Degradation of Dissolved Organic Carbon in Permeable Coastal Sediments

This study addresses the decomposition of dissolved organic carbon (DOC) in highly permeable coastal sand sediments. DOC fluxes from shelf sediments (~180 Tg C yr-1) are significant, roughly equal to the DOC flux from rivers (~200 Tg C yr-1) and to the rate of carbon burial in marine sediments (~160 Tg C yr-1) (Burdige et al., 1999). DOC thus plays an important role in the global cycles of carbon and nitrogen (Hedges, 1992) and understanding the processes that control DOC dynamics is critical to clarify this role. The small concentrations of lignin in the ocean and the 13C-enriched composition of marine DOC suggest relatively rapid degradation of terrestrial DOC, however, the mechanisms by which the large volumes of DOC released to coastal waters are rapidly degraded following discharge are poorly understood (Hedges, 1992). Photooxidation can decompose DOC to low molecular weight substances (Kieber et al., 1989; Mopper et al., 1992), and this process may account for the removal of 20 to 30% of DOC in coastal waters (Mopper and Kieber, 2002). Microbial activity in the water column and shelf sediments degrades DOC, but the importance of these processes are not well constrained. Approximately 70% of the shelf sediments are relict sands (Riggs et al., 1996), and in the shallow coastal zone a large fraction of these sands are highly permeable and permit circulation of water through the interstitial space (Marinelli et al., 1998; Huettel et al., 1996; Huettel and Gust, 1992). In this shallow environment, strong boundary currents caused by wind, waves, and tides force bottom water loaded with DOC through the sediment ripples and upper surface layers of the sand. Abundance and diversity of microbes in permeable sediments exceed that of the overlying water column (Hunter et al., 2006), and the question arises whether the filtration through the sediment affects the decomposition of DOC and water column DOC dynamics. I tested the working hypotheses that rapid DOC transport along relatively short pathways through the sand significantly enhances the degradation of the DOC and that the sedimentary flushing tightly links sedimentary and water column DOC concentrations. The main objectives of the research were: 1) To measure degradation rates of DOC in percolated permeable sediment and to compare the rates to those in the water column; 2) To assess the magnitude and variability of DOC concentrations in water and pore water at two nearshore environments of the northeastern Gulf of Mexico with permeable sediments, and to determine the links between the sedimentary and water column DOC dynamics; 3) To investigate the processes that control the DOC distribution in the surface layer of the permeable bed. I combined time series and chamber experiments conducted in-situ with laboratory column reactor and flume experiments. While the laboratory experiments were designed to assess DOC degradation rates and processes controlling DOC distribution under well-defined conditions, the field experiments were conducted to gain insight on DOC fluxes and seasonal trends at two coastal sites differing in their DOC input and hydrodynamic characteristics. The flume experiment examined the distributions of DOC resulting from degradation of phytoplankton deposited on permeable sediments exposed to unidirectional flow. The results show that DOC from algal cell degradation in the sediment surface layer is rapidly transported deeper into the sediment by advective pore water flows and concentrated in specific zones dictated by the pore water flow field. The DOC profiles showed highest concentrations in the upper 2 cm of the sediment, lower concentrations below that layer and in some cases increasing DOC concentrations in the layers below the flushing zone, resulting in a concave profile shape. The laboratory column reactor experiments demonstrate that fresh DOC originating from phytoplankton, as well as older DOC from terrestrial sources, is rapidly degraded (2.15 to 124.04 µmol l-1 h-1) while passing short distances through permeable sands by the microbial community and that degradation rates in the sediment exceeded those in the water column, approximately 7-fold for my experimental settings. The measurements of water column and pore water DOC concentrations at St. George Island produced the first DOC time series for two shallow Gulf of Mexico coastal environments showing DOC ranges and temporal dynamics. These time series indicate that during the winter season, when hydrodynamic forcing is strongest, water column and sedimentary DOC concentrations are coupled, while no such link could be recorded during the calmer periods of the year. The similarity between the in-situ DOC profiles and those measured in the laboratory flume suggest that plankton deposition, combined with advective pore water transport processes and slow upward diffusion of refractory DOC from deeper layers, shape the concave DOC concentration profiles. The field measurements with advection chambers reveal seasonal variation of the magnitude and direction of DOC fluxes and showed that the permeable coastal sands can act as a sink (winter) or a source (summer) for DOC.