Organic matter and dissolved inorganic nitrogen distributions in estuarine muddy deposits

Organic matter (OM) and dissolved inorganic nitrogen (DIN: nitrite, nitrate and ammonium) in the sediments as well as in the water column of two temperate estuaries, the Scheldt Estuary in Belgium and the Netherlands, and the Fraser Estuary in Canada, were investigated. Three representative stations, differing in salinity and representing areas of fast sedimentation, were selected in each estuary. Samples were taken during periods of high and low river discharge. The results show, in both estuaries, that the vertical distributions of OM and DIN in a sediment layer are affected by the instability, caused by episodic resuspension and re-deposition, of the uppermost sediment layer. The findings of this study suggest a hypothesis, next to biogeochemical processes, that the OM and DIN distributions in upper sediment layers are influenced by sedimentary processes in the estuarine environment. The same sedimentary processes even in different estuaries affect OM and DIN distributions in an equivalent way. Correspondingly, the similarity or difference in OM and DIN distribution to a certain extent reflects the sedimentary dynamics. River runoff and sediment resuspension and sedimentation have important impacts on sediment behaviour and thus regulate OM and DIN distributions and shape their vertical profiles in the sediments. As a reflection, the coupling of sediment resuspension followed by redeposition can be deduced from the vertical profile of DIN in the bottom sediments which, in turn, can provide a time-integrated periodic record of the most recent sedimentary history.