Dissolved Organic Matter Quality and Bioavailability Changes Across an Urbanization Gradient in Headwater Streams

Abstract

Landscape urbanization broadly alters watersheds and stream ecosystems, yet the impact of nonpoint source urban inputs on the quantity, quality, and ultimate fate of dissolved organic matter (DOM) is poorly understood. We assessed DOM quality and microbial bioavailability in eight first-order Coastal Plain headwater streams along a gradient of urbanization (i.e., percent watershed impervious cover); none of the streams had point source discharges. DOM quality was measured using fluorescence excitation-emission matrices (EEMs) coupled with parallel factor analysis (PARAFAC). Bioavailability was assessed using biodegradable dissolved organic carbon (BDOC) incubations. Results showed that watershed impervious cover was significantly related to stream DOM composition: increasing impervious cover was associated with decreased amounts of natural humic-like DOM and enriched amounts of anthropogenic fulvic acid-like and protein-like DOM. Microbial bioavailability of DOM was greater in urbanized streams during spring and summer, and was related to decreasing proportions of humic-like DOM and increasing proportions of protein-like DOM. Increased bioavailability was associated with elevated extracellular enzyme activity of the initial microbial community supplied to samples during BDOC incubations. These findings indicate that changes in stream DOM quality due to watershed urbanization may impact stream ecosystem metabolism and ultimately the fate of organic carbon transported through fluvial systems.

Effects of human land use on the terrestrial and aquatic sources of fluvial organic matter in a temperate river basin (The Meuse River, Belgium)

“…More specifically, human disturbance commonly alters fluvial OM toward a more microbial/algal and less plant/soil-derived character. This pattern was first observed for DOM (Cronan et al 1999;Giling et al 2014;Graeber et al 2012;Hosen et al 2014;Masese et al 2016;Parr et al 2015;Williams et al 2010Williams et al , 2016Wilson and Xenopoulos 2009;Yang et al 2012), and recently extended to POM (Boëchat et al 2014;Duan et al 2014;Le Meur et al 2017;Lu et al 2014a, b;Ngugi et al 2016).…”

“…Greater inputs of microbial/algal components in the DOM pool in human-impacted catchments were evidenced by highest FI and b:a ratios (McKnight et al 2001;Williams et al 2010) and by the higher contribution of C3, C5 and C6 components. C3 and C5 components have been previously found to occur in catchments under agricultural influence and are both associated with biological activity and transformation of terrestrial OM (Stedmon and Markager 2005;Williams et al 2010Williams et al , 2013Yamashita et al 2010) while C6 is referred as an anthropogenic pool of organic molecules generated in streams that trend to occur mainly in urbanized aquatic ecosystems (Hosen et al 2014;Williams et al 2016). The preferential occurrence of these components in agricultural or urban catchments is confirmed by our study as %C3 and %C5 were better correlated with croplands while %C6 was better correlated with urban areas (Table 4).…”

“…Impact of agricultural practices on the terrestrial sources of fluvial OM Gradual loss of aromatic plant/soil-derived OM along a gradient of human disturbance-evidenced for example by the decreasing relative contribution of aromatic components C1, C2 and C4-can result from several processes such as (1) disconnection from terrestrial sources in urban areas (Hosen et al 2014), (2) difference in litter quality inputs between forest leaves and crop residues, and (3) greater photodegradation of aromatic molecules due to reduced canopy in urban and agricultural catchments (McEnroe et al 2013). In parallel, an increase in microbial activity and/or algal primary production in human-impacted aquatic ecosystems due to higher nutrient content in waters and greater sunlight exposure as a result of reduced canopy is often invoked as the main processes leading to the enrichment in microbial/algal components (Masese et al 2016;McEnroe et al 2013;Williams et al 2010;Wilson and Xenopoulos 2009;Yang et al 2012).…”

“…7). C6 is associated with the biological production of DOM within aquatic ecosystems and is generally described as being of anthropogenic origin because it seems to be identified primarily in urban catchments (Hosen et al 2014;Parr et al 2015;Williams et al 2016). The absence of C6 in groundwaters (Fig.…”

“…Reduction of inputs of terrestrial OM due to hydrological disconnection with terrestrial sources and/or in-stream removal and greater in-stream primary production promoted by increasing light penetration and nutrient availability have been invoked as the main mechanisms interacting collectively to explain this pattern (Hosen et al 2014;McEnroe et al 2013;Parr et al 2015;Wilson and Xenopoulos 2009). However, because DOM and POM are rarely investigated together, further studies are required in order to assess if human land use impacts systematically and similarly the autochthonous production of these OM pools as well as potential exchange between them.…”