Coastal salt marshes like those at the VCR LTER are geologically young formations and experience rapid change. Changes in sea level trigger systematic hydrologic and ecologic response characterized by the migration of marsh zones coinciding with increased inundation. As areas of marsh experience increased flooding, the vegetation characteristic to low, mid, and high marsh zones also migrate in response to the change. Along with the obvious changes in plant species as marshland migrates into upland areas, less obvious changes in the organic matter content of the sediments also occurs. The low organic matter content upland soils give rise to high organic matter content high marsh sediments which in turn give rise to low organic matter content low marsh sediments. These changes in sediment organic matter content may be directly attributed to altered patterns of tidal flooding frequency or may be related to differences in the susceptibility of each zone's characteristic vegetation to decay.
Microbial mineralization of organic matter consists of the conversion of particulate organic carbon to dissolved organic carbon (DOC), a form assimilated and metabolized by microbial cells. Therefore, the quantity and quality of DOC in sediment pore water may be an indicator of the rate of the susceptibility of sediment organic matter to mineralization or burial. For example, low concentrations of labile DOC would suggest rapid turnover of DOC and organic material mineralization, while high concentrations of refractory DOC would indicate slow turnover, long residence times, and the potential for organic material accumulation. Preliminary observations of sediment pore water DOC concentrations and the ability of bacteria to convert the DOC to cell biomass in four vegetation types representing a tidal inundation gradient in areas of the Phillips Creek Marsh suggest the following: i.) sediment pore water concentrations increase as inundation frequency decreases (Fig. 1) and ii.) that DOC in frequently flooded sediments is more readily converted to bacterial biomass than in less frequently flooded sediments (Fig. 2).
During the summer of 1995, experiments will be conducted to determine if the differences in DOC concentration and conversion of DOC to bacterial cells is a result of differences in tidal inundation frequency, the type of plant materials characteristic of the four marsh zones, or a combination of both inundation frequency and type of organic material.
Figure 1. Sediment pore water concentrations of dissolved organic carbon (DOC) collected from suction lysimeters at 10 cm depth. Samples were collected from a tall Spartina alterniflora (TS), a short S. alterniflora (SS), a Juncus romerianus (JR), and a S. patens (SP) zones of the marsh where tidal inundation occurs twice each day, once each day, on spring tides, and irregularly during storm events, respectively. Concentrations are the mean of replicate samples collected from three lysimeters. Error bars represent one standard deviation from the mean.
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Figure 2. Increase in bacterial abundance in response to incubation with dissolved organic carbon (DOC) collected from suction lysimeters at 10 cm depth in 4 marsh zones. The bioassay used equal volumes of a bacterial inoculum collected from the marsh creek adjacent to the DOC sampling locations in Phillips Creek marsh (see Fig. 1, Wu and Blum) and DOC collected from the zones as described in Fig. 1, above. Bacteriovores were removed from the creek water by filtration through an 8 um pore diameter filter. DOC was filter-sterilized. Acridine orange direct counts of bacterial were done at the beginning and end of the 12-hour incubation. The change in bacterial abundance values (initial cell number - final cell number) are the mean of three replicate bioassays. Error bars represent one standard deviation from the mean.
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