SPATIAL AND TEMPORAL VARIABILITY IN MARSH WATER COLUMN INTERACTIONS IN A SOUTHEASTERN USA SALT-MARSH ESTUARY

We measured the exchange of inorganic nutrients and particulate matter between the Spartina alterniflora marshes and the adjacent estuary of Cumberland Island, Georgia, USA, beginning in January 1991. Tidal fluxes were quantified using throughflow flumes at 3 sites within 10 km of one another. These sites represent a spatial gradient in geologic age, sediment characteristics, marsh topography and elevation, and exposure to open water that is often found in transgressive marsh-barrier complexes. They exhibited large variability in frequency and duration of tidal inundation and in susceptibility of the marshes to wind and wave erosion. The flumes were sampled seasonally and on consecutive days, and we present flux data from 7 samplings. We also investigated short-term temporal variability in nutrient and particulate fluxes by sampling one flume 4 times in 6 days. Temporal variability in total and organic suspended sediment fluxes, which was largely related to quickly changing wind and wave conditions, was greater than spatial variability measured during the same time. Dissolved constituent fluxes were generally more variable across space, suggesting that day-to-day variability in dissolved nutrient exchanges was not a major contribution to spatial variability. Dissolved inorganic nutrient fluxes (as ammonium, nitrate+nitrite, and soluble reactive phosphorus) followed a spatial pattern of highest nutrient uptake at the geologically young marsh site. This marsh also consistently imported dissolved organic carbon. This site has the lowest absolute elevation of the 3 sites and a ramp-like topographic profile, and its young geologic age suggests that it is also ecologically immature. Fluxes of dissolved constituents at this site were negatively related to the area of marsh inundated, switching to export when large areas of the young marsh were inundated for long periods of time. This marsh also generally exported total and organic sediments; data from the other 2 sites were more variable. Sediment fluxes from the older marsh sites were positively related to slack high tide water level and area inundated, switching from particulates release to uptake only when the highest portions of these marshes were inundated. Most Cumberland Island marshes thus appear to take up sediments only when tidal heights exceed about 2.3 m above National Geodetic Vertical Datum, corresponding to tides where the moon is within 20 % of new or full phase, Our data also suggest definite differences in the way the geologically young marshes interact with the inundating water column compared to geologically older marshes in the same estuarine system.