The volume of water removed from marsh sediments by evapotranspiration and drainage during low tide is replaced at least in part by air drawn into the sediment. The volume of gas advected into salt marsh sediments was measured in situ by recording the displacement of air in manometers connected to the sealed headspace of core tubes placed in the sediment. Measurements, made during June and July in short form Spartina alterniflora marshes in South Carolina, indicated that as much as 4.0 1 m-, of gas was drawn into the sediment, depending on marsh location and climatic variables. Gas advection rates were equivalent to water losses of 1.5 to 4.0 % of the total sediment water above the water table and corresponded to water table depths that fell to as much as 34 cm during low tide. Transpiration rates of S. alterniflora accounted for about 22% of the gas volume drawn into the sediment. Analyses of gas bubbles expelled from the sediment during tidal inundation showed that O2 was consumed in excess of CO, advected. Excess O2 consumption may result because of sulfide oxidation; this possibility is supported by decreasing pore-water pH during low tide exposure. Alternatively, some CO, produced in the sediment may remain in the pore water in dissolved form. Considering both advective and diffusive gas fluxes, a total of 56.3 mm01 m-, of CO, (4.9 by advection, 51.4 by diffusion) was evolved from the sediment during low tide compared to a total 0, consumption of 32.9 mm01 m-,. The mean 0, consumption from gas drawn into the sediment during low tide was 10.9 mm01 m-,. This compares to a mean diffusive 0, consumption by surface sediment of 22.0 mm01 m-, during 10 h of low tide exposure.

Nitrogen exchange within the Bly Creek basin (North Inlet, South Carolina, USA) was studied during 34 tidal cycles between 20 June 1983 and 19 June 1984. Estimates of tidally mediated transport were made along with annual estimates of nitrogen input to the basin via streamwater, groundwater, and rainwater. Withln the basin, effects of the vegetated marsh, oyster reef community, and the tidal creek on material transport were assessed. There was a small, but statistically insignificant (a = 0.05) NH,+ export from the basin through the tidal creek of 433 kg N yr-' (0.65 g N m-' yr-l); inputs into the basin via streamwater, groundwater, and rain totalled 78 kg NH,+-N yr-'. The NH4+ mass balance suggests the basin is not a source or a sink for this constituent to the surrounding estuary. However, since the vegetated marsh surface imports ca l500 kg NH4+-N yr-l, the tidal creek subsystem must act as a source. It is suggested that remobilization of NH4+ within the tidal creek water column and/or benthic community produces the ammonium necessary to satisfy the input of this constituent to the vegetated marsh. There was also a small, insignificant import of nitrate + nitrite into the basin through the tidal creek of 158 kg N yr-' (0.23 g N m-'yr-'). The vegetated marsh is capable of removing all the nitrate + nitrite potentially imported into this system via tidal water, streamwater, groundwater, and rain. There was a significant (a = 0.05) dissolved organic nitrogen (DON) export from the basin of 7782 kg N yr-' or 11.7 g N m-2 yr-l, whereas there was a statistically insignificant import of particulate nitrogen (PN) (1541 kg N yr-' or 2.3 g N m-' yr -l ) . The vegetated marsh withm the basin appears to be the main source of DON material to the basin as a significant amount of DON was exported both during tidal inundation and via runoff during tidal exposure. Due to the lack of statistical significance in the tidally mediated PN flux, it is difficult to state whether the basin is a source or sink for this constituent. However, a negative association between tidally mediated PN flux and maximum tidal height suggests that the marsh surface is important in removing PN during high tide conditions.