A multi‐tracers analysis of sources and transfers of particulate organic matter in a tropical reservoir (Petit Saut, French Guiana)

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.

Seasonal occurrence of anoxygenic photosynthesis in Tillari and Selaulim reservoirs, Western India

“…Such observations have previously been reported from the Black Sea and other anoxic zones (Sinninghe Damste et al, 1993;Chen et al, 2001;Squier et al, 2002). A recent study of pigment composition in Petit Saut, a tropical reservoir in French Guiana, showed the dominance of Chlorophyceae (Chl a, Chl b and lutein) in the oxic epilimnion, and anoxygenic photo-autotrophic bacteria below the oxycline as revealed by BChl c and BChl d (Junet et al, 2009). An example of how the community structure of phytoplankton is dependent on nutrient availability is provided by the results of Zhu et al (2010) from Taihu Lake (China) where Chlorophyta were found to be predominant at high concentrations of nitrogen and phosphorus, whereas cyanobacteria dominated in waters having low concentrations of these nutrients.…”

Do models of organic carbon mineralization extrapolate to warmer tropical sediments?

“…The x-and y-axes representing OC mineralization rates are on a log scale. perimeter ratios; and the quality of the OC available for mineralization, which is potentially more processed due to partial mineralization prior to entry into the reservoir (De Junet et al 2009). However, it is important to highlight that reservoirs had generally lower temperature ranges than other systems (Fig.…”

Organic carbon burial efficiency in a subtropical hydroelectric reservoir

“…The usually high nutrient availability of fluvial inflow and the gradual decrease in turbulence towards the dam lead to rivers-dam gradients of, for example, sediment accumulation rates and sedimentary OC sources in reservoirs (De Junet et al, 2009). The combined effect of an increase in the relative contribution of more labile autochthonous-derived OC (further discussed below) and a decreasing sediment accumulation rate result in a decrease in OCBE towards the dam (p = 0.01; r = 0.69).…”

“…While spatial variation in CO 2 emissions from reservoirs' surface to the atmosphere have been described (Roland et al, 2010), the effect of heterogeneity on the sediment-carbon flows is unknown, even though reservoir heterogeneity is clearly reflected in sediment characteristics (De Junet et al, 2009). The main purpose of this study was to determine the OCBE in a subtropical hydroelectric reservoir, and to evaluate the importance of variable factors driving it.…”

“…Organic carbon in hydroelectric reservoirs can originate from several sources (De Junet et al, 2009). First, carbon derived from flooded soil and vegetation may fuel the system's metabolism for long periods, leading to elevated greenhouse gas (GHG) emissions, especially during the initial ∼ 10 years after damming (Abril et al, 2005;Tremblay et al, 2004;Barros et al, 2011).…”