Former agricultural fields that are converted to wetlands can serve as either a source or a sink of P to connecting downstream water bodies. Understanding the P dynamics associated with these flooded fields can lead to appropriate mitigation strategies. We investigated P dynamics in flooded celery fields that are adjacent to and connect with a major tributary that eventually flows into Lake Michigan. We measured the P concentrations and loads in the tributary upstream and downstream of the flooded celery fields. Mean total P (TP) load was 2.6× greater downstream than upstream of the flooded celery fields. Turbidity, chlorophyll a, and soluble reactive P (SRP) concentration all were significantly greater at the downstream sampling site than at the upstream site. Mean water-column TP concentrations in the flooded celery fields ranged from 368 to 810 µg/L, compared to mean TP concentrations of 29 and 46 µg/L at the upstream and downstream sampling sites, respectively, in the tributary. Equilibrium P concentrations of flooded celery-field sediments were significantly greater than the water-column SRP concentrations, a result suggesting that the sediments were a source of P to the overlying water. Last, sediment pore-water concentrations of P were very high based on in-situ pore-water samplers, with maximum values of ∼1.6 mg/L. Our results show that wetlands converted from former agricultural fields can be a significant source of nutrients to downstream receiving water bodies. We recommend that the concentrations and release rates of sediment nutrients be measured before areas are converted to wetlands to ensure they serve as nutrient sinks instead of nutrient sources.