virtually all of the salt-marsh algae are motile and migrate vertically. By 

 choosing a time and place when the algae have migrated to the surface, they 

 can be scraped off almost quantitatively. Respiration of the scraped area can 

 then be compared with an adjacent undisturbed area. When this is done, all 

 the respiration measured, within the limits of the method, is non-algal. 

 Therefore the data for gross algal production must also serve as approxima- 

 tions of net algal production, and the difference between the two is probably 

 less than ten percent. Respiration of the sediments community amounts to 

 about 100 grams of carbon per square meter per year. Short-term rates are 

 13 milligrams of carbon per square meter per hour under water and from 

 lO(spring) to 50 milligrams (winter) in air. 



Table 1, 



Weighted mean gross algal production, milligrams of carbon fixed 

 per square meter per day, for periods of two months. 



Month: i-n in -IV v-vi vu-vm IX -X xi-xn 



Production: 443 237 435 780 -- 714 



Net production of the sediments community {excluding Spartina and 

 macroorganisms such as snails and crabs) is about 100 grams of carbon per 

 square meter per year. This is the production by algae less the respiration 

 of the algae and the heterotrophic organisms in the sediments: bacteria, 

 nematodes, protozoa, etc. 



The amount of solar radiation reaching the sediments proves to be one 

 factor limiting algal production in salt marshes. Radiation was nneasured 

 with a Whitney photometer both in air and under water. When the marsh is 

 exposed to air, light intensity is above the optimum for photosynthesis except 

 under dense stands of Spartina. This will limit photosynthesis, particularly 

 in summer. When the marsh is under water, light intensity is within the op- 

 timal range over much of the marsh. This shows us why production in air is 

 at a maximum in winter when radiation is at a minimum, while production 

 under water is at a maximum in summer when radiation is near its maximum. 



Photosynthesis varies with temperature when the algae are under water 

 and light is optimal, but it does not vary with temperature when the algae are 

 in air, because light is above the optimum. Temperature as well as light in- 

 tensity may limit photosynthesis under water in winter and enhance it in sum- 

 mer. This may explain why the maximum rate of photosynthesis under water 

 is in late summer rather than spring. 



There is no evidence that the supply of carbon dioxide is limiting to 

 underwater photosynthesis, but it may sometimes be limiting to photosynthesis 

 in air. The pH of the surface of the marsh sediments rises from an early 



89 



