Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Hugh W. Ducklow


Bacterial community dynamics were investigated over seasonal and basin scales within the York River, VA, estuary. Various parameters characterizing bacterioplankton dynamics were measured weekly at a single station (March 1996 through May 1997) and monthly at six stations (June 1996 through May 1997) spanning the entire salinity gradient (0 - ca. 20 psu over 60 km). Bacterial abundance and production were found to be high throughout the estuary. Bacterial abundance ranged from 4.4 x 108 to 1.3 x 1010 cells-liter-1Incorporation of 3H-thymidine ranged from 10 to 863 pmol-liter-1-hr-1 while 3H-leucine incorporation rates ranged from 25 to 1963 pmol-liter -1-hr-1. Clear seasonal trends were apparent. The highest values of abundance and incorporation were found during the summer months. On a basin scale, bacterial properties were strongly related to changes in salinity. Although there is a great deal of variability from month to month, two opposing trends were consistently found: bacterial abundance increased from freshwater to the mouth of the river, while incorporation rates decreased from freshwater to the mouth. These patterns imply a strong landward gradient in specific growth rates. Growth rates determined by a two-dimensional box model were indeed higher upstream, but net growth rates were highest near the mouth. This explains how biomass can accumulate downstream even though production decreases. The box-model was also used to investigate the role of circulation in the distribution of bacterial cells. Dispersion rates were greater than net growth rates, indicating that dispersion controlled bacterial distribution during the study period. However, gross growth and removal rates were generally greater than dispersion rates, indicating the potential importance of biological processes to bacterial community structure within the estuary. BIOLOG plates were used to determine if differences in bacterial community structure or metabolic capabilities occurred over time or space. Two distinct bacterial communities separated by temperature were found over the course of a year at the VIMS pier. Four distinct communities separated by temperature and salinity were found over the course of a year along the salinity gradient. Thus, temperature, salinity, circulation and removal processes are the dominant processes controlling bacterial dynamics in the York River estuary.



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