To understand carbon and oxygen dynamics in sediments with deep oxygen penetration, we investigated eight locations (160-318-m depth) throughout Lake Superior. Despite the 2-4 weight percent organic carbon content, oxygen penetrated into the sediment by 3.5 to . 12 cm at all locations. Such deep penetration is explained by low sedimentation rates (0.01-0.04 cm yr 21 ), high solubility of oxygen in freshwater, and a shallow (, 2 cm) bioturbation zone. In response mainly to oxygen variations in the bottom waters, the sediment oxygen penetration varied seasonally by as much as several centimeters, suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged. The oxygen uptake rates (4.4-7.7 mmol m 22 d 21 , average 6.1 mmol m 22 d 21 ) and carbon mineralization efficiency (, 90% of deposited carbon) were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates. The reactivity of organic carbon was found to decrease with age similarly to the power-law documented in marine environments. The burial flux of carbon into the deep sediment (0.7 mmol m 22 d 21 ) was 2.5% of the previously estimated primary production. Maximum volume-specific carbon degradation rates were 0.3-1.5 mmol cm 23 d 21 ; bioturbation coefficient near the sediment surface was 3-8 cm 2 yr 21 . These results indicate that carbon cycling in large freshwater systems conforms to many of the same trends as in marine systems.

In coastal marsh ecosystems, porewater salinity strongly affects vegetation distribution and productivity. To simulate marsh porewater salinity, an integrated, spatially explicit model was developed, accounting for tidal inundation, evaporation, and precipitation, as well as lateral and vertical exchanges in both surface waters and the subsurface. It was applied to the Duplin River marsh, Sapelo Island, USA, over a 3-year period, which covered both drought and wet conditions. Simulated porewater salinity in the low and high marsh correlated with Duplin River salinity, with evapotranspiration and precipitation leading to substantial variations in porewater salinities across seasons, in particular in the high marsh. The model revealed substantial interannual variability in marsh soil conditions, and—due to its process-based approach linked to external forcings—can be used to explore effects of sea level rise and changes in hydrological forcings on marsh soil conditions.

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