PROJECT SUMMARY
EFFECTS OF PHOTODEGRADATION
ON THE COMPOSITION, OPTICAL PROPERTIES AND BIOAVAILABILTY OF DOM IN ESTUARIES
Drs. Liz Minor
and Ken Mopper
Funded by the
NSF
River-transported dissolved organic matter (DOM) is
the main source of terrestrial dissolved organic carbon in the sea. Chemical,
photochemical and microbial processes that occur in estuaries can dramatically
alter the molecular composition, reactivity and optical properties of riverine
DOM before it is discharged into the oceans.
The main goal of this study is to
quantify variations in the molecular size distributions, molecular-level
characteristics, microbial availability, and optical properties of riverine and
estuarine DOM as a function of light exposure and initial DOM composition.
The proposed experiments will test the following hypotheses:
1) Photochemical alteration of
aquatic DOM will lead to measurable shifts in molecular-weight distributions
and functional group composition. The
types and magnitudes of the shifts will depend upon the chemical characteristics
of the initial DOM.
2) Photochemical alterations of
the DOM will change its bioavailability.
Whether the bioavailability increases or decreases will depend upon the
chemical characteristics of the initial and photochemically altered material.
A multiple molecular-level approach, including
direct temperature-resolved mass spectrometry (DT-MS), pyrolysis-gas
chromatography-mass spectrometry (PyGCMS), Fourier transform infrared
spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR), will be
used to characterize light-mediated changes in molecular-level composition
within estuarine DOM and representative source materials believed to contribute
significantly to estuarine DOM. DT-MS
and PyGCMS will be performed on both ultrafiltered size fractions (UDOM) and
size-exclusion chromatography (SEC) fractions isolated from the water samples.
Changes in DOM structure, composition, functional groups and oxidation as
revealed by spectroscopic, chromatographic, and MS techniques will then be
related to photochemically induced changes in DOM bioavailability
(incorporation and respiration).
This study will be the first attempt to evaluate in
detail the role of photochemical degradation/alteration processes on modulating
the composition, microbial availability, and optical properties of DOM
discharged from rivers and estuaries to ocean margins. By combining mass
spectrometry, FTIR, NMR, and optical property measurements with microbial
availability determinations, we should be able to identify the functional
groups and higher order structures responsible for varying both the optical
properties and the bioavailability of the DOM pool. The use of SEC and ultrafiltration to provide
information on DOM size will indicate whether the size spectrum shifts
significantly during photodegradation and whether this shift impacts DOM
bioavailability.