Composition and fate of dissolved organic carbon derived from phytoplankton detritus in coastal marine sediments

Abstract

The influence of fresh phytoplankton detritus on carbon mineralization was studied by burying phytoplankton detritus (40 g C m-2) in the surface layer (0 to 2 cm) of sediment cores collected at an intertidal location in Odense Fjord, Denmark. The enriched sediment showed increased carbon mineralization during the first 20 d of incubation at 15°C compared to control cores. The depth integrated sulfate reduction rates (5.9 ± 1.6 mmol m-2d-1) were not significantly different from the control sediments (4.0 ± 1.4 mmol m-2d-1), and were responsible for less than 26 % of the total sediment metabolism (44.9 to 95.4 mmol m-2d-1). About 10% of the added detritus was mineralized during the 1 mo incubation. A parallel anaerobic incubation of the sediment with a similar amendment showed rapid carbon mineralization and high accumulation of dissolved organic compounds (DOC) and short-chain fatty acids. About 19% of the added phytoplankton detritus was mineralized within 1 mo in this incubation. The effect of sediment depth and temperature on carbon mineralization was studied by incubation of amended and unamended surface sediment (0 to 2 cm) and deep sediment (8 to 10 cm) at a winter (5°C) and a summer temperature (15°C). Sulfate reduction was the main terminal mineralization process accounting for 71 to 100% of the CO2 production, and attained its highest rates in the surface sediment at 15°C. Acetate was an important component of the DOC pool, especially in the deep layer, where up to 100% of the DOC pool consisted of acetate. The net particulate organic carbon decomposition (DOC and total CO2 production) was similar in all amended sediments, whereas the terminal mineralization (sulfate reduction) was related to sediment depth and temperature. This indicates that the initial hydrolysis and fermentation processes are dependent on the organic matter source rather than temperature.