algal blooms and tropical storms pose ongoing challenges.
- Charlotte Harbor, a vital seagrass habitat, faces frequent red tide blooms and acidification events linked to elevated nutrient levels and freshwater inflow.
- The Caloosahatchee River, a river-fed estuary, exhibited unique dynamics, including high total alkalinity (which helps buffer pH changes) alongside elevated CO2 levels, which may exacerbate acidification.
During blooms, K. brevis cell densities ranged widely—from 500 to over 3 million cells per liter—indicating significant variability influenced by factors like nutrient input and freshwater inflow.
“This link between localized acidification events and HABs highlights the importance of maintaining ecosystem health to mitigate further risks,” said Dr. Kimberly Yates, Senior Research Oceanographer at USGS.
By analyzing how red tide affects water chemistry, researchers have provided valuable insights into the biological and chemical processes driving acidification. Seasonal changes, such as increased carbon and alkalinity during dry periods, further emphasize the complexity of these interactions.
“Our study clearly highlights the important link between red tide and ocean acidification, but also indicates a need for much more clarity on the impacts of this connection,” said Dr. Michael P. Crosby, President and CEO of Mote Marine Laboratory. “Continuous sampling and sensor deployment are essential to understanding the relationship between K. brevis and acidification.”
The findings highlight the critical need for adaptive management strategies to protect Florida’s estuaries from the dual threats of harmful algal blooms and acidification.
The full study, titled Nutrient and Carbonate Chemistry Patterns Associated with Karenia brevis Blooms in Three West Florida Shelf Estuaries (2020–2023), is available in Frontiers in Marine Science.