Graduation Year


Document Type




Degree Granting Department

Marine Science

Major Professor

Gabriel A. Vargo.


stable isotope, nitrogen, carbon, phytoplankton


Blooms of the red tide dinoflagellate Karenia brevis occur annually on the west Florida shelf. In the late summer/early fall months, background concentrations increase from 10³ cells L⁻¹ to excesses of 10⁶ cels L⁻¹. Blooms are most common between Tampa Bay and Charlotte Harbor, and may be maintained for months. The region's hydrography may play a role in the initiation, maintenance and termination of blooms. The west Florida shelf is depauperate in inorganic nutrients. Inorganic nitrogen and phosphorus rarely exceed the limits of detection, whereas dissolved organic nitrogen is often present at concentrations of 15 to 20 µ M. Because K. brevis exhibits ability to utilize both organic nitrogen and phosphorus, the organic pool may serve as an important nutrient source. The source of nutrients for K. brevis blooms is the focus of much scientific research. Nitrogen is considered to be the limiting nutrient in marine waters and may have several sources.

Potential sources of inorganic and organic nitrogen are estuarine outflow, atmospheric deposition, upwelling, dissolved organic nitrogen released from N₂ fixing cyanobacteria, diatom blooms, decaying seagrasses, fish or other organic matter. The natural abundance stable isotopic signatures of particulate bloom material (δ¹⁵N and δ¹³C) associated with K. brevis blooms during 1998 to 2001 was analyzed and compared with known isotopic values of potential nutrient sources. Data was analyzed from blooms occurring from 1998 to 2001. Extensive analysis of the 2001 bloom showed that the δ¹⁵N of bloom material ranged from 2% to 5%. δ¹³C bloom material ranged from -22% and -17%. Non-bloom material was considerably more variable in both δ¹⁵N and δ¹³C. δ¹³C values were higher near shore than offshore during the 2001 bloom, suggesting lower disolved inorganic carbon levels due to high temperature and/or high biomass.

δ¹⁵N of bloom material fell within the range of the δ¹⁵N values of potential nitrogen sources. It appears that K. brevis utilizes the available nitrogen sources opportunistically, and that isotopically more depleted sources are more important. More enriched sources such as upwelled nitrate or sewage nitrogen can be excluded as significant sources based on the isotopic data.