Butyltins in the waters of Saguenay Fjord (Canada): a threat for a semi-enclosed aquatic ecosystem?
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.