Fate of natural and anthropogenic dissolved organic carbon in the macrotidal Elorn estuary (France)

Abstract

Dissolved organic carbon (DOC) was studied, during two consecutive years, in the Elorn estuary (France) to assess its inputs and behaviour, and the possible processes, especially bacterial degradation, which may influence its fate in the estuary. Except in the case of sudden floods, riverine DOC concentration varied slightly (1.76–2.73 mg l −1) during the period of study and is apparently not related to river flow variations. In seawater, marked seasonal variations were observed with a minimum concentration (0.77 mg l −1) in March and maximum concentration in mid-summer (1.38 mg l −1). Because of industrial inputs (∼200 t yr −1), in the upper estuary, DOC concentrations are erratic and higher than those encountered in the river. The net flux of DOC to the Bay of Brest amounts to ∼600 t yr −1). The contribution of primary production to the DOC pool in the Bay of Brest is almost equivalent to the total input from the major land sources. It appeared that none of the endogenic additions or removal processes affected significantly the concentration of DOC during estuarine transport; this confirms the conservative behaviour. The difference between the net and gross fluxes are in good agreement with anthropogenic inputs. First-order degradation kinetic constants range from 0.006 to 0.011 day −1 for k 1 and from 0.0013 to 0.0036 day −1 for k 2. Constants usually increase with decreasing salinity. Only one degradation step (one constant) is found for salinity above 27, whatever the season. Refractory DOC, remaining after 4–5 months incubation, accounts for up to 90% of DOC.