Conservative behaviour of riverine dissolved organic carbon in the Severn Estuary: chemical and geochemical implications

The distribution, variability and chemical behaviour of dissolved organic carbon (DOC) was investigated over 2$\text{1}\text{2}$ years in the Severn Estuary and Bristol Channel, UK. The concentrations of riverine DOC (3.1–7.8 mg C l^?1) covaried with river flow and were invariably conservative in this turbid slowly flushing (~200 days) estuary, indicating that any microbial degradation, chemical flocculation or adsorption processes do not affect the flux of riverine DOC through the estuary. The DOC inputs from the Severn (1.7–2.7 × 10¹⁰ g Cyr^?1) and other rivers (2.6–3.4 × 10¹⁰ g Cyr^?1) are the principal sources of DOC in the estuary and correspond to an export of 0.7–1.1% of the terrestrial productivity from the river catchment to the ocean. This export rate is in accord with recent predictions derived from global compilations of organic inputs from rivers and would imply that the global flux of riverine DOC could be as high as 7.8 × 10¹⁴ g Cyr^?1 which is 5 times greater than some previous estimates.The geochemical significance of a conservative delivery of riverine DOC to the ocean is that irrespective of which flux estimate is considered, such river inputs would make a significant contribution (~SO%) to oceanic DOC, and that the steady-state oceanic DOC flux would have to be significantly greater than present estimates (2.9 × 10¹⁴ g Cyr^?1) which are based on a mean radio carbon age of 3400 yr.An alternative, more realistic DOC flux model, which assumes a polydisperse age distribution about the mean age, is shown to yield the higher oceanic DOC fluxes required. Flocculation and adsorption processes would remove less than 10% and 0.2% respectively of riverine DOC in estuaries.