Grantee Research Project Results
The Role of DOC in the Degradation of Sedimentary Organic Matter: A Laboratory Incubation ExperimentEPA Grant Number: U914944
Title: The Role of DOC in the Degradation of Sedimentary Organic Matter: A Laboratory Incubation Experiment
Investigators: Hee, Carol A.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1996 through July 23, 1998
Project Amount: $68,000
RFA: STAR Graduate Fellowships (1996) RFA Text | Recipients Lists
Research Category: Fellowship - Oceanography , Fellowship - Aquatic Ecology and Ecosystems , Academic Fellowships
The overall goal of this research project is to determine the role of porewater dissolved organic carbon (DOC) in the degradation of organic matter. The specific objectives of this research project are to: (1) measure water column dissolved organic nitrogen (DON) concentrations as a function of season and location; (2) determine concentrations of major subpools of DON; and (3) determine rates of bacterial utilization of DON. The main question addressed in this research project is whether DOC is a reactive intermediate in the degradation of particulate organic carbon to metabolic end products. This research project will test two hypotheses: (1) DON is a significant component of the nitrogen pool in the Neuse River Estuary; and (2) transformations involving DON play a major role in the nitrogen cycle.
DON concentrations will be determined for surface, middepth, and bottom-water samples collected monthly from six long-term water-quality monitoring sites along a salinity gradient in the Neuse River. Total dissolved nitrogen concentrations will be measured by high-temperature combustion and chemiluminescent detection. NH4+ and NO3- plus NO2- will be determined spectroscopically. DON will be determined as the difference between the total and inorganic pools. Although much of DON is uncharacterized, amino acids and humic substances have accounted for the largest identifiable components of bulk DON. To quantify the contribution of dissolved free amino acids and dissolved combined amino acids, I propose to measure both pools using an o-phthalaldehyde derivatization technique. The contribution of humic substances to the DON pool will be determined using resins, which separate humic substances based on hydrophobicity. In addition, ultrafiltration will be used to investigate the molecular weight distribution of DON.
Remineralization of DON by heterotrophic bacteria releases ammonia, which can stimulate primary production. However, because most studies of bacterial utilization have focused on the uptake of the few identifiable constituents of the DON pool, the biological availability of the bulk DON pool remains unquantified. I propose to examine uptake of amino acids and size fractions of the DON pool using incubation techniques. Efforts during Year 1 of the project will focus on determining the spatial and seasonal variations of DON concentrations. This information will provide the foundation for the determination of appropriate times and study sites for Objectives 2 and 3.
Slurried sediment from Cape Lookout Bight, NC, was either maintained sulfate reducing or permitted to go methanogenic. Phytoplankton with a distinct isotopic signature (del 13C= -32) was added to half of these slurries. The distribution of organic carbon among the various carbon pools was determined by measuring concentrations of POC, DOC, organic acid, dissolved inorganic carbon (DIC), CO2 (g) and CH4 (g) on subsamples taken throughout a year. Additionally, POC and DOC stable carbon isotopic compositions were determined. Thus, the transformation from phytoplankton particles (i.e., POC to phytoplankton-derived DOC, CO2 [g], and CH4 [g]) could be monitored by following concentration and isotope changes.
This research project will lead to a more complete nitrogen budget, and will provide a better understanding of the key processes that control eutrophication in estuaries.