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 flu xes were quantified using throughflow flumes at 3 sites within 10 km o f one another. These sites represent a spatial gradient in geologic ag e, sediment characteristics, marsh topography and elevation, and expos ure to open water that is often found in transgressive marsh-barrier c omplexes. They exhibited large variability in frequency and duration o f tidal inundation and in susceptibility of the marshes to wind and wa ve erosion. The flumes were sampled seasonally and on consecutive days , and we present flux data from 7 samplings. We also investigated shor t-term temporal variability in nutrient and particulate fluxes by samp ling one flume 4 times in 6 days. Temporal variability in total and or ganic suspended sediment fluxes, which was largely related to quickly changing wind and wave conditions, was greater than spatial variabilit y measured during the same time. Dissolved constituent fluxes were gen erally more variable across space, suggesting that day-to-day variabil ity in dissolved nutrient exchanges was not a major contribution to sp atial variability. Dissolved inorganic nutrient fluxes (as ammonium, n itrate+nitrite, and soluble reactive phosphorus) followed a spatial pa ttern 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 al so ecologically immature. Fluxes of dissolved constituents at this sit e were negatively related to the area of marsh inundated, switching to export when large areas of the young marsh were inundated for long pe riods of time. This marsh also generally exported total and organic se diments; data from the other 2 sites were more variable. Sediment flux es from the older marsh sites were positively related to slack high ti de water level and area inundated, switching from particulates release to uptake only when the highest portions of these marshes were inunda ted. Most Cumberland Island marshes thus appear to take up sediments o nly when tidal heights exceed about 2.3 m above National Geodetic Vert ical Datum, corresponding to tides where the moon is within 20 % of ne w or full phase, Our data also suggest definite differences in the way the geologically young marshes interact with the inundating water col umn compared to geologically older marshes in the same estuarine syste m.