Inferring Ecosystem Function from Dissolved Organic Matter Optical Properties: A Critical Review

Abstract

Over the last 30 years, the optical property community has shifted from conducting dissolved organic matter (DOM) measurements on new complex mixtures in natural and engineered systems to furthering ecosystem understanding in the context of past, present, and future carbon (C) cycling regimes. However, the appropriate use of optical properties to understand DOM behavior in complex biogeochemical systems is of recent debate. This critical review provides an extensive survey of DOM optical property literature across atmospheric, marine, and terrestrial biospheres using a categorical approach that probes each biosphere and its subdivisions. Using this approach, a rubric of ecosystem variables, such as productive nature, C cycling rate, C inputs, and water quality, sets the foundation for interpreting commonly used optical property DOM metrics such as fluorescence index (FI), humification index (HIX), and specific ultraviolet absorbance at 254 nm (SUVA₂₅₄). Case studies and a meta-analysis of each biosphere and subdivision found substantial overlap and characteristic distributions corresponding to ecosystem context for FI, HIX, and SUVA₂₅₄, signifying chromophores and fluorophores from different ecosystems may be more similar than originally thought. This review challenges researchers to consider ecosystem connectivity when applying optical property results rather than making traditional "if this, then that" results-style conclusions.

Keywords: Absorbance; DOM; atmospheric; carbon quality; chromophores; fluorescence; fluorophores; marine; reactivity; source material; spectroscopy; terrestrial.

Figure 1

Schematic diagram of biosphere dissolved organic matter (DOM) pool categories (atmospheric, marine, and terrestrial) and subdivisions of each accompanied by their defining ecosystem characteristics from the perspective of carbon cycling. Ecosystem characteristics include relative terms for productive nature, carbon cycling rate, donor control (relating to inputs received) and extent, and water quality (WQ). Note: subdivision categories that share relatively similar ecosystem characteristics are grouped together (e.g., lakes and ponds).

Figure 2

Atmospheric biosphere subdivision box plots and distributions of their dissolved organic matter calculated optical properties of (a) fluorescence index, (b) humification index, and (c) Specific UV Absorbance at λ = 254 nm (SUVA₂₅₄). The number of data points is indicated next to each distribution (n = #). Data is presented for subdivisions in which peer-reviewed values were available.

Figure 3

Marine biosphere subdivision bos plots and distributions of their dissolved organic matter calculated optical properties of (a) fluorescence index, (b) humification index, and (c) Specific UV Absorbance at λ = 254 nm (SUVA₂₅₄). The number of data points is indicated next to each distribution (n = #). Data is presented for subdivisions in which peer-reviewed values were available.

Figure 4

Terrestrial biosphere subdivision box plots and distributions of their dissolved organic matter calculated optical properties of (a) fluorescence index, (b) humification index (HIX), and (c) Specific UV Absorbance at λ = 254 nm (SUVA₂₅₄). The number of data points is indicated next to each distribution (n = #). Data is presented for subdivisions in which peer-reviewed values were available.

Figure 5

Box plots and distributions of (a) fluorescence index (FI), (b) humification index (HIX), and (c) specific UV absorbance at λ = 254 nm (SUVA₂₅₄) for dissolved organic matter (DOM) in each biosphere including atmospheric, marine, and terrestrial. The conceptual diagram in the upper right-hand corner depicts the environmental biogeochemical gradient and subsequent changes to FI, HIX, and SUVA₂₅₄. DOM shifts to a lower FI, more humified (high HIX), and higher SUVA₂₅₄ as it becomes more processed by various environmental aspects along the gradient. Typical endmember DOM isolates, Suwannee River I and Pony Lake Fulvic Acids (SRFA and PLFA), overlay each distribution plot.^,,,−

Figure 6

Humification index plotted with fluorescence index for (a) each dissolved organic matter (DOM) biosphere and corresponding biosphere subdivisions: (b) atmospheric, (c) marine, and (d) terrestrial. Each polygon represents an area in which peer-reviewed, coupled FI and HIX values span. Biosphere categories and subcategories are referenced from Figure 1, and data is organized in Table S1.