the food web indicates the important components of 

 the system and the seasonal dynamics of energy pro- 

 duction. 



Table 4.1. Estimates of organic export 

 from wetlands. 



the wetlands at tlie end of summer is not a 

 indicator of the real productivity of the system. 



Table 4.2. Summary of annual net shoot 

 production by six marsh 

 plants (Gosselink et al. 1977). 



true 



Productivity. On the Gulf of Mexico coast, the 

 year-round wami temperatures and the continuous 

 nutrient subsidy from flooding waters combine to 

 yield remarkably high production, judging from 

 annual production of several important marsh plants 

 in southeastern Louisiana (table 4.2). These yields far 

 exceed those of other systems, even heavily fertilized 

 agricultural crops such as sugarcane (fig. 4-7). Since 

 on the Gulf coast many plants grow and die con- 

 tinuously throughout the year, the yield, i.e., the 

 harvestable biomass at the end of summer, is only 

 about one third of the total net production for most 

 marsh species. Thus the large stand of plants covering 



Best estimate from several methods, rounded to 

 nearest metric ton. 



Control of Primary Productivity. Since primary 

 production is the source of organic energy in an eco- 

 system, it is appropriate to examine the major con- 

 trols of this production. While most dry land plants 

 "saturate" with light (reach maximum photosynthesis 

 rates) at about one-half of full sunlight, many marsh 

 plants are adapted to increase their rate of energy 

 capture as long as light intensity increases (Black 

 1971). These higher rates of energy capture are not 



o 

 a 



X 



0) 



o 



c 



CO 

 O) 



30 



20 



Brackish 

 marsh 



Salt 

 marsh 



Swamp 

 forest 



Impounded 

 marsh 



Fresh 

 marsh 



1 



20 40 60 80 100 120 



Frequency of inundation (times/yr ) 



140 



160 



Figure 4-6. Hypothetical relationship between frequency of inundation and organic export from wetlands 

 (inundation frequencies from Byrne et al. 1976, export rates from Day et al. 1973 and 1977). 



160 



