What once was a low grassy meadow, is turned into a tall reedy thicket. Common reed blocks tidal flows even further, reducing estuarine organisms' access to the salt marsh. The export of organic matter is also reduced or eliminated where tidal exchange is restricted or cut off by a tide gate, flapper valve, or dike. Together changes in vegetation and the reduction of the detrital food base cause a major shift in wildlife use and a decrease in species diversity. A reduction of tidal flow also causes a dramatic change in soil chemistry.
Sea water contains large quantities of sulfur.
In the normally anoxic marsh soil conditions, it is reduced by soil bacteria into sulfide. The sulfide forms insoluble pyrites with iron. When restricted areas receive insufficient tidal flushing, the soil sulfides become exposed to air where they become reoxidized to sulfate. The result is the formation of sulfuric acid. Under these conditions, the soil acidity is so extreme, with pH values as low as 1.5 the vegetation can no longer survive.