Shifts in the dominant populations of ammonia-oxidizing b-subclass Proteobacteria along the eutrophic Schelde estuary

in its protonated form), which can be oxidized to nitrite (NO 2 -) by chemolithotrophic ammonia-oxidizing bacteria. Ammonia oxidation is the first, and often rate-limiting, step in the removal of nitrogen from environmental systems (Prosser 1989), and nitrification is quantitatively important in the estuary, in terms of both oxygen and ammonia consumption (Soetaert & Herman 1995a). Estuarine environments contain gradi-