Aquatic Organic Matter Fluorescence - Aquatic Organic Matter Fluorescence - page 98

3.3.5 Dissolved Organic Carbon vs. Fluorescence Relationships
A number of studies have examined relationships between fluorescence intensities of
protein-like or humic-like fluorophores and DOC concentration in freshwater ecosystems
(Baker, 2002 ; Cumberland and Baker, 2007 ; Hudson et al., 2007 ; Baker et al., 2008 ). The
fluorescence intensity versus DOC relationship has been observed to differ in gradient
(i.e., the fluorescence per gram of carbon) and in the strength of the correlation coefficient
dependent on the source of the DOC and which fluorophore is being related to DOC con-
centration (Cumberland and Baker, 2007 ; Baker et al., 2008 ). Typically, the strongest cor-
relations between fluorescence intensity and DOC have been observed at study sites where
natural DOM is dominant (i.e., sites that have little anthropogenic impact) and greater fluo-
rescence per gram of carbon is reported in samples dominated by HMW, aromatic DOM
(e.g., peat catchments, wetlands) (Cumberland and Baker, 2007 ; Baker et al., 2008 ). There
is clear potential for future studies to utilize DOC versus fluorescence intensity relation-
ships, as has been shown for CDOM absorption coefficient versus DOC relationships, to
derive fluxes of DOC from freshwater systems that have been appropriately ground-truthed
(Spencer et al., 2009b ). The prospect of deploying in situ fluorometers to derive real-time
DOC fluxes as well as improve spatial and temporal resolution is an exciting prospect,
particularly in watersheds that exhibit significant short-term export events (e.g., freshets,
storms) (Saraceno et al., 2009 ; Spencer et al., 2009b ).
3.4 Fluorescence in Groundwater
3.4.1 Introduction
Groundwater organic matter fluorescence has been investigated systematically since the
work of Smart et al. ( 1976 ). They had previously observed “background” fluorescence var-
iations during dye trace experiments in limestone aquifers, and the 1976 paper described
this natural fluorescence signal in a combination of groundwater, rain, and surface water
samples. Smart et al. observed a peak excitation at 340-350 nm, with emission spectra
exhibiting broad peaks at 400-460 nm, equivalent to what is now considered peak C. In
the experiment, samples were stored for up to 9 days in both light and dark conditions
and filtered and unfiltered to assess sample stability. Temperature dependence, pH effects,
and potential metal quenching effects were also considered. Smart et al. ( 1976 ) demon-
strated a strong correlation between fluorescence intensity and dissolved organic carbon
concentration. Although the relationship was specific to sample source, and their work was
widely cited by the wider aquatic organic matter fluorescence community (e.g., Stewart
and Wetzel, 1980 , 1981 ; Zepp and Schlotzhauer, 1982; Laane, 1982 ) but the use of fluo-
rescence to characterize and quantify organic matter in groundwater was largely forgotten
by that community.
A revitalization of interest in groundwater fluorescent organic matter came about after
the observation that cave stalagmites contain annual fluorescent laminae (Baker et al., 1993 ;