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T h e d istribution o f nitrification has b e en m easu red w ith the 111 '(() ; in co rp o ra tio n m eth o d in the Seine River a n d its estuary during sum m er conditions. T h e Seine R iver below Paris receives large am ounts o f am m onium th ro u g h w astew ater discharge. In the river itself, this am m onium is only slowly nitrified, while in the estuary nitrification is ra p id a n d com plete. We show th a t this contrasting behavior is re la te d to the d iffere n t hydrosedim entary conditions o f the two system s, as nitrifying b acteria are associated w ith su sp en d ed particles. In the river, particles and th eir attached bacteria e ith e r rapidly settle o r have a sestonic behavior. B ecause o f the sh o rt residence tim es o f the w ater m asses, the slow growing nitrifying pop u latio n has no tim e to develop sufficiently to nitrify the available am m onium. T h e estuary is characterized by strong tidal dynam ics. Particles settle and are re su sp e n d ed continuously w ith the strong c u rre n t inver sions o f ebb and flood. As a result o f these dynam ics, particles a n d th e ir attach ed nitrifying bacteria experience longer residence tim es in a tem p o rary su sp en d ed state th an the w ater m asses them selves, providing to slow growing nitrifying bacteria the oppo rtu n ity to develop a large pop u latio n capable o f nitrifying all the available am m onium .

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