Spatial Variation in the Optical Properties of Dissolved Organic Matter (DOM) in Lakes on the Canadian Precambrian Shield and Links to Watershed Characteristics

Abstract

In the present study, we explored the use of various optical parameters to detect differences in the composition of the dissolved organic matter (DOM) in a set of lakes that are all located on the Canadian Precambrian Shield, but within which Cu and Ni speciation predictions were previously shown to diverge from measured values in some lakes but not in others. Water samples were collected with in situ diffusion samplers in 2007 (N = 18 lakes) and 2008 (N = 8 lakes). Significant differences in DOM quality were identified between the sampling regions (Rouyn-Noranda, Québec and Sudbury, Ontario) and among lakes, based on dissolved organic carbon concentrations ([DOC]), specific UV absorbance (SUVA 254 ), fluorescence indices (FI), and excitation-emission matrix (EEM) fluorescence measurements. Parallel factor analysis (PARAFAC) of the EEM spectra revealed four components, two of which (C3, oxidized quinone fluorophore of allochthonous origin, and C4, tryptophan-like protein fluorescence of autochthonous origin) showed the greatest inter-regional variation. The inter-lake differences in DOM quality were consistent with the regional watershed characteristics as determined from satellite imagery (e.g., watershed-to-lake surface area ratios and relative percentages of surface water, rock outcrops vegetative cover and urban development). Source apportionment plots, built upon PARAFAC components ratios calculated for our lakes, were used to discriminate among DOM sources and to compare them to sources identified in the literature. These results have implications for other areas of research, such as quantifying lake-to-lake variations in the influence of organic matter on the speciation of trace elements in natural aquatic environments.

Trace metal speciation predictions in natural aquatic systems: incorporation of dissolved organic matter (DOM) spectroscopic quality

“…Although a detailed description of the study sites can be found in Mueller et al [20] a brief description is given here. Lake water was collected from lakes in two different regions on the Canadian Shield: near Rouyn-Noranda in north-western Quebec and near Sudbury in north-central Ontario.…”

“…pH and concentrations of DOC and major cations) is known to be minimal within many of the sampled lakes. [3] Lake water was collected passively using equilibrium diffusion sampling as described in Fortin et al [3] and Mueller et al [20] Briefly, 250-mL polypropylene jars (Nalgene, Nalge Nunc International Corporation, Rochester, NY) topped with custom-made plastic lids fitted with a 0.2-mm filter membrane (HT Tuffryn Membrane, Pall Canada Ltd, Ville St-Laurent, QC, Canada) were filled with Milli-Q (Millipore, EMD Millipore, Billerica, MA) ultrapure water (.18 MO cm) and fixed to plastic rods that were anchored to the lake bottom. The samplers were installed at a depth of 1 m from the lake surface and left to equilibrate for 13 to 14 days.…”

“…Details concerning the quantitative and qualitative characterisation of DOM can be found in Mueller et al [20] Briefly, 20-mL subsamples were transferred to amber borosilicate vials (I-Chem Brand, Thermo Fisher Scientific, Rockwood, TN) for total organic carbon analysis (TOC-V CPH , Shimadzu Scientific Instruments, Columbia, MD). Subsamples (1 mL) were transferred to 3-mL glass vacutainers (Kendall Monoject, Mansfield, MA) for total inorganic carbon analysis by gas chromatography (Varian 3800 GC, Agilent Technologies Canada Inc.).…”

“…[major cations], pH) of the lakes was described in Mueller et al [20] for 2007 and 2008. Only the 2008 data are considered here, since the metal speciation measurements date from that year.…”

“…Wide ranges in general water chemistry (pH, Ca, Mg and DOC concentrations) were found among the eight lakes sampled (Table 1), reflecting both regional geological differences and local differences at the watershed scale. [20] The lakes in both sampling regions have low concentrations of calcium ([Ca]: 68 to 464 mM) and magnesium ([Mg]: 27 to 274 mM), contributing to generally soft water. Lake Bethel, from the Sudbury region, had the highest pH and the highest Ca and Mg concentrations of all the lakes; for the IET measurements, all other lake water samples were thus adjusted upwards to these Ca concentrations (see Ion exchange technique (IET) above).…”