Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Nicola A Mcenroe,Paul C Frost,Petr Porcal,Clayton J Williams,Marguerite A Xenopoulos,Tomoya Iwata

doi:10.1371/journal.pone.0080334

Nicola A Mcenroe, Paul C Frost + Show 4 more

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https://doi.org/10.1371/journal.pone.0080334

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Abstract

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L-1 across the ponds with an average value of 5.3 mg C L-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.

Highlights

In the last few decades, urbanization has been associated with widespread loss of natural and agricultural areas [1]
While parallel factor analysis (PARAFAC) component C6 was generally the most abundant (23 - 52%) and C4 was the least abundant (2 - 5%), the relative abundance of the individual components varied among ponds (Figure 3)
We examined the quantity and chemistry of dissolved organic matter (DOM) in ponds embedded within highly urbanized environments and report the change in its optical chemistry after irradiation

Summary

Introduction

In the last few decades, urbanization has been associated with widespread loss of natural (wetlands and forest) and agricultural areas [1]. This landscape conversion has been accompanied by greater imperviousness of watersheds to infiltration by surface waters and altered hydrological cycles (e.g., [2,3]). One study based on 26 urban stormwater ponds reported high rates of microbial activity and biogeochemical processes suggesting that, at current pond density, they could play an important role in regional and global carbon (C) cycles [7]. Shallow freshwater systems are typically characterized by disproportionately high rates of nutrient processing compared to larger bodies of water [8]. Allochthonous derived DOM has been shown to fuel surprising quantities of microbial production in small aquatic ecosystems and to support freshwater food webs beyond that provided by primary production alone (e.g., [12,13,14,15])

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