Nutrients That Feed Red Tide

Florida red tide, Karenia brevis, Mote Marine Laboratory
Florida Fish and Wildlife Conservation Commission, ECOHAB project, Niskin bottlesNiskin bottles submergedDr. L. Kellie Dixon, mesocosmMote Marine Laboratory, Dr. Emily R. Hall, filter water samplesDr. Cynthia Heil, Bigelow Laboratory for Ocean Sciences
December 2014
Co-authored by staff from: Bigelow Laboratory for Ocean Sciences, the Florida Fish and Wildlife Conservation Commission, Mote Marine Laboratory and University of Maryland.
Highlights from Scientists’ study of red tide caused by the harmful algae Karenia brevis

The “food” sources for Florida red tides are more diverse and complex than previously realized, according to five years’ research on red tide and nutrients published as a special issue of the journal Harmful Algae.

The project was funded by the National Oceanic and Atmospheric Administration’s ECOHAB program* and involved seven institutions.

The researchers studied four red tide blooms caused by the harmful algae Karenia brevis to understand which nutrients supported them and how significantly coastal pollution might contribute.

Study partners documented 12 nutrient sources in southwest Florida waters — including some newly associated with K. brevis. Results supported the consensus that blooms start 10-40 miles offshore, away from the direct influence of land-based nutrient pollution, but once moved inshore blooms can use both human-contributed and natural nutrients.
The project documented the microbiology, physiology, ecology and physical oceanography factors affecting Florida red tides in new detail.

Florida red tides — which occur naturally in the Gulf of Mexico and most frequently off southwest Florida — are elevated concentrations of the microscopic algae K. brevis, whose toxins can kill marine life, make shellfish toxic and cause respiratory irritation in humans. These red tide blooms occurred centuries before the mid-to-late twentieth century population boom along Florida’s coast. Now, with more residents and visitors in Florida, blooms can significantly affect public health and the economy.
Public information and short-term forecasts help mitigate red tide impacts, but ongoing research is critical to help resource managers understand and potentially reduce nutrients available to blooms.

“Data go a long way toward increasing our understanding,” said Dr. Cynthia Heil, Senior Research Scientist at Bigelow Laboratory for Ocean Sciences in Maine, who co-edited the special issue of Harmful Algae and was formerly with FWC’s Fish and Wildlife Research Institute. “This report, which includes data from four different red tides and numerous laboratory studies and modeling efforts by biological, chemical and physical oceanographers, shows the collaborative efforts needed to understand why Florida red tides are so frequent and harmful in this region.” 

Co-editor Dr. Judith O’Neil, Research Associate Professor at the University of Maryland Center for Environmental Science, added, “We learned that K. brevis is an adaptable and flexible organism. We identified 12 different sources of nutrients that it can take up and use. One of the most interesting things that hadn’t previously been taken into account is this organism’s ability to not just use sunlight, like plants, but to also consume other single-celled organisms as a nutrient source. Additionally, its migratory behavior and directed swimming allows K. brevis access to nutrient sources everywhere it finds them — at the surface, bottom and throughout the water column.” 
According to the study, K. brevis can get the nutrients nitrogen and/or phosphorus from the following sources (bold sources were newly linked to K. brevis blooms through the ECOHAB project):

  • Undersea sediments
  • Decaying fish
  • Water flowing from estuaries
  • Deposits from the atmosphere
  • Nitrogen from the air transformed, or “fixed,” into a more useable form by the naturally occurring bacteria Trichodesmium (which is a type of “cyanobacteria” that uses energy from sun to make food, like plants. They can form blooms.)
  • Waste from zooplankton — small aquatic animals visible to the naked eye
  • The “sloppy eating” and waste of smaller zooplankton only visible under a  microscope
  • Picoplankton — tiny life forms that K. brevis consumes
  • Bacteria transforming nitrogen in the water into more useful forms
  • Light creating available nutrients from natural, dissolved compounds like tannins in the water
  • Decay of Trichodesmium blooms (newly documented as a long-term nutrient source for K. brevis blooms)
  • Nitrogen from the air “fixed” by other cyanobacteria that are NOT Trichodesmium

Many of these sources were individually more than enough to support observed blooms, but no single source was deemed solely responsible.

Trichodesmium provided the most nitrogen, but not all, for K. brevis blooms developing offshore. Nearer to shore and within estuaries, major nitrogen sources included estuary water carrying land-based nutrients to sea, underwater sediments, dead fish and other sources.

Some coastal sources — estuary water, atmospheric deposits and underwater sediments — carry natural nutrients plus some enhanced levels from human activity. With other nutrient sources — such as microscopic life forms — connections with human activities are less direct, so it is harder to predict how they might be influencing red tides.

“Nature is messy, but this project has put several new pieces in place,” said Dr. Kellie Dixon, Senior Scientist at Mote Marine Laboratory and Co-Principal Investigator for the ECOHAB project. “Until now we had not looked at this many of the 12 sources and their specific quantities simultaneously. Some of the sources, like nutrients released from the sediments, had never been measured in southwest Florida’s coastal waters until we studied them for ECOHAB.”

The project incorporated physical oceanography to help describe how blooms are brought to shore.

“Until now, effective management of harmful algal blooms caused by K. brevis was complicated because we didn’t know enough about how different nutrient sources and forms taken up by K. brevis interacted with the physical environment,” said Matt Garrett of FWC's Fish and Wildlife Research Institute, who managed the ECOHAB project. “This project provides data that can help inform management recommendations on how to control nutrient sources and possibly improve forecasting models.”

The special issue of Harmful Algae includes the following management recommendations:

  • Maximize efforts to reduce potentially controllable nutrient inputs and sources that contribute to K. brevis blooms.
  • Monitor for known physical conditions that favor/disfavor the initiation, transport and export of K. brevis blooms in the southwest Florida region.
  • Identify and provide necessary funding at state and federal levels to maintain the southwest Florida coastal observing system infrastructure on an operational basis.

*Research funded by: NOAA’s ECOHAB: Karenia program — a five-year collaborative project funded by NOAA’s Coastal Ocean Program. ECOHAB: Karenia was led by FWC’s Fish and Wildlife Research Institute and involved Co-Principal Investigators from the College of William and Mary’s Virginia Institute of Marine Science (VIMS); Mote Marine Laboratory; University of Miami; Old Dominion University; University of Maryland Center for Environmental Science; Horn Point Laboratory; and University of South Florida.

Take Action

You can make a difference. Follow these steps to create a positive future for the ocean.

Take Action
  • Get familiar with the facts and the sources of updates about Florida red tide — they can help you know what to do during a bloom.
  • Follow guidelines from the document “Harmful Algal Blooms in the Gulf of Mexico: A Primer” (Available for download here):
  • Fertilizer runoff fuels many harmful algal blooms. (Though Florida red tides start offshore, away from coastal nutrient sources, these blooms can drift toward shore and then use both natural nutrients and man-made ones in runoff.) You can reduce your runoff by minimizing fertilizer use and learning proper fertilizer practices.
  • Wetlands help reduce nutrients before they enter the ocean. Support the protection and restoration of natural wetlands and construction of artificial wetlands.
  • Support best wastewater management and treatment practices in your community.
  • Support red tide research by making a donation. Research helps support better monitoring, forecasting and public information during red tides. Donate to Mote’s red tide research.
  • Notify FWC about fish kills: 1-800-636-0511 (Fish Kill Hotline) or
  • Report human health effects from red tide to the 24/7 Florida Poison Control Information Center: 1-888-232-8635