Copper addition by organic matter degradation in the freshwater reaches of a turbid estuary

This study reports on the relationship between copper (Cu) behavior and organic matter (OM) transformation along the turbidity gradient in the freshwater reaches of the Gironde Estuary. During a one-year survey, surface water and suspended particulate matter (SPM) were sampled at least monthly at three sites along the Garonne Branch, representing the main fluvial branch of the Gironde Estuary. Additionally, a longitudinal high resolution profile was sampled along the Garonne Branch, covering the turbidity gradient from the river water endmember to the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of Cu in both the dissolved phases (< 0.2 μm, Cu0.2 and < 0.02 μm, Cu0.02) and particulate Cu (CuP) clearly suggested Cu0.2 addition during summer, that increased the Cu0.2 concentrations by a factor ~ 2, mainly manifested by an increase in the Cu0.02 fraction. At the annual timescale (2004), this internal Cu reactivity increased Cu0.02 fluxes in the Garonne Branch by ~ 20% (3.6 t year−1), with the equivalent of ~ 2.9 t year−1 derived from the CuP fraction and ~ 0.7 t year−1 from the colloidal (0.02–0.2 μm) fraction, without involving and/or affecting the CuC18 (hydrophobic metal-organic complexes) fraction.Combining data on Cu speciation with the results obtained by several independent techniques (DOC and POC measurements, 3D-fluorescence, and TEM) suggested close relationships between Cu behavior and OM transformation/restructuration along the turbidity gradient in the Garonne Branch. The observed Cu0.02 addition was related to increasing humification (humification index HIX increased from 9 to 12, network formation) and labile OM degradation (Iγ/Iα ratio decreased from 0.70 to 0.44), going along with decreasing DOC and POC concentrations. Mass-balances suggest that in the studied system, degradation of OM may account for the release of ~ 25 μmol potentially bioaccessible Cu0.02 per mole of particulate organic carbon mineralized.

Research Highlights
► Quality of water entering into the Gironde Estuary is good to mediocre for Cu.
► Addition of dissolved Cu (< 0.02 μm) was observed in the Garonne fluvial estuary.
► Cu addition is related to transformation and degradation of organic matter.
► Transformation and degradation of organic matter release potentially bioavailable Cu.
► This process may increase Cu bioavailability in this vulnerable aquatic system.