Many aquatic ecosystems are experiencing a decline in their oxygen (O-2) content and this is predicted to continue. Implications of this change on several properties of bacterioplankton (heterotrophic prokaryotes) remain however are poorly known. In this study, oxic samples (similar to 170 mu M O-2 = controls) from an oligohaline region of the Scheldt Estuary were purged with N-2 to yield low-O-2 samples (similar to 69 mu M O-2 = treatments); all were amended with C-13-glucose and incubated in dark to examine carbon incorporation and cell size of heterotrophic prokaryotes, and relationships between organic matter (OM) degradation and phosphate (P) availability in waters following O-2 loss. Stable isotope (C-13) probing of phospholipid fatty acids (PLFA) and flow cytometry were used. In samples that have experienced O-2 loss, PLFA biomass became higher, prokaryotic cells had significantly larger size and higher nucleic acid content, but P concentrations was lower, compared to controls. P concentration and OM degradation were positively related in controls, but uncoupled in low-O-2 samples. Moreover, the dominant PLFA 16:1 omega 7c (likely mainly from Gram-negative bacteria) and the nucleic acid content of heterotrophic prokaryotic cells in low-O-2 samples explained (62-72 %) differences between controls and low-O-2 samples in P amounts. Shortly after incubations began, low-O-2 samples had consistently lower bacterial PLFA C-13-enrichments, suggesting involvement of facultatively anaerobic metabolism in carbon incorporation, and supporting the view that this metabolic pathway is widespread among pelagic bacteria in coastal nutrient-rich ecosystems. Estimates based on C-13-enrichment of PLFAs indicated that grazing by protozoa on some bacteria was stronger in low-O-2 samples than in controls, suggesting that the grazing pressure on some heterotrophic prokaryotes may increase at the onset of O-2 deficiency in nutrient-rich aquatic systems. These findings also suggest that physiological responses of heterotrophic prokaryotes to O-2 loss in such ecosystems include increases in cell activity, high carbon incorporation, and possibly phosphorus retention by cells that may contribute to reduce phosphate availability in waters.