Annual cycle of humic substances in a temperate estuarine system affected by agricultural practices

Abstract

, et al.. Annual cycle of humic substances in a temperate estuarine system affected by agricultural practices. Geochimica et Cosmochimica Acta, Elsevier, 2013Elsevier, , 106, pp.231-246. 10.1016Elsevier, /j.gca.2012 Annual cycle of humic substances in a temperate estuarine system affected by agricultural practices AbstractAlthough widely studied for their chemical structures and properties (e.g., metal complexation, growth stimulation of planktonic species), humic substances (HS) have been very poorly quantified in fluvial and estuarine waters. In this monthly basis study, we determined HS concentrations (by Adsorptive Square Wave Cathodic Stripping Voltammetry) along the entire river-seawater gradient of the Penzé estuary (NW France), with the aim to characterize the export of these compounds. In this watershed where agricultural activities are predominant, manuring activities were identified as being the main source of dissolved organic carbon (DOC) and HS. HS concentrations varied usually within a narrow range in fluvial waters, i.e., 1.8 ± 0.4 mgC L À1 (150 ± 40 lM), but increased significantly as the first flood of autumn occurred (>4 mgC L À1 in river and upper estuary). At this time, HS accounted for a very high proportion of DOC (>80%). As evidenced by the increasing contribution of HS to DOC, and by the increasing contribution of small colloidal HS species; this autumnal flood increase should be attributed to a greater retention and transformation of organic matter on soils over the hotter, drier, and lighter period preceding the first autumnal flood. In the mixing zone, HS displayed mostly conservative behaviour, although some removals were occasionally observed. Overall, our study suggests that preservation of HS could be relatively important during their transfer across macrotidal temperate estuaries, at least in systems affected by agricultural practices.

Bioavailability of riverine dissolved organic matter in three Baltic Sea estuaries and the effect of catchment land use

“…Humic substances, predominantly formed by the breakdown of terrestrially derived organic matter, dominate as a major carbon fraction in freshwater DOM (Dittmar and Kattner, 2003). They may account for over 80 % of DOC leaching from agricultural soils, depending on the prevailing hydrological conditions (Waeles et al, 2013). Such humic substances are generally resistant to biodegradation in estuaries and the greatest changes in their quality and degradation only take place once they reach fully marine conditions (Kisand et al, 2013).…”

On the control of copper colloidal distribution by humic substances in the Penzé estuary

“…Skrabal et al (2000) have however proposed that they could correspond to biogenic compounds (exudates from bacteria/macrofauna or microbial metabolites), or to the alteration of pre-existing organic substances. The fact that HS behave conservatively in our system (Waeles et al, 2013), even in spring, is more in favor of a biotic nature for these compounds. More specific work is needed for describing sediment-porewater exchanges of Cu species in estuarine areas of preferential sedimentation of the continental OM.…”

“…Guo and Santschi, 2007). In the Penzé system we showed that humic substances which accounted for a high proportion of DOC were mainly observed as colloidal species (Waeles et al, 2013). Their control on Cu concentration and Cu colloidal distribution is discussed below.…”

“…In this system highly impacted by agricultural practices, manuring activities were identified as being the main source of HS (Waeles et al, 2013). Thus, the physical and chemical speciation of Cu in our system is expected to be partly related to that in manure and that in amended soils.…”

“…HS were analyzed by Adsorptive Square-Wave Cathodic Stripping Voltammetry at a static mercury drop electrode (potentiostat/galvanostat l-AutolabIII unit controlled by GPES 4.9 software) using the method developed by Quentel et al (1987). This simple and rapid method, based on adsorption of the complex formed by HS with molybdenum and originally developed for marine waters, has also proved its usefulness in freshwaters (Chanudet et al, 2006) and estuarine waters (Waeles et al, 2013). Deposition of HS as a Mo(V)-HS complex at the working electrode is performed after addition of Mo(VI) (0.105 lmol L À1 ) to the acidified (0.01 mol L À1 HCl) sample and under a potential of À300 mV (vs Ag/AgCl, 3 mol L À1 KCl).…”