SEASONAL-VARIATIONS IN PRODUCTION AND CONSUMPTION RATES OF DISSOLVED ORGANIC-CARBON IN AN ORGANIC-RICH COASTAL SEDIMENT

Dissolved organic carbon (DOC) concentrations in anoxic marine sedimen ts are controlled by at least three processes: (1) production of nonvo latile dissolved compounds, such as peptides and amino acids, soluble saccharides, and fatty acids, via hydrolysis of particulate organic ca rbon (POC); (2) conversion of these compounds to volatile fatty acids and alcohols by fermentative bacteria, and (3) consumption of volatile fatty acids and alcohols by terminal bacteria, such as sulfate reduce rs and methanogens. We monitored seasonal changes in concentration pro files of total DOC, nonacid-volatile (NAV) DOC, and acid-volatile (AV) DOC in anoxic sediment from Cape Lookout Bight, North Carolina, USA, in order to investigate the factors that control seasonal variations i n rates of hydrolysis, fermentation, and terminal metabolism. During t he winter months, DOC concentrations increased continuously from 0.2 m M in the bottomwater to approximately 4 mM at a depth of 36 cm in the sediment column. During the summer, a large DOC maximum developed betw een 5 and 20 cm, with peak concentrations approaching 10 mM. The mid-d epth summertime maximum was driven by increases in both NAV- and AV-DO C concentrations. Net NAV-DOC reaction rates were estimated by a diage netic model applied to NAV-DOC concentration profiles. Depth-integrate d production rates of NAV-DOC increased from February through July, su ggesting that net rates of POC hydrolysis during this period are contr olled by temperature. Net consumption of NAV-DOC during the late summe r and early fall suggests reduced gross NAV-DOC production rates, pres umably due to a decline in the availability of labile POC. A distinct subsurface peak in AV-DOC concentration developed during the late spri ng, when the sulfate depletion depth shoaled from 25 to 10 cm. We hypo thesize that the AV-DOC maximum results from a decline in consumption by sulfate-reducing bacteria (due to sulfate limitation) and a lag in the development of an active population of methanogenic bacteria. A di agenetic model that incorporates a lag period in the sulfate reducer-m ethanogen transition successfully simulates the timing, magnitude, dep th, and shape of the AV-DOC peak.