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We quantified the fate and transport of watershed‐derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole‐ecosystem ¹⁵N labeling experiment. ¹⁵N‐NH₄⁺ was added to the floodwater entering a 3,477 14 m2 tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO₃⁻, NO₂⁻, N₂O, N₂, NH₄⁺, and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br² budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added ¹⁵N‐NH₄⁺ was retained or transformed. Nitrate was the most …