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

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.

Keywords: EEM-PARAFAC; Florida coastal Everglades; absorbance; dissolved organic matter; fluorescence; subtropical wetland; ultrahigh resolution mass spectrometry.

Figure 1

Everglades sampling sites along Shark River Slough (SRS2, SRS4, SRS6) and Taylor Slough (TS2, TS7, TS10). Gray arrows show the flow of water along the Shark River Slough, which flows from the north to the southwest into the Gulf of Mexico, and Taylor Slough, which flows south into Florida Bay.

Figure 2

EEMs of PARAFAC components for Everglades DOM. Fluorescence spectra are labeled with conventional peaks A_C, C, A_C+, C+, A_M, M, B, and T (defined in the text).

Figure 3

Van Krevelen distributions of molecular formulae positively correlated with each Everglades PARAFAC component (corresponding EEMs are shown in Figure 2).

Figure 4

Molecular formulae positively correlated with (A) SUVA₂₅₄, (B) HIX, (C) FI, and (D) freshness index.

Figure 5

Molecular formulae positively correlated with (A) S_R, (B) S_275−295 and (C) S_350−400, and negatively correlated with (D) S_R, (E) S_275−295, and (F) S_350−400. Panels (B) and (C) show formulae assigned to mass spectral peaks which become more abundant as spectral slopes became steeper. Panels (E) and (F) show formulae assigned to mass spectral peaks which become more abundant as spectral slopes became shallower.

Figure 6

A heat map of the number of molecular formulae which tracked positively with multiple PARAFAC components and/or optical indices. Blocks are colored along a gradient from blue (few or no shared formulae) to red (many shared formulae). Red blocks indicate that very similar molecular families are associated with two optical properties.

Figure 7

Molecular mass (first column), H/C ratio (second column) and modified aromaticity index (AI-mod; third column) distributions of formulae associated with optical properties. Plots (A–C) show the distributions of formulae which positively correlated with PARAFAC components. Plots (D–F) show the distributions for optical properties which track with allochthonous or terrestrial-type DOM (formulae positively correlated with SUVA₂₅₄, HIX and formulae negatively correlated with S_275−295, S_350−400, and S_R). Plots (G–I) show the distributions for optical properties which track with autochthonous or microbial-type DOM (formulae positively correlated with FI, freshness index, S_275−295, S_350−400, and S_R).