Associations Between the Molecular and Optical Properties of Dissolved Organic Matter in the Florida Everglades, a Model Coastal Wetland System.

Sasha Wagner,Rudolf Jaffé,Aron Stubbins,Kaelin Cawley,Thorsten Dittmar

doi:10.3389/fchem.2015.00066

Abstract

Optical properties are easy-to-measure proxies for dissolved organic matter (DOM) composition, source, and reactivity. However, the molecular signature of DOM associated with such optical parameters remains poorly defined. The Florida coastal Everglades is a subtropical wetland with diverse vegetation (e.g., sawgrass prairies, mangrove forests, seagrass meadows) and DOM sources (e.g., terrestrial, microbial, and marine). As such, the Everglades is an excellent model system from which to draw samples of diverse origin and composition to allow classically-defined optical properties to be linked to molecular properties of the DOM pool. We characterized a suite of seasonally- and spatially-collected DOM samples using optical measurements (EEM-PARAFAC, SUVA254, S275−295, S350−400, SR, FI, freshness index, and HIX) and ultrahigh resolution mass spectrometry (FTICR-MS). Spearman's rank correlations between FTICR-MS signal intensities of individual molecular formulae and optical properties determined which molecular formulae were associated with each PARAFAC component and optical index. The molecular families that tracked with the optical indices were generally in agreement with conventional biogeochemical interpretations. Therefore, although they represent only a small portion of the bulk DOM pool, absorbance, and fluorescence measurements appear to be appropriate proxies for the aquatic cycling of both optically-active and associated optically-inactive DOM in coastal wetlands.

Highlights

Dissolved organic matter (DOM) is an integral component of aquatic systems
Excitation and emission maxima for the current four component parallel factor analysis (PARAFAC) model are detailed in Table 1 where they are related to conventional peak assignments and FCE model components
The group of molecular formulae which tracked with Specific UV absorbance at 254 nm (SUVA254) and humification index (HIX) in the Everglades (Figures 4A,B) overlap with those identified as photolabile in other studies (Kujawinski et al, 2004; Gonsior et al, 2009; Stubbins et al, 2010)

Summary

Introduction

Dissolved organic matter (DOM) is an integral component of aquatic systems. DOM is fundamentally involved in many environmental processes, such as the binding of metals (Haitzer et al, 2002), transport of pollutants (Schwarzenbach et al, 2003), attenuation of light (Morris et al, 1995), and cycling of nutrients (Opsahl and Benner, 1997). Optical spectroscopic techniques (e.g., absorbance and fluorescence) are a quick and relatively inexpensive means for assessing DOM quality (Fellman et al, 2010; Coble et al, 2014). Optical properties have been established as efficientlymeasured proxies for DOM source and reactivity (Stedmon et al, 2003; Jaffé et al, 2008; Hernes et al, 2009). Current studies commonly use EEMs combined with parallel factor analysis (PARAFAC) to assess the environmental dynamics of DOM in diverse aquatic ecosystems (Jaffé et al, 2014). Very little is known about the optically-inactive compounds that track with chromophores and fluorophores as they enter and are processed within aquatic ecosystems

Methods

Results

Discussion

Conclusion