Bacterioplankton dynamics in the York River estuary: primary influence of temperature and freshwater inputs

Abstract

Bacterial community dynamics were investigated over seasonal and basin scales within the York River estuary, Virginia. Variables describing bacterioplankton dynamics were measured at 6 stations spanning the entire salinity gradient (0 to ca. 20 psu over 60 km). Samples were collected monthly from June 1996 through May 1997 and every other month from June 1997 through May 1998. Bacterial abundance and production were high throughout the estuary. Bacterial abundance ranged from 4.4 × 10 8 to 1.3 × 10 10 cells l -1 . Incorporation of 3 H-thymidine ranged from 10 to 863 pmol -1 h -1 while 3 H-leucine incorporation rates ranged from 25 to 1963 pmol l -1 h -1 . A strong rela- tionship between bacterial properties and temperature was found with clear seasonal trends. On a basin scale, bacterial properties were strongly related to changes in salinity, suggesting that fresh- water inputs and estuarine circulation controlled the distribution of bacterial abundance and activity in the river. Although there was a great deal of variability from month to month, 2 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, and thus a close match between production and removal near the freshwater endmember and throughout the estuary.