Organic matter processing in tidal estuaries

Processing of organic matter in tidal estuaries modifies its transfer from the river to the sea. We examined the distribution and the elemental and isotopic composition of organic matter in nine tidal estuaries along the Atlantic coast of Europe (Elbe, Ems, Thames, Rhine, Scheldt, Loire, Gironde, Douro and Sado). Short-residence time estuaries such as the Douro and Rhine estuaries have low suspended matter (SPM) contents with variable organic carbon (OC) concentrations, but rather uniform δ15N values and C/N ratios. Carbon isotope ratios vary from − 20 to − 22‰ in the marine end-member to − 26 to − 30‰ in the river end-member with conservative mixing in between. Tidal-dominated estuaries have high SPM contents, rather uniform OC contents and C/N ratios, and uniform and intermediate δ13C values (− 24 to − 26‰). Organic matter in these intermediate to long residence time estuaries is extensively modified such that δ15N values may show enrichments up to 20‰ due to (heterotrophic) processing. Pools of dissolved and particulate organic matter appear to be involved in dynamic exchange reactions with their partitioning mainly governed by SPM concentration. The interaction of tidally induced cycles of deposition and erosion with this repartitioning of organic matter between dissolved and particulate pools due to changes in SPM concentration can be viewed as a chemical reactor with optimal solid–liquid exchange. As a consequence, riverine organic matter is extensively modified in tidal estuaries before it is transferred to the sea.