Tuesday, January 19, 2010
101 Stanley Thomas Hall
Tulane University (Uptown)
Refreshments will be served
Geoffrey A. Sinclair, Louisiana Universities Marine Consortium (LUMCON)
Physical and Chemical Constraints on the Benthic-Pelagic Coupling of Karenia brevis
Multiple hypotheses have been presented to explain how nutrients are delivered to populations of the harmful dinoflagellate, Karenia brevis, in oligotrophic water columns in the Gulf of Mexico. Vertical migration behavior coupled with the physiology of K. brevis may help alleviate bottom-up controls and permit populations to persist as vegetative cells near the sediment-water interface throughout the year. Aggregations of natural Karenia brevis populations near the sediment-water interface suggest that cells may derive nutrients from the sediment in oligotrophic water columns. How cells interact with the sediment, however, remains uncertain. Video of cells near the sediment-water interface suggest that cells may either access nutrients that flux out of the sediment or migrate into the sediment pores where higher nutrient concentrations exist. Experiments to test the ability of K. brevis to migrate into the sediment were devised using chambers divided by a 100 μm mesh overlain with a thin layer of sediment collected from the Gulf of Mexico. Since the diel vertical migration of K. brevis typically displays a nocturnal descent, experiments tested migration response during the day and night and with and without a sub-sediment nutrient source. In order to determine the ability of K. brevis to exploit elevated nutrients associated with the sediment, we examined the diel rates of uptake and assimilation of different nitrogen substrates (NH4+, NO3-, Urea). Uptake and assimilation rates were measured during the day with the light intensity under which the cells were grown (< 30 μmol quanta m-2 sec-1) and in the dark at night. Growth under a 12:12 light/dark cycle produced average growth rates of ~ 0.15 divisions day-1 for all the experiments. The chamber experiments suggest that while the sediment affects the progress of descending cells, migration occurs through thin layers of sediment and increases in response to elevated nutrient concentrations below the sediment. Since all cells found below the sediment had significantly higher C/N ratios than those remaining above the sediment, the motivation for migration appears related to a cell’s internal biochemical state. The flexible exploitation of sediment-derived nutrients combines with the diel uptake and assimilation of a variety of N substrates to support low growth rates. The ability to maintain low growth rates in low light environments while accessing sediment derived nutrients permits K. brevis to persist as vegetative cells near the sediment-water interface. This ability to maintain slow growing populations associated with the sediment may substitute for life cycle strategies that involve encystment under unfavorable conditions that are observed in other dinoflagellates. Populations of vegetative cells associated with the sediment may provide seed populations that are advected onshore, under upwelling favorable conditions, to environments that promote cell aggregation and growth on higher light and opportunistic nutrient sources. The combination of cell aggregation and growth would result in the near-surface harmful algal blooms that are observed near-shore.
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