The first is that estimates of total 

 C0 2 production from marsh sediments 

 indicate there is little loss of reduced 

 sulfur from the marsh. All of the final 

 decomposition processes produce carbon 

 dioxide as an end product, so the total 

 C0 2 produced is a measure of total 

 decomposition. Oxygen is consumed when 

 the decomposition is via respiration and 

 also when the decomposition products 

 (e.g., sulfides and methane) are 

 reoxidized. So if the C0 2 produced is 

 balanced by the 2 consumed, there is 

 little net loss of carbon produced from 

 the system (Figure 25; Howes et al. 1984). 

 The C0 2 production is higher than 2 

 consumption early in the season because 

 reduced sulfur compounds are being 

 accumulated; but later the relationship is 

 reversed as the reduced sulfur is 

 reoxidized at the mud surface. 



The second point is that carbon 

 isotopes, especially when combined with 

 sulfur isotopes, can tell us something 

 about the marsh food web. Carbon-13 is a 

 natural stable isotope of carbon present 

 in small amounts in all carbon compounds 



1 



- C0 2 production 

 Oiyqen uptake 



a s 



MONTH 



Figure 25. Annual cycle of carbon dioxide 

 production and oxygen consumption in Great 

 Sippewissett Salt Marsh. The total 

 production and consumption are equal. The 

 offset between the curves indicates the 

 accumulation of sulfide when decomposition 

 (as measured by C0 2 ) exceeds oxidation 

 early in the year, and the reverse later 

 in the season when accumulated sulfides 

 are oxidized (Howes et al. 1981). 



on earth. Its abundance in samples of 

 organic matter can be analyzed and 

 compared to a standard called PDB Chicago, 

 which is a fossil cephalopod (a 

 belemnite). The analytical results are 

 expressed as d 13 C, the fraction of 

 carbon-13 compared to carbon-12 in the 

 sample divided by that in the standard 

 minus one times 1000: 



d 13 C = 



L3 C/ 12 C sample 

 13 C/ li; C standard 



1,000 



Negative values come from samples in which 

 there is less 13 C than there is in the 

 standard. Organisms should have less 13 C 

 in their tissues than is present in their 

 carbon source because it takes a little 

 more energy to build a compound with 

 carbon weighing 13 atomic units than it 

 takes to build with carbon weighing only 

 12 units. The bicarbonate in seawater has 

 d 13 C of about ppt; CO 2 in the atmosphere 

 has a value on the order of -7 ppt. 

 Sparti na and other plants with the C-4 

 photosynthetic pathway (see Sect. 3.1) 

 have values from -12 to -14 ppt. Most 

 temperate terrestrial plants, which have 

 the C-3 pathway, have values of -22 to -34 

 ppt. Phytoplankton range from about -20 

 to -30 ppt, benthic diatoms in the marsh 

 from about -15 to -18 ppt. 



One would expect animals to have a 

 d 13 C value that reflects the food that 

 they eat (subject to some minor 

 constraints); for example, animals that 

 feed principally upon Spartina detritus 

 might be expected to have d 13 C values of 

 -12 to -14 ppt. Haines (1976a, b) and Dow 

 (1982) found this to be true for organisms 

 like the marsh grasshopper, which feeds 

 upon living Spartina . A similar value was 

 found in some of the omnivorous crabs in 

 the Georgia marshes. These crabs are very 

 close to Spartina in the food web and at 

 times feed directly on decaying Spartina 

 leaves. The grass shrimp in the Georgia 

 marshes also have values that are similar 

 to Spartina , which is consistent with a 



detritus. Fundulus 



values suggest that 



from a mixture of 



diet of Spartina 

 heterocl itus d 13 C 

 comes 



their carbon 

 Spartina and benthic 

 animals that form the main 

 prey) (Kneib et al . 1980). 



algae carbon (via the 

 part of their 



42 



