No abstract

We measured the organic carbon (OC) content, the isotopic composition, the C/N ratios and the photosynthetic pigment composition of suspended matter, sediments, sediments traps and epiphytic and epilithic biofilms at the tropical Sinnamary River system (French Guiana). Our sampling included the mid-stream reservoir lake (Petit Saut) and the estuary on the Atlantic coast. These tracers were complementary and allowed identifying different sources of particulate organic matter (POM) in the system. We found a d 13 C-C/N signature of POM close to that of soils and litters collected in the surrounding forest, both for water column and sediment at the upstream station as well as for the sediment in a littoral zone of the reservoir, which thus indicated a terrestrial origin. Plankton communities at the centre of the reservoir were dominated by Chlorophyceae (chlorophyll a (Chl a), chlorophyll b (Chl b) and lutein) in the oxic epilimnion and by anoxygenic phototrophic bacteria, namely Chlorobiaceae (bacteriochlorophyll d (BChl d) and bacteriochlorophyll c (BChl c)) at and below the oxycline (6 m depth). In addition, this planktonic material was slightly 13 C-depleted due to a contribution of methanotrophic bacteria. Phytoplankton and bacterioplankton were the major source of settling material collected in the traps at all depths in the centre of the reservoir. In the traps, POM was subject to intense degradation, as revealed by C/N and isotopic data and by the presence of pheopigments. In the river downstream of the dam, Chl b, lutein, BChl c and d originating from the reservoir progressively decreased downstream as the result of mineralization. At the estuarine mouth, fucoxanthin showed the presence of diatoms and the d 13 C-C/N signature matched that of POM carried by the Amazonian coastal mobile mud belt. By analysing sedimentation rates in the reservoir and its outflow into the river, we were able to provide a first estimates of POM transfers in this system during the sampling period. POM IN A TROPICAL RESERVOIR 255 Figure 2. Typical chromatograms at 440 and 664 nm showing most of the pigments quantified in the Petit Saut samples. (A) Surface sediment at Station 1; (B) suspended matter at À6 m depth at Station 4 A. DE JUNET ET AL. Ã Indicates that lutein and BChl d coeluted in the samples and lutein concentrations were corrected from the BChl c and d concentration as detailed in the text.