. The ecology of Tijuana Estuary, California : a national estuarine research reserve . MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR 1977 •' 1978 -' Figure 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 ). It is likely that the inorganic nitrogen that is imported
. The ecology of Tijuana Estuary, California : a national estuarine research reserve . MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR 1977 •' 1978 -' Figure 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 ). 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. 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
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