Results show accumulation of ammonium in pore waters in the fall.
Changes in pore water ammonium concentrations from September to October fit the constant source solution of Fick's second law of diffusion, so that increasing ammonium could be related to diffusion from a constant source at the sediment-water interface. Environmental conditions for the diffusional transport into the top 10 cm depth of the sediment column are discussed. The coincidence of calm hydrodynamic conditions and high inputs of fresh organic matter to the sediment would allow the development of an interfacial viscous layer with enhanced metabolism. The storage of reactive substances in this layer is proposed as a mechanism that may cause sediments to be enriched with dissolved substances. On the other hand, transition from nonstationary October ammonium profiles to a new stationary state was shown to occur 30 d after the breakdown of the viscous layer.