Lakes fulvic acid

According to their acidity, humic substances are hydrophobic and are split into two groups humic and fulvic acids. Humic acids are stable molecules originating from the ageing of organic matter. They are responsible for water coloration and represent 40-60% of natural organic matter in rivers and lakes. Fulvic acids are smaller than the humic acids and are generally less aromatic than humic acids extracted from the same pool of DOM. 

South Fork fulvic acid Castle Lake fulvic acid 5.5 2.8 ... 

Figure 9.13. Seasonal variation in (a) chlorophyll a, (b) DOC, (c) percentage fulvic acid, (d) whole water (solid line) and fulvic acid (dashed hue) McKnight Fluorescence Index (FI), and (e) whole water redox index (RI) at the outlet to Green Lake 4, an alpine lake in the Colorado Rocky Mountains. The vertical dashed lines indicate the sustained rain event, and the shaded areas represent the timing of the peak in chlorophyll a at the outlet. PLFA represents the FI of the Pony Lake fulvic acid, a microbial end member, and SRFA represents the FI of the Suwannee River fulvic acid, a terrestrial end member. (Adapted from MiUer et al., 2009.)...

DOM is derived from autochthonous sources such as phytoplankton and photosynthetic bacteria (16) at Big Soda Lake near Fallon, Nevada. This lake is alkaline (pH 9.7) and chemically stratified. It contains DOC concentrations as high as 60 mg/L and dissolved salt concentrations as high as 88,000 mg/ L (17). The DOM in this lake is colorless. The fulvic acid fraction was isolated by adsorption chromatography (Amberlite XAD-8 resin) (18) and by zeo-trophic distillation of water from N,N-dimethylformamide (19). Average molecular model synthesis was achieved in a manner similar to that used for fulvic acid from the Suwannee River. The characterization data are presented in Table I and the structural model is presented in Structure 2. 

Table I. Molecular Data Used for Average Structural Model of Fulvic Acid from Big Soda Lake...

Structure 2. One of several average structural models of fulvic acid from Big Soda Lake, Nevada. 

Fulvic acid was isolated in Big Soda Lake above and below the chem-ocline, which occurs at 34-m depth. Water near the lake surface has moderate salinity and is oxygenated, whereas water below the chemocline is hypersaline and anoxic (17). In spite of these environmental differences the chemical character of the fulvic acid from above or below the chemocline did not vary, as determined by elemental analyses and NMR spectrometry. 

The 14C age determination of the fulvic acid isolated rom water near the lake surface was 2300 years before the present, whereas the 14C age was 4900 years before the present for the fulvic acid isolated from water below the chemocline. These old ages for both fulvic acids from Big Soda Lake are in marked contrast to that reported for fulvic acid from the Suwannee River, less than 30 years before the present (11). The refractory nature of this type of fulvic acid derived from phytoplankton and photosynthetic bacteria is significant for carbon-cycling studies. 

Fulvic acid from the Suwannee River was used to calibrate peak-height ratios for aromatic carbon content. The application of this method to fulvic acid samples with known aromatic plus olefinic carbon content from various environments is shown in Table II. Aromatic plus olefinic carbon percentages calculated by the peak-height ratio method using lH NMR data closely agree with these percentages computed from 13C NMR data, with the exception of the Big Soda Lake samples. 

H NMR data from these seven sites are presented by spectral peak-height ratios in Table III. The sites were listed in order of increasing aromatic plus olefinic carbon percentages. Fulvic acids from all the lake samples are much lower in aromatic plus oleflnic carbon content than those from river samples. These results confirm the hypothesis that autothonous inputs result in dissolved humic substances that have a low aromatic plus oleflnic carbon content. The lake samples also are lower in the ratios of peak 2 (carboxylated chains and aliphatic ketones), peak 3 (carbohydrates), and peak 4 (phenolic tannins and lignins) to peak 1 (branched methyl groups and alicyclic ali-phatics) than are the river samples. 

Fulvic acid isolated from Island Lake provided direct spectral evidence of protons attached to aliphatic alicyclic rings. The H NMR spectrum of this sample (Figure 3) has a broad medium-intensity peak near 1.8 ppm. The chemical shifts of protons on aliphatic alicyclic rings are commonly split into two broad peaks centered at 1.1 and 1.8 ppm because of their variable boat and chair configurations (23). 

The Sagavanirktok River is intermediate in aromatic carbon content. This river drains the bogs on the Arctic tundra (allochthonous inputs) and several lakes (autochthonous inputs). Samples from the Suwannee and Calcasieu rivers are very similar in peak-height ratios with the exception of peak 2 1 ratio, which is much lower for the Calcasieu River sample. The data in Table II indicate that the Calcasieu River fulvic acid has a greater ring content... 

Fulvic acid from Island Lake near Oshkosh, Nebraska 0.565 0.232 0.022 0.050 0.44 9.5... 

Fulvic acid from Hidden Lake Creek near Soldotna, Alaska 0.596 0.184 0.046 0.056 0.80 10.7... 

The stable isotopes 13C and 15N as well as the radioactive isotope 14C have the potential to serve as tracers for sources of fulvic acid if there is sufficient difference in isotopic signature between the possible organic sources in the catchment of a lake, stream, or wetland. The measurement of 13C is particularly attractive because the measurement is relatively inexpensive and can be very precise. However, using 13C measurements to successfully resolve sources can be difficult because of the overlap of 13C values for different DOC precursor materials (Schiff et al., 1997 Raymond and Bauer, 2001). The measurement of 13C is reported as 813C in parts per thousand (%o),... 

Measurements of 14C have rarely been used as tracers to study sources of individual DOC fractions. Aiken et al. (1996) used 14C measurements of fulvic acid in a permanently ice-covered closed basin lake in Antarctica to investigate sources of DOC in the lake. Sampling along a depth profile in the lake showed that the fraction of modern carbon decreased with depth and confirmed that the high concentrations of fulvic acid in the bottom waters represented an old source, with an age of around 3000 years, which was diffusing into the upper water column (Fig. 3). 

FIGURE 3 Depth profiles for 14C age and sulfur-to-carbon (S/C) ratios for fulvic acid samples isolated from Lake Fryxell, a permanently ice-covered lake in the McMurdo Dry Valleys, Antarctica (from Aiken et al., 1996). In the upper water column, the fulvic acids have a modern signal. This young fulvic acid derives from perennial algal mats in glacial meltwater streams. At depth, the fulvic acids are quite old (about 3000 years) and are possibly derived from organic material in sediments. The ratio of S/C increases with depth as conditions in the water column become anoxic. 

We found consistent differences in the fluorescence properties of fulvic acids from streams where fulvic acids are terrestrially derived and from lakes where fulvic acids are microbially derived (McKnight et al., 2001). The upper maximum in microbially derived samples is more sharply defined and... 

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