sediments and is replaced by air at low 

 tide determines how much of the upper 

 layer alternates between aquatic and 

 aerial conditions. 



The exposure of the marsh surface to 

 rainfall can rapidly change the salinity 

 both at and below the surface. The 

 freshwater can penetrate furthest in 

 openings such as crab burrows along the 

 edges of the creeks, 

 oxygen and salinity 

 principal stresses to 

 organisms must adapt. 



These changes in 



are some of the 



which many marsh 



The hydrology of salt marsh sediments 

 is not well-studied. Hemond and Burke 



(1981) measured the infiltration of about 

 1 cm of water into a marsh sediment as the 

 tide flooded, and an exfiltration of about 

 0.8 cm on the following ebb tide. Hemond 



(1982) has preliminary measurements of 

 pressure changes in marsh sediments of up 

 to 70 cm H 2 which drive water movement 

 during tidal cycles to an extent 

 determined by peat porosity. Large 

 amounts of water evaporate from plants 

 which are even more important than the 

 tides in controlling water movement 

 through marsh sediments during the growing 

 season (Dacey and Howes 1984). Water 

 movement into marsh sediments controls the 

 supply of dissolved substances (such as 

 plant nutrients and sulfate); water 

 movement out of marsh sediments controls 

 the supply of oxygen to sediment 

 organisms. Both sulfate and oxygen are 

 involved in the decomposition cycle and 

 formation of detritus in salt marshes. 



Since marshes in areas of rising sea 

 level are depositional systems, they 

 accumulate both sediments and materials 

 sorbed to sediments. Marshes also serve 

 as collectors for materials that act as 

 sediments in the water. Pieces of plastic 

 and other non-degradable materials 

 discarded into the sea collect in the 

 drift lines or wrack zone on marshes. 

 Pollutants, such as heavy metals and 

 hydrocarbons, that are attached to 

 particles deposited by tidal waters or 

 that fall directly from the air also 

 accumulate on the marsh surface. 



Salt marshes, then, are systems 

 subject to both marine and terrestrial 

 conditions in a fairly regular alternating 

 fashion. Marshes are well-watered with 

 seawater; their sediments are anoxic and 

 have an active sulfur cycle. 



At Great Sippewissett Salt Marsh in 

 Massachusetts, biologists from Woods Hole 

 have been experimenting with salt marshes 

 since 1970, principally by fertilizing 

 small marsh plots and following the 

 consequences. Figure 4 shows the layout 

 of the experiments and the levels and 

 types of fertilizer used. The rest of 

 this report concentrates heavily upon the 

 extensive Massachusetts data to help 

 elucidate how salt marshes function, 

 though similar or related experiments done 

 at other places are also drawn upon. 



DOSAGE OF SEWAGE 



SLUDGE-BASED 

 FERTILIZER ON PLOTS 



LF = lowest (8g/m 2 /week) 



HF = middle (25 g/m 2 /week) 



XF = highest (75 g/m 2 /week) 



IOm 



S "*\ experimental 



I i " plots 



Figure 4. Layout of the experimental 

 plots at Great Sippewissett Salt Marsh 

 (from Valiela et al. 1975). 



10 



