PRODUCTIVITY OF ALGAE IN SALT MARSHES 



by 



Lawrence R. Pomeroy 

 University of Georgia 



The surface layers of the sediments in salt marshes contain a diversi- 

 fied population of algae. Pennate diatoms of many genera and species are 

 abundant. Dinoflagellates, green, and blue -green algae are also present. Al- 

 though the algal population of the marshes is inconspicuous, and the standing 

 crop is small, the rate of growth of the population is rapid, and growth con- 

 tinues throughout the year. The amount of energy transformed by algal photo- 

 synthesis is a significant contribution to the total primary production of the 

 salt-marsh ecosystem. 



Photosynthesis of the marsh algae was measured under bell jars filled 

 with filtered, boiled estuarine water (by change in oxygen content of the water). 

 When the sediments were exposed to air at low tide, photosynthesis was 

 measured by pressing transparent plastic boxes into the sediments, passing 

 air through the boxes continuously, and measuring the change in carbon di- 

 oxide content of the air with CO2 -absorption columns. 



Measurements of photosynthesis were made at various elevations on 

 the side of a natural levee beside the Duplin River, a tidal drainage creek. 

 These were taken to represent production in other areas of the marsh having 

 similar elevation, density of Spartina, and temperature conditions. Under- 

 water production reaches a peak rate of about 200 milligrams of carbon fixed 

 per square meter of marsh surface per hour (gross algal production: the 

 total primary product of photosynthesis) during August and drops to about 

 50 milligrams in winter. For periods when the marsh is exposed to air a 

 peak rate of about 150 milligrams is reached in January. Production drops 

 nearly to zero in March, and gradually rises again through the rest of the 

 year. 



Daily rates of production were estimated from the short-term (1 hour) 

 production measurements by correcting for day length and the amount of time 

 various parts of the marsh are exposed to air and flooded with water during a 

 tidal cycle (Table 1). From the daily production estimates an estimate of 

 annual production was made which was weighted according to the amount of 

 marsh in the study area having certain elevations and Spartina density. The 

 mean annual production for Georgia salt marshes was estimated to be 200 

 grams of carbon per square meter (gross algal production). 



To estimate net algal production (total primary product of photosyn- 

 thesis less the amount required for respiration by the photosynthetic organ- 

 isms) it is necessary to separate the amount of respiration of algae from that 

 of the heterotrophic organisms in the sediments. This can be done, because 



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