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

Abstract

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, Cu(0.2) and <0.02 μm, Cu(0.02)) and particulate Cu (Cu(P)) clearly suggested Cu(0.2) addition during summer, that increased the Cu(0.2) concentrations by a factor ~2, mainly manifested by an increase in the Cu(0.02) fraction. At the annual timescale (2004), this internal Cu reactivity increased Cu(0.02) fluxes in the Garonne Branch by ~20% (3.6 t year(-1)), with the equivalent of ~2.9 t year(-1) derived from the Cu(P) fraction and ~0.7 t year(-1) from the colloidal (0.02-0.2 μm) fraction, without involving and/or affecting the Cu(C18) (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 Cu(0.02) 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 Cu(0.02) per mole of particulate organic carbon mineralized.