Date of Award

Winter 2012

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean/Earth/Atmos Sciences

Committee Director

Margaret R. Mulholland

Committee Member

Alexander Bochdansky

Committee Member

Harold G. Marshall

Committee Member

Diane K. Stoecker


Karenia brevis, the toxic dinoflagellate responsible for massive red tides in the Gulf of Mexico (GOM), causes fish kills, shellfish poisoning, and acute respiratory irritation in humans. Bloom initiation and maintenance have been linked to the physical environment as well as various nutrient input mechanisms. To date, efforts to quantify nitrogen (N) sources fueling K. brevis blooms in the GOM have not included mixotrophic grazing although many dinoflagellates, including K. brevis, are known to be capable of mixotrophy. This dissertation reports field and laboratory results demonstrating that natural bloom populations and K. brevisisolates from the West Florida Shelf (WFS) can ingest a WFS Synechococcus isolate. Maximum K. brevis ingestion rates were measured within the first 2 to 6 hours in laboratory incubations augmented with Synechococcus prey and rates ranged from 7.2 to 48.0 Synechococcus K. brevis -1 hr-1. I calculated a lower feeding threshold of 1.86 × 104 Synechococcus ml-1 , which is the prey concentration necessary for K. brevis to ingest this prey organism.

To determine whether dissolved N or light affected ingestion rates for Karenia brevis on Synechococcus, grazing was measured in N-replete and -deplete cultures and during the day and night when incubation lights were on or off, respectively. Ingestion rates ranged from 2.7 to 7.2 Synechococcus K. brevis-1 hr-1 and there were no significant differences in ingestion rates between treatments. I calculate that the N-specific uptake rates from Synechococcus prey were on the order of 10-2 to 101 μmol N l-1 hr-1. I also demonstrate for the first time that K. brevis is able to ingest Prochlorococcus (27.3 ± 8.3 Prochlorococcus K. brevis -1 hr-1) and heterotrophic bacteria (0.1 - 3.1 bacteria K. brevis-1 hr-1), although the latter are likely underestimates as I tried to minimize contamination by heterotrophic bacteria in K. brevis cultures.

Karenia brevis ingestion rates on live and heat-killed Synechococcus were not statistically different, 23.4 ± 18.1 and 21.38 ± 12.6 Synechococcus K. brevis-1 hr-1, respectively. This allowed me to examine prey uptake versus photosynthetic or amino acid C uptake in the same incubation bottles where grazing was measured. C-specific uptake from Synechococcus ingestion ranged from 11.2 to 38.8 pmol C K. brevis -1 hr-1, which was 7.5 to 22.4 times greater than photosynthetic C uptake in parallel incubations.

Ingestion rates by Karenia brevis on Synechococcus measured during cruises to the WFS during three blooms were 0.04 to 15.5 Synechococcus K. brevis-1 hr -1, which falls within the range found in laboratory studies. The highest ingestion rates by K. brevis on the WFS were measured in 2009 despite low ambient concentrations of Synechococcus. N-specific uptake from Synechococcus ranged from 0.05 to 13.86 μmol N l-1 hr-1 during laboratory and field experiments. Grazing on Synechococcus, as well as other possible picoplanktonic prey, can contribute substantially to the N budget for K. brevis growth in the GOM, which has been reported between 0.056 to 0. 267 μmol N l-1 d-1 for moderately sized (105 cells l-1) blooms growing autotrophically.