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. Many rivers and estuaries are net heterotrophic as indicated by CO2 supersaturation and efflux, which in turn indicates that terrestrial DOM is being respired at high rates. High rates of respiration imply that terrestrial DOM undergoes structural degradation and loss of source specificity. Thus, without a better understanding of the chemical and photochemical alterations that riverine components undergo in estuaries, we may be incorrectly interpreting the chemical signatures (i.e., source specificity) and optical properties of DOM ultimately discharged to the coastal zone.
In this study, researchers at the Old Dominion University will attempt 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, the investigators expect 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.
