Biogeochemical characters of organic matter in the water column and sediments of a brackish ecosystem: the Tau lagoon - Seasonal variations
The Thau lagoon along the French Mediterranean coast of the Gulf of Lions has unusual characteristics. It is sometimes subjected to anoxic conditions, known as "malaigues", which result from the accumulation of organic matter during the warmer period. Throughout the year this organic deposition, associated oyster farming and terrigenous inputs, contributes to biogeochemical exchanges between the water column and the underlying deposits. In this same environment, high-resolution analytical techniques (HPLC ; PY-GC-MS) were used to analyze the distribution and nature of the organic matter and to determine its origin and behaviour in the water column and sediments. Total suspended matter (TSM) was determined by filtration of water samples pumped up from different levels of the water column and filtered onto glass fiber filters (GF/F grade) previously heated at 450 °C for 4 hours. Particulate organic carbon (POC) was determined on the same samples with a Leco CS 125 analyzer after removal of inorganic carbonates by treatment with a H2 SO4 (2N) solution. Dissolved organic carbon (DOC) was determined on the filtrates using a Shimadzu TOC 5000 analyzer. The determination of polysaccharides in the TSM was achieved by a colorimetric method involving a H2 SO4 (3N) solution and anthrone reagent (Gallali 1972). Phenolic compounds were determined by high performance liquid chromatography (HPLC) after cupric oxide alkaline oxidation of TSM samples. The oxidized samples were acidified (HCl, 2N) and subjected to liquid-liquid extraction with ethyl acetate (Hartley & Buchan 1973; Hedges & Ertel 1982). The limit of detection is 10-4 g and the precision of the method is about 2% for each compound. Separation and quantification of phenolic monomers was carried out by HPLC (Hartley & Buchan, 1973 ; Serve et al., 1983). Of a total of 28 identified products, eleven represent the monomers constituting lignin and are taken into account according to Hedges & Parker (1976), Hedges & Mann (1979) and Hedges & Ertel (1982). The products of oxidative hydrolysis of lignin belong to the following three series : 4-hydroxybenzyl "H" (p-hydroxybenzoic acid, p-hydroxybenzaldehyde, p-hydroxyacetophenone), 3-methoxy-4-hydroxybenzylic "V" (Vanillyl) and 3,5-methoxy-4-hydroxybenzylic "S" (Syringyl). Each of these three series presents an alkyl side chain with 1, 2 or 3 carbon atoms. The compounds in C6-C1 can be acids or aldehydes, those in C6-C2 are ketones and those in C6-C3 are acids. The latter, having a phenylpropenic structure, belong to the Cinnamyl "C" series (ferulic acid, p-coumaric acid). Separation of phenols was carried out on a Merck analytical column (250 mm long x 4 mm in diameter) with a Lichrosorb reversed phase C18 stationary phase of 5 μm granulometry, equipped with a precolumn (40 mm long) containing the same phase. Elution was achieved with ternary eluents (water, acetonitrile, acetic acid), in a high pressure binary gradient (Charrière 1991). The eluted products were determined qualitatively, by comparison of their retention times with those of commercial products (detection in UV at 275 nm), after a co-injection if necessary, and quantitatively by an internal standard method (phloroglucinol : 1,3,5-benzenetriol and p-anisic acid : p-methoxybenzoic acid). Analysis of the major classes of organic compounds was carried out by coupled pyrolysis - gas chromatography - mass spectrometry. A CDS 1000 pyrolysis probe was directly fitted with a Perkin-Elmer 8700 gas chromatograph (GC) equipped with a TR-WAX capillary column (length: 30 m, diameter: 0.32 mm, phase thickness: 0.50 μm). Pyrolysis temperature was 700 °C for 10 s and the column temperature was programmed from 60°C to 240 °C at a rate of 6 °C/min according to Puigbo et al. (1989). Pyrolysis fragments were identified by coupling the GC to a HP 5989 mass spectrometer. Twenty three major peaks were selected on the pyrochromatograms and each selected compound was expressed as a percentage of the sum of the surface of these 23 peaks Pyrolysis products were grouped into five main families, each of them including similar molecules or closely related chemical structures: aromatic hydrocarbons, nitrogenous compounds, sugars, phenols and amino sugars. The survey of all these parameters showed some characteristic differences over the four seasons. Summer appears as a period when the biological production reaches maximum levels in the water column. At that time, organic matter is stratified with high levels of accumulation in the deeper layers. DOC is also abundant throughout the water column and organic compounds belonging to the class of sugars decrease according to depth. Autumn corresponds to Mediterranean storms and typical rainfalls. Terrestrial inputs increase in this season and degradative processes affect the organic matter that was produced in large quantities in the summer by the autotrophic organisms of the lagoon. DOC is recycled and reflects the degradation of autochthonous organic material. Winter, with reduced TSM levels related to low terrestrial inputs, is characterized by a homogenization of the water column and a weak biological activity. Lignin-derived phenols are abundant and correspond to a period of low biological activity. In contrast, in the spring the biological activity recovers, as indicated by the high sugar content of the DOC and by a homogenization of the water column. Under the oyster beds, an increase of organic matter is observed in the water column as well as in the sediments. However, the active degradation processes in summer and autumn reduce considerably the amount of the settling organic matter. The results of these processes are variable according to whether the stations are under or outside of the oyster beds. Degradation in the water column leads to a decrease of biodegradable compounds in the sediments and an increase in resistant compounds like phenols and aromatic hydrocarbons. These mineralization processes increase with depth in deposits, as reflected by higher proportions of aromatic compounds. The relationship between nutrients and organic matter, the latter constituting both their source and their sink, appears in sediments under oyster beds, where the inorganic nutrients accumulate at the surface.