Other Georgia marsh animals have d 13 C 

 values that are considerably lower, much 

 closer to the values for phytoplankton, 

 benthic diatoms, and terrestrial C-3 type 

 plants than they are to Spartina . Haines 

 (1978) suggested that these carbon isotope 

 data support the idea that particulate 

 organic detritus in Georgia estuarine 

 waters comes from offshore phytoplankton 

 production rather than from the marsh, and 

 that the marsh is actually accumulating 

 organic matter from offshore phytoplankton 

 production rather than exporting detritus 

 to the estuaries. Dow (1982) reviews the 

 difficulties in using carbon isotopes 

 alone to determine the origin of the food 

 of marsh organisms. 



Peterson et al. (1984) have included 

 an analysis of the sulfur isotope, 34 S, in 

 their interpretation of food webs in Great 

 Sippewissett Salt Marsh for added 

 resolution in determining food sources 

 because, compared to plankton, Spartina is 

 depleted in 34 S. Peterson et al . (1984) 

 found that the mud snails Ilyanassa 

 obsoleta and Fundulus heterocl itus were 

 very close to Spartina in both carbon and 

 sulfur isotopes. Marsh mussels, Geukensia 

 demissa , varied in isotopic composition 

 showing that they fed principally on 

 phytoplankton near the marsh entrance to 

 the bay and about equally on Spartina 

 detritus and phytoplankton in the 

 innermost reaches of the marsh. 



43 



