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This article is part of the Spring 2022 issue of the Earth Science Matters Newsletter.

study region map of watersheds in the Upper Mississippi River Basin
Figure 1. Study region, land cover, and sampling sites (white dots). The streams/rivers and their watersheds (outlined in black) are: (A) Allequash Creek (forest), (B) Flambeau River (forest), (C) Chippewa River (forest), (D), Como Creek (agriculture), (E) Trout Creek (agriculture), (F) Red Cedar River (agriculture), (G) Bassett Creek (urban), (H) Minnehaha Creek (urban), (I) Shingle Creek (urban). From Fig.1 in (Vaughn et al., 2021)

Across the United States, lands historically covered by forests and wetlands have been converted for agricultural uses and urban population centers. One impact of these land use changes is altered water quality in streams and rivers that drain affected watersheds due to modified vegetation, soils, hydrology, and management practices. Streams and rivers are vitally important for human and ecosystem health and services. As such, improved understanding of the effects of land use change on various aspects of water quality is needed.

One such aspect of water quality is dissolved organic matter (DOM). DOM is composed of a group of complex chemical compounds that includes dissolved organic carbon (DOC), and nitrogen- and sulfur-containing organic compounds that are foundational in aquatic nutrient cycling and food webs. DOM is delivered to streams and rivers through runoff and groundwater from the surrounding terrestrial environment. It can also be produced within streams and rivers by microbes and aquatic vegetation.

To better understand the impacts of human land use on stream and river DOM, research scientists from USGS and Florida State University measured and compared DOM molecular composition of small streams and rivers draining predominantly forest, agriculture, or urban land use in the Upper Mississippi River Basin. Nine streams and rivers with varying drainage areas were selected based on the dominant land-cover classification in their watersheds (determined from the 2016 National Land Cover Database; figure 1).

graph of dissolved organic matter molecular diversity by land use type
Figure 2. Dissolved organic matter molecular diversity, as defined by number of formulae identified in samples, is significantly higher in streams draining agricultural and urban watersheds compared to streams draining forested watersheds. Solid circles and the thick horizontal lines represent the mean and median for each land use type. From Fig. 2 in (Vaughn et al., 2021)

Three streams were in the Minneapolis-St. Paul metropolitan area of Minnesota (“urban”), and six streams/rivers were in rural Wisconsin: three in agricultural areas (“agriculture”) and three in forest-wetland areas (“forested”) having very little urban development. Water samples were collected several times over one and a half years targeting the full range of seasons (spring, summer, fall, winter). Samples were analyzed for DOC concentration using a standard carbon analyzer, and for DOM molecular composition using an ultrahigh-resolution analytical technique capable of identifying the presence of tens of thousands of chemical formulae, providing a unique fingerprint of DOM for each stream sample.

Clear and distinct differences in the molecular composition, or fingerprint, of DOM were evident in streams and rivers draining different land use types. First, the molecular richness of DOM (defined by the number of different chemical formulae identified) was significantly higher for agricultural and urban streams compared to forested streams (figure 2). Second, of the 27,400 formulae identified across all samples, the majority (~76%) were associated with just agricultural and urban land uses (figure 3). In other words, certain compounds were more likely to be found in some land use types than the others. For example, in comparison to forest stream DOM, agricultural stream DOM had higher associations with nitrogen-containing compounds, and urban stream DOM was enriched in sulfur-containing compounds,

These results reveal how land use imparts a chemical fingerprint that is carried in stream and river dissolved organic matter. The components of DOM, organic carbon and nutrients, play important roles in eutrophication, hypoxia, binding of metals for downstream transport, and production and emission of greenhouse gases. Knowing what specific chemical compounds make up the fingerprints of land use helps us understand how land management activities impact water quality, and the subsequent implications for carbon and nutrient cycling in streams and rivers impacting ecosystem and human health.

correlations between relative abundance of molecular formulae and land use in watershed
Figure 3. Spearman-rank correlations between the relative abundance of assigned molecular formulae and (b) percent forest in the watershed, (c) percent agriculture in the watershed, (d) and percent urban in the watershed. Axes represent the atomic ratios of Hydrogen to Carbon (H/C) and Oxygen to Carbon (O/C) of each compound. Colors represent the correlation coefficient between the relative abundance of each formula and percent land use, with warmer color indicating positive correlations and cooler color indicating negative correlations. Meaning warmer points represent compounds that were more likely to occur in a given land use type. These resulting graphs represent the unique fingerprint of Dissolved Organic Matter types in certain land use types. Lines approximately delineate compound groups (CA = condensed aromatic compounds; HUP = highly unsaturated and phenolic compounds; PP = polyphenolic compounds). From Fig. 6 in (Vaughn et al., 2021)

The paper, “Anthropogenic landcover impacts fluvial dissolved organic matter composition in the Upper Mississippi River Basin”, was recently published in the journal Biogeochemistry.

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