Along the western Antarctic Peninsula, marine bacterioplankton respond to the spring phytoplankton bloom with increases in abundance, production and growth rates, and a seasonal succession in bacterial community composition (BCC). We investigated the response of the bacterial community to experimental additions of glucose and ammonium, alone or in combination, 1°W). Changes in bulk propertiesand BCC in ammonium-amended carboys were small relative to controls, compared to the glucoseamended treatments. The BCC in Day 0 and Day 10 controls and ammonium treatments were > 72% similar when assessed by denaturing-gradient gel electrophoresis (DGGE), length heterogeneity polymerase chain reaction (LH-PCR) and capillary electrophoresis single-strand conformation polymorphism (CE-SSCP) fingerprinting techniques. Bacterial abundance increased 2-to 10-fold and leucine incorporation rates increased 2-to 30-fold in the glucose treatments over 6 d. The BCC in carboys receiving glucose (with or without ammonium) remained > 60% similar to that in Day 0 controls at 6 d and evolved to < 20% similar to that in Day 0 controls after 10 d incubation. The increases in bacterial production rates, and the changes in BCC, suggest that selection for glucose-utilizing bacteria was slow under the ambient environmental conditions. The results suggest that organic carbon enrichment is a major factor influencing the observed winter-to-summer increase in bacterial abundance and activity. In contrast, the BCC was relatively robust, changing little until after repeated additions of glucose and prolonged (~10 d) incubation.