Long-term research begun in 1984 in North Inlet estuary, SC was designed originally to test the effect of sulfide in sediment on salt marsh primary production. Annual net primary production (ANPP) of the marsh grass Spartina alterniflora is measured by taking a monthly census of tagged-stem populations on permanent plots. Stem heights are measured, and allometric equations are used to compute stem biomass. ANPP is quantified by summing the incremental changes in biomass. Additionally, the elevation of the marsh has been measured monthly with SET devices, and sediment pore water has been analyzed monthly for nutrients.
A series of surprising observations explain how salt marshes respond to sea-level rise: ANPP responds to anomalies in mean sea level; primary production at North Inlet has been rising; and marsh elevation increased after raising ANPP by fertilizing the marsh. This led to the development of a theory of marsh equilibrium: Feedback among sediments, tides, and vegetation establish the productivity and relative elevation of the marsh in equilibrium with sea-level. There is an optimal rate of sea-level rise and a tipping point. Vertical accretion of the marsh surface is dictated by the concentration of suspended sediment, the duration and depth of the flood, and production of new biovolume. New biovolume arises from the turnover and preservation of roots and rhizomes. ANPP is a function of relative elevation and is possible only within a limited range of the intertidal zone, with an optimum at a mid-point. At elevations above the optimum, plants respond positively when sea level rises. This increases sedimentation and biovolume growth, and reestablishes equilibrium. Plants also appear to orchestrate the generation of nutrients, probably by stimulating sulfate reducing bacteria. Biogeochemical cycles in North Inlet, SC marshes show a high degree of order. The cycles are synchronous and in phase with monthly plant growth rates. The marsh ecosystem behaves as a well-ordered cybernetic system that compensates for external forcing, including sea-level rise.