Butylétains dans les eaux du fjord du Saguenay (Canada) : menace pour l’écosystème d’un milieu semi-fermé ? Butyltins in the waters of Saguenay Fjord (Canada): a threat for a semi-enclosed aquatic ecosystem?

Résumé

Dans le cadre de travaux sur le comportement des composés des butylétains dans les milieux côtiers froids, des échantillons d’eau, de matière particulaire en suspension et de seston (phyto- et zooplancton) ont été prélevés à huit (8) stations le long du fjord du Saguenay (Canada) et dans la baie des Ha! Ha! en mai 2001. Les concentrations en butylétains totaux (MBT + DBT + TBT) étaient significativement plus élevées en surface (26 à 206 ng Sn L^-1) que dans les échantillons de fond (7 à 30 ng Sn L^-1). Les niveaux trouvés à l’embouchure du fjord étaient deux fois plus élevés que ceux observés dans son axe principal et cinq fois plus élevés que ceux dans la baie des Ha! Ha! dont les concentrations variaient de 40 à 55 ng Sn L^-1 avec les plus élevées à proximité de Port‑Alfred. Le tributylétain (TBT) est toujours le composé minoritaire, que ce soit dans les eaux de surface (de 1 à 5 %) ou dans la couche d’eau profonde (5 à 24 %). Le dibutylétain (DBT) domine dans les deux masses d’eau avec des proportions d’environ 85 % pour la surface et de 34 à 90 % au fond. Le monobutylétain (MBT) est présent dans toute la colonne d’eau avec des pourcentages fluctuant de 4 à 15 % dans les eaux de surface et de 2 à 46 % dans les eaux profondes. Les butylétains sont également présents dans tous les échantillons de seston. En surface, les concentrations des métabolites (DBT + MBT) sont plus élevées (25 à 59 ng Sn g^-1) que celles du TBT (10 et 20 ng Sn g^-1). Dans la couche de fond, le TBT est majoritaire dans le seston avec des concentrations similaires entre les stations d’environ 30 ng Sn g^-1. Les facteurs de bioconcentration obtenus à partir des données du seston confirment que les niveaux de TBT dans l’eau sont suffisants pour induire une bioaccumulation par étape au sein de la chaîne alimentaire. Enfin, les concentrations en TBT dans la colonne d’eau semblent bien au-dessus du niveau susceptible de perturber l’écosystème en causant des effets chroniques sur la reproduction de plusieurs organismes ou en affaiblissant leurs systèmes immunitaires.

Mots-clés:

• Butylétains,
• distribution spécifique,
• devenir des butylétains,
• colonne d’eau,
• milieu côtier froid,
• toxicité

Abstract

In order to understand the fate of butyltin compounds in cold coastal ecosystems, samples of water, suspended particulate matter and seston (phyto- and zoo-plankton) were taken in the Saguenay Fjord, a deep and narrow glacial valley filled with seawater from the St. Lawrence Estuary (Canada) and used as a navigation channel to reach an upstream industrial area. On one hand, this study was designed to evaluate the butyltin contamination level of waters of the Fjord. On the other hand, the samples were used to better understand the behaviour of tributyltin (TBT) and its breakdown products (dibutyltin (DBT), monobutyltin (MBT)) in the water column and its associated ecosystem. In addition, this study enabled us to establish if concentrations found in the Fjord may represent a toxic threat for the biota.

The water sampling was carried out at eight stations and various depths along the Saguenay Fjord and in the Baie des Ha! Ha! in May 2001. During this same expedition, samples of seston were also collected at 5 and 70 m depth for the same stations. Butyltins were extracted according to a well defined protocol and quantified by gas chromatography - mass spectrometry (GC-MS) on an Ion Trap GC/MS operated in single ion monitoring (SIM) mode. In the water column, concentrations of total butyltins (MBT + DBT + TBT) were significantly higher in surface samples (26 to 206 ng Sn L^-1) than in deep samples (7 to 30 ng Sn L^-1). The levels found at the mouth of the Fjord were two times higher than those observed along its main axis and five times higher than those in the Baie des Ha! Ha! where concentrations varied from 40 to 55 ng Sn L^-1 with the highest values observed near Port Alfred. Tributyltin (TBT) was always a minor component in water surface (from 1 to 5%) as well as in the deep water layer (5 to 24%). Dibutyltin (DBT) dominated in the two water masses with proportions of approximately 85% for surface and from 34 to 90% at the bottom. Monobutyltin (MBT) was present in all waters, contributions fluctuating from 4 to 15% in surface water and from 2 to 46% in deep water. Butyltin compounds were also present in all seston samples. In the surface layer, concentrations of metabolites (DBT + MBT) were higher (25 to 59 ng Sn g^-1) than TBT itself (10 to 20 ng Sn g^-1). However, TBT dominated in the deep samples (70 m depth) with similar concentrations among stations of approximately 30 ng Sn g^‑1.

This study presented the first evidence of the widespread butyltin contamination of Saguenay Fjord with levels typically reported for contaminated coastal areas. The shipping traffic was considered as the main source of continuous butyltin inputs, essentially TBT, but industrial activities located along the St. Lawrence Estuary were also suspected of generating the release of significant quantities of DBT, and thus contributing to the total reservoir of butyltins. The particular oceanographic conditions of the Saguenay Fjord (tidal cycles, strong stratification of water column) were responsible for the variation of concentrations among stations but also among depths. In addition, results observed in this study (high bioconcentration factors) confirmed the affinity of TBT for particulate matter compared to its metabolite, DBT. The chemical properties of TBT (low solubility, high water-sediment and water-octanol partition coefficients) are such that the processes of sorption and bioaccumulation in organisms are favoured. Bioconcentration factors calculated from seston data confirmed that the levels of TBT in water were sufficient to induce a stepwise bioaccumulation throughout the food chain. Consequently, suspended particular matter and organisms were the two compartments responsible for the TBT elimination from the water column to sediment. On the other hand, because of its higher solubility and a lower water-sediment partition coefficient, DBT was the major compound in the dissolved phase and its behaviour was influenced by hydrographic conditions. Lastly, TBT concentrations in the water column seemed well above the level likely to disturb the ecosystem by causing chronic effects on the reproduction of several organisms or by disturbing their immune systems.

Keywords:

• Butyltins,
• specific distribution,
• fate of butyltin,
• water,
• cold coastal ecosystems,
• toxicity