which have demonstrated their high rates of pri- 

 mary production. Primary productivity is often 

 measured in units of grams of carbon produced 

 per square meter of ground per year or in grams 

 dry weight per square meter per year. Since car- 

 bon makes up shghtly less than one-half of the 

 dry weight of these marine plants, one could mul- 

 tiply grams of carbon by approximately 2 to 

 convert to grams dry weight. Well-known studies 

 of the Georgia Spartina alterniflora marshes by 

 Smalley (1959) and Odum and Fanning (1973) 

 have produced productivity estimates as high as 

 3990 g dry wt/mVyr- Keefe (1972) and Turner 

 (1976) reviewed the various salt marsh producti- 

 vity studies and concluded that production usually 

 declines on a gradient from south to north along 

 the east coast of North America. In North Caro- 

 lina, primary productivity of Spartina alterniflora 

 marshes usually falls in the range of 329 to 1296 g 

 dry wt/m^/yr while Juncus roemerianus produc- 

 tion lies between 560 and 1960 g dry wt/m^ /yr 

 (Keefe 1972). Stiven and Plotecia (1976) em- 

 ployed a multiple regression model to analyze the 

 importance of several factors on marsh produc- 

 tivity using data from 23 east coast marshes. They 

 found that along the east coast of North America, 

 vegetational species, latitude, growing season, 

 temperature range, and mean tidal height explained 

 69% of the variation in reported productivities. 



Virtually all of these studies rely upon what is 

 termed the harvest method of estimating net pri- 

 mary productivity. At the time of peak standing 

 crop (usually in the fall), sample plots are har- 

 vested and the plants are dried and weighed. As- 

 suming that marsh plants undergo incremental 

 growth to a peak height without loss of any signi- 

 ficant portions during the year, this peak biomass 

 would represent the total production for the full 

 year. Such an assumption obviously produces an 

 underestimate of true productivity. Kirby and 

 Gosselink (1976) demonstrated that the under- 

 estimate was large indeed in a Louisina Spartina 

 alterniflora marsh. By using the so-called Wiegert- 

 Evans "old-field" method of estimating primary 

 productivity, which involves estimating the death 

 and loss of vegetation during the year, they 

 demonstrated that true annual rates of primary 

 production are about two and one-half times the 

 harvest estimates. Although the published harvest 

 estimates seem relatively high for marsh plants, 

 actual values are even higher. 



Just how great the primary production of 

 marsh plants is has been discussed by Odum 

 (1959). Most marshes are more productive than 

 cultivated and highly managed terrestrial crops. 

 The world's average production for corn fields is 

 412 g carbon/m^./yr, while rice is 497 g car- 

 bon/mVyr (Odum 1959). In the U.S., hayfields 

 are highly productive, but they average ony 420 

 g carbon/m^ /yr. The most productive parts of the 

 seas occur in upwelling areas, such as off the coast 

 of Peru (Ryther 1969); however, these upwellings 

 too are not as productive on an areal basis as a 

 salt marsh. 



Seagrass beds also show relatively high pro- 

 ductivities in many areas. Values for annual pro- 

 duction of Zostera range from approximately 5 to 

 600 g carbon/mVyr (Phillips 1974). In North 

 Carolina, Zostera productivity has been measured 

 near Beaufort at approximately 330 to 340 g car- 

 bon/mVyr (Dillon 1971,Penhale 1977). Mixed in 

 with the eelgrass in this area is another seagrass, 

 Halodule , and a brown alga, Ectocarpus, which 

 together contribute another 73 to 300 g car- 

 bon/mVyr (Dillon 1971, Penhale 1977). If these 

 estimates are representative of the subtidal seagrass 

 beds in North Carolina's estuarine systems, then it 

 is clear that an acre of a North Carolina seagrass 

 bed is also more productive than rice, corn, and 

 the other terrestrial crops listed by Odum (1959). 

 In the sounds and estuaries of North Carolina, 

 seagrass beds are prominent and clearly important 

 producers of detritus, some of which is processed 

 on mud and sand flats. 



As is suggested by its low standing crop, the 

 productivity of phytoplankton in estuarine sys- 

 tems has long been thought to be relatively low. 

 For instance, Marshall (1970) estimated that 

 phytoplankton contributed only 50 g carbon/ 

 m^ /yr to New England's subtidal shoal waters, 

 compared to a contribution of 125 g carbon/ 

 m^ /yr for all macrophytes. In the Newport River 

 estuary at Beaufort, North Carolina, Williams 

 (1966) and Thayer (1971) estimated that phyto- 

 plankton produce about 110 g carbon/m^ /yr. 

 Ragotzkie (1959) measured oxygen uptake in 

 light and dark bottles (a conventional means of 

 estimating net phytoplankton productivity) in the 

 Duplin River estuary of Georgia and found phyto- 

 plankton production to be negligible. 



15 



