Long-Term Studies of Salt Marsh Primary Production
Long-term measurements of salt marsh primary production and sediment chemistry (funded by NSF LTREB) have been monitored monthly on permanent plots that were established in North Inlet salt marshes in 1982. The objective is to quantify the variability and assess the relative importance of physicochemical parameters that regulate productivity. The existing long-term data on demography (e.g. stem survival, density dependent growth and mortality, etc.), productivity, and sediment chemistry are unique and have led to new insights about the regulation marsh productivity. These data demonstrate that there is significant interannual variation in net aboveground production that is controlled proximally by soil salinity, and that there are complex interactions involving sea level anomalies, flooding frequency, and the solute balance of intertidal sediments that extend to the entire estuary.
To the right, the upper graph is a time series of mean sea level from Charleston Harbor. Notice that monthly mean sea level varies seasonally with a range of about 30 cm. Sea level is lower in the winter months and higher during summer due to seasonal changes in temperature of the sea and thermal expansion of water. Mean sea level during July and August, peak months for the growth of marsh plants, is also plotted. Note that there is considerable interannual variation in the July-August mean. Changes in mean sea level of this magnitude have a great effect on the flooding frequency of salt marshes, and this affects the salinity of the soil, particularly during the summer when the evaporation of water is high. The soil salinity during the warm summer months can rise to levels that reduce the growth of the marsh plants.
The lower figure to the right shows the annual production of marsh grass (Spartina alterniflora) from two sites within North Inlet. The interannual variation in production correlates very well with mean summer sea level. When summer sea level is abnormally low, soil salinity increases and the productivity of the plants decreases. Shown in the upper panel is a time series of mean sea level from Charleston Harbor.
