Fig 1.
Locations of sampling sites and ecozones.
River locations (Grand River, ON; Mackenzie River, NT) are labelled at the mouth of the river.
Table 1.
Dissolved organic matter composition as described by chemical, absorbance, and molecular-size based measures used in this study.
Fig 2.
Compositional measures versus total DOM concentration.
Measures include A) SUVA, specific ultraviolet absorbance at 255 nm, B) S275-295, spectral slope between 275 to 295nm, C) DOC:DON, and D) HSF; humic substances fraction. Colours represent geographical sampling locations (HZ: Lake Hazen Watershed, NU; DL: Daring Lake, NT; WK: Wekweètì, NT; YK: Yellowknife, NT; MR: Mackenzie River, NT; ELA: IISD-Experimental Lakes Area, ON; TLW: Turkey Lakes Watershed, ON; NW: Nottawasaga River Watershed, ON; GR: Grand River, ON; LP: Long Point, ON; BBK: Black Brook Watershed, ON) while shapes represent hydrologic environments. Light grey circles represent two other published DOM characterization studies conducted at similar scales [16, 44].
Fig 3.
Principal component analyses for samples from different ecozones.
Grey dots represent individual samples, while vectors represent absorbance (red), elemental (green), and LC-OCD (purple) compositional measures.
Fig 4.
Composition Wheels for DOM from different hydrologic settings within each geographical site.
Axes are a numerical value normalized relative to the maximum and minimum encountered within the dataset for each parameter. Parameters for each axis found in bottom left. The orientation of the composition wheel (CW) axes are arbitrary. Different samples from the same hydrological and geographic setting are plotted within the same CW. Two ‘end member’ compositions are highlighted in sites with many samples to help visualize the continuum of DOM mixtures: groundwater-like DOM (low DOC:DON, high SUVA and HSF) or photolyzed DOM (high S275-295).
Fig 5.
Changes to a composition wheel resulting from photolytic and microbial degradation (left) and differences in terrestrial versus aquatic DOM sources (right). Changes to dissolved organic matter (DOM) (grey) with photolytic (orange; 12 to 18 day exposure to sunlight) or microbial (purple; 30-day experiment) degradation based on experimental incubations of natural DOM samples [51] (left). The mean decrease in DOC concentration was 18% during photolysis and 10% during microbial degradation experiments (n = 11, 9; respectively). Hypothetical differences in DOM sources (right) are based on various leachate experiments in literature [22, 37, 55–59].
Fig 6.
Conceptual model of DOM evolution along the aquatic continuum using Composition Wheels.
A conceptual model of a few select processes and sources of dissolved organic matter (DOM) along the aquatic continuum. Two main hydrologic flowpaths are illustrated (A-B-C and A-b-c) and originate from the same source ‘A’. However, each flowpath undergoes different processing within the environment. Arrows represent the transport or addition of DOM among groundwaters (dark purple) and surface waters (light purple), while lowercase text highlights some of the specific processes and sources encountered. The wetland ‘b’ represents an area of organic matter degradation and DOM accumulation within organic-rich groundwaters, resulting in the transport of large, aromatic DOM components into surface water ‘c’. Conversely, DOM is rapidly transported down the river ‘B’ and into a large lake ‘C’, resulting in much higher exposure to sunlight. The use of Composition Wheels to illustrate differences in sources and processing is shown on the bottom panel, comparing DOM from an organic-rich source with longer exposure to sunlight (A-B-C) versus the same DOM with less sunlight exposure and greater microbial degradation (A-b-c).
Fig 7.
Surrogate parameters for the DOM composition wheel.