with the marsh, 

 every 2 weeks 

 During the summer, 

 actively growing, 

 applied nitrogen 

 applied phosphate 

 waters (Valiela 



Nutrients were added once 

 as sewage sludge solids, 

 when the grasses were 

 only 6% to 20% of the 

 and 6% to 9% of the 

 were lost in tidal 

 et al. 1973). When 

 nutrients were added as a dilute solution 

 via a spray irrigation system, about 90% 

 of both nitrogen and phosphorus were 

 retained during the growing season; in 

 spring and fall, 25% to 40% were lost to 

 ebbing tidal waters. 



The biggest effect of the addition of 

 sewage to this salt marsh was the 

 stimulation of marsh productivity by 

 nitrogen. The increased production of 

 grasses and algae stimulated production of 

 the herbivores, detritivores, and the rate 

 of plant decomposition. There were also 

 changes in marsh structure. Spartina 

 alterniflora plants changed from the short 

 to the tall form. The stems became 

 thicker and the plants more widely 

 spaced—features characteristic of the 

 tall form of the grass that grows on creek 

 banks (Valiela et al. 1978a). This change 

 made the surface of the marsh more 

 accessible to predatory fishes which were 

 then better able to maneuver between the 

 more widely spaced stems. 



Added nitrogen also increased the 

 nitrogen content of grass tissues by about 

 1% (Figure 28; Vince et al. 1981). This 

 was enough to make the grass leaves more 

 attractive as food for geese and voles. 

 In fertilized plots, voles cut off as much 

 as 30% of the Spartina , although they ate 

 only a little of the base of each piece 

 cut. Their effects were nearly absent in 

 control plots (Valiela et al. 1985). 

 There were even more dramatic increases in 

 the abundance of insect herbivores in the 

 fertilized plots (Figure 29). The 

 detritus formed was also enriched in 

 nitrogen and increased the production of 

 detritivores (e.g., marsh amphipods and 

 snails) by 2 to 5 times (J.M. Teal, 

 unpubl. observ.). In Great Sippewissett 

 Salt Marsh, even the largest additions of 

 nitrogen (2.5 g N/m 2 /week) did not seem to 

 damage the marsh system. However, there 

 was a change in the relative abundance of 

 Spartina and Distichlis . (Figure 30). 

 Spartina alterniflora exhibited maximum 

 production at relatively low N levels, 



• XF = 75 g/m 2 /week 

 ■HF = 25 g/m 2 /week 

 *L F= 8 g/m 2 /week 



5 - 







3 - 



2 - 



1 - 



O 



□ C= Control 



oU = Urea, 



nitrogen 

 equivalent 

 to HF 



ft? 



s 



O 



_L 



_L 



J 



MAY JUNE JULY AUG SEPT 



MONTH 



Figure 28. Total nitrogen content of 

 Spartina alterniflora grown in experi- 

 mental plots at Great Sippewissett Salt 

 Marsh through the growing season. Values 

 are mean percent dry weight ± standard 

 error. 



while Distichlis spicata 

 increase production as 

 addition rate was increased. 



continued to 

 the nitrogen 



Spartina production increased over 

 time, but there was a relative decrease in 

 standing crop after the first 4 years 

 (Figure 31). Valiela et al . (1985) 

 suggested that this decrease may have been 

 caused by increased water loss by tran- 

 spiration of the more vigorous plants 

 which led in turn to increased soil 

 salinity, or by increased herbivory in 

 fertilized plots. Both processes might 

 also have led to the formation of the 

 patches of glasswort, Sal icornia europaea . 

 Sal icornia is an opportunistic annual 

 plant species that became a conspicuous 

 part of the marsh in the second and third 

 years following high levels (c. 2,000 

 kg/ha/yr) of nitrogen addition. This 

 species disappeared as it was selectively 



49 



