a Succulent 



MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR 



1977- —' — 1978—' 



MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR 



1977 •' 1978 -' 



Figure 4.6. Net flux of total inorganic 

 nitrogen dissolved in the tidal waters for a) 

 the succulent-dominated study site and b) 

 the mixed cordgrass-succulent study site of 

 Winfield (1980). Reprinted with permission 

 from T. Winfield. 



Fluxes of organic nitrogen have not been 

 measured but can be assumed from Winfield's 

 data, which show a net export of dissolved 

 organic carbon (Section 4.5.3). It is likely 

 that the inorganic nitrogen that is imported 

 with the incoming tide is fixed into organic 

 matter and released to channel waters as 

 particulate and/or dissolved organic nitrogen. 

 The marsh functions as a transformer of 

 inorganic to organic matter. 



Imported nutrients could enter the marine 

 food chain as amino acids and detrital particles 

 become available to consumers. During 

 sewage spills, concentrations of organic 

 nitrogen are much higher, and both water 

 quality and estuarine organisms are severely 

 damaged. 



4.4.2 Nitrogen Additions to Salt Marsh 

 Vegetation 



We have long been aware of spatial and 

 temporal variability in marsh plant growth, 

 especially for cordgrass. While soil salinity 

 reductions that accompany flooding (Chapter 5 

 and Zedler 1983b) have been shown to be 

 important in controlling growth, flooding does 

 not explain all of the growth dynamics. 

 Furthermore, freshwater influence is not 

 independent of nutrient influxes, as the 

 previous section shows. When the Tijuana 

 River flows, there are many changes in total 

 water chemistry. 



Before Covin's (1984) study at Tijuana 

 Estuary, the influence of nitrogen on salt 

 marsh vegetation was in question. D. Turner 

 (SDSU; unpubl. data) had fertilized 

 pickleweed-dominated vegetation at San Diego 

 River Marsh and found large increases in 

 vascular plant productivity. However, when 

 Nordby added the same concentrations of urea 

 to cordgrass transplants at Tijuana Estuary, 

 he failed to see a growth response (Nordby et 

 al. 1980). The latter experiment took place 

 during 1980, when major floodwater may 

 have enriched the marsh soils with nitrogen 

 or stimulated local nitrogen recycling. These 

 conflicting results stimulated Covin to develop 

 a detailed investigation of nitrogen-plant 

 interactions. 



Covin's first step was a broad survey of 

 soil nitrogen in 1981, using 67 of the 102 

 lower marsh monitoring stations at Tijuana 

 Estuary (Chapter 5). Soil nitrogen proved to 

 be variable within stations, among stations, 

 and among transects. There was only a hint 

 that cordgrass growth was related to soil 

 nitrogen concentrations; the transect with 

 highest soil nitrogen had the highest total stem 

 length (a biomass estimate) of cordgrass. 



Reasoning that nutrient uptake by 

 cordgrass might be enhanced by nitrogen 

 concentrations or reduced by competitive 

 uptake on the part of pickleweed, Covin set up 

 two-way experiments with both nitrogen 

 (turea) and presence of competitors 



90 



