Dissolved Organic Carbon in a Large Macrotidal Estuary (the Humber, UK): Behaviour During Estuarine Mixing

Dissolved organic carbon (DOC) was measured during four cruises to the Yorkshire Ouse–Humber Estuary, covering contrasting seasons. Despite the high turbidity and the complete flushing times in the ‘Humber properʹ (downstream from Trent Falls), DOC was not sensitive to bacterial oxidation and/or chemical precipitation at any time. More than 96% of the variability in the DOC profiles are simply explained by conservative mixing of the two end-members. The average DOC concentration at zero salinity (C*), was 20% higher than levels measured in the landward end of the Yorkshire Ouse (C0). C* showed an inverse relationship with river discharge, illustrating a significant contribution from sources other than natural processes in the catchment area. The difference between C* and C0 was due to a conspicuous DOC peak observed year-round about the turbidity maximum zone of the tidal Ouse (upstream from Trent Falls). Both anthropogenic waste discharges and biogeochemical production within the turbidity maximum are compatible with the observed inverse relationship with river flows.