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From cultivated soils to coastal waters, a history of organic matter

The sea is enriched by nutrients carried by rivers, but little is known about the transformations undergone by organic matter when it transits from freshwater to seawater. In the estuary of a small Breton river affected by agricultural practices, scientists scrutinize humic substances…


Dissolved organic matter accounts for more than 90% of the organic carbon in the oceans, where it plays a major biogeochemical role. It is mainly supplied by continents et undergoes important transformations in the estuaries and the coastal waters, but its composition and its fate are still poorly known. One of its components, humic substances, is made of highly complex molecules deriving mainly from the decay of terrestrial plants. In aquatic environments, they enhance solubility of organic pollutants and pesticides, stimulate phytoplankton production and provide food for estuarine sedimentary microorganisms. But here again, in spite of their important role, little is known about their behavior during their transport to the sea. This study investigated the fate of humic substances when they transit through an estuary.

Satellite view of the Penzé estuary (source: Géoportail)

 The estuary of the Penzé River was chosen as it can be regarded as a typical temperate system affected by agriculture. It collects water from a 141-km² watershed mainly occupied by farming activities: 70% of the area is cultivated and 102,000 pigs, 1,600,000 poultry and 17,500 cattle are raised. The estuary extends over about ten kilometers; it retains water for 2 to 15 days according to the tide and the river discharge. Water samples were collected in twelve stations covering the whole freshwater–seawater mixing zone, every month in 2008 and three times in 2009. Measurements were done on temperature, salinity, suspended particular matter, particular organic carbon, dissolved organic carbon, photosynthetic pigments and humic substances. The annual cycle of water discharge in the Penzé River is representative of Breton rivers, with a maximum in winter (November to March) and the lowest levels between April and October. Some samples were collected in periods of strong increase of water discharge, particularly in November 2008 after two days of heavy rainfall.


Annual cycles of dissolved organic carbon (left) and humic substances (right) along the salinity gradient (from 0 to 34 g/l). The highest values of concentrations are in red.

The measures made in 2008 show that the concentrations of humic substances decreased downstream, as a result of the increasing dilution of freshwater into seawater. The seasonal cycle of dissolved organic carbon shows peaks in winter (January, February and November) and in spring (May). Humic substances were abundant only in November, and always in the upstream part of the estuary; they then accounted for 80% of the dissolved organic carbon. In 2009, humic substances were dominated by species of high molecular weight in May and medium-sized ones later; but at all dates the salinity gradient had no influence on the relative proportions of size classes within humic substances, suggesting no major reaction under estuarine mixing.

Waeles 2013 Fig4en.jpg
Concentrations (color) and proportions (grey) of humic substances according to their molecular weight, in July and November 2009.


Two models describing the mixing of water masses (dilution of freshwater and its dissolved substances) were applied to the decrease of humic substances concentration along the salinity gradient. Even if sudden decreases suggest losses of humic substances at mid-estuary or upstream, quantities are generally conserved: if they exist, losses of humic substances are probably limited. This model made possible the estimation of the fluxes through the Penzé watershed: 360 ± 60 tons of dissolved organic carbon per year, among which 220 ± 40 of humic substances.

Although mobilization of humic substances occurs under leaching of soils, a significant increase of their concentration was observed only in situations of increasing water discharge (January, May and November 2008, November 2009). Their abundance in November of both years can be explained by the accumulation of falling leaves and other plant debris during the long dry period of the preceding months, and the fall tillage which is known to increase soil sensitivity to erosion. But their origin is not only vegetal: the number of poultry, pigs and cattle living on the catchment area and the expected amount of waste material they produce lead to the conclusion that, through the application of manure on cultivated soils, animal production activities could contribute a significant proportion, if not the majority, of the exported dissolved organic carbon to the estuary.

Other factors played a role in the observed seasonal evolution of humic substances, such as adsorption on soil mineral particles, consumption by micro-organisms, manure composting and seasonal bans on application of fresh or composted manure. The change in molecular size distribution of humic substances between May and July 2009 could be partly related to a more important biological activity on bigger molecules and to a photochemical degradation.

In this watershed subject to high agricultural pressure, manure application is the main provider of dissolved organic carbon and humic substances, whose concentrations in the estuary are closely linked to hydrological conditions. The flux of humic substances is highest during the first autumn floods (~30 % of the yearly flux within one month); they then account for about 80 % of the dissolved organic carbon. This could result from the retention and the transformation of organic matter within the soil during a hot, dry and sunny period. With the exception of a few occasional removals, humic substances are globally conserved during their transfer through the estuary. This preservation should be also substantial in the coastal area (English Channel) where strong tidal currents efficiently mix the whole water column, thus limiting both the occurrence of these compounds in the sunlit zone and their photochemical degradation.


The paper

Waeles M., Riso R., Pernet-Coudrier B., Quentel F., Durrieu G., Tissot C., 2013. Annual cycle of humic substances in a temperate estuarine system affected by agricultural practices. Geochimica et Cosmochimica Acta 106 (2013) 231–246.
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The authors

This work was conducted by scientists from four French laboratories: Laboratoire des sciences de l’environnement marin (Lemar, IUEM), Géomer (LETG, IUEM), Laboratoire de chimie, électrochimie moléculaires et chimie analytique (UBO) and PROTEE (Université du Sud Toulon-Var)


The journal

Published by Elsevier, Geochimica et Cosmochimica Acta is the common journal of The Geochemical Society and The Meteoritical Society. It publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry: physical chemistry of gases, aqueous solutions, glasses, and crystalline solids, igneous and metamorphic petrology, chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth, organic geochemistry, isotope geochemistry, meteoritics and meteorite impacts, lunar science and planetary geochemistry.


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