Live Weight 



Standing Dead 

 Weight I 



MAMJ J ASONDJ FMAMJ J A SON D J FMAMJJ A SON 

 ' 1976 1 1977— — ' 1978 ' 



Figure 4.4. Live biomass and dead standing 

 crop for salt marsh vascular plants. Vertical 

 bars are ±_1 standard error, n = 25. 

 Reprinted from Winfield (1980) with 

 permission. 



Species-specific differences in standing 

 crop are easiest to see in the 1977 harvest 

 data, for which the number of sampling 

 stations was constant. The annual pickleweed 

 (Salicornia bigelovii) had the largest 

 temporal increase (13.4-fold), while shore 

 grass had the least (1.4-fold). Year-to-year 

 differences in August biomass were also large. 

 Variability, then, is the rule; it is high from 

 species to species, season to season, and year 

 to year. Onuf (1987) reached the same 

 conclusion in his analysis of pickleweed 

 biomass at Mugu Lagoon. 



4 . 4 NUTRIENT INTERACTIONS 



Our understanding of the nutrient cycles at 

 Tijuana Estuary is limited. Most of the work 

 has focused on nitrogen, because nitrogen has 

 long been accepted as the major limiting 

 factor in coastal ecosystems. Smith (1984) 

 has challenged that dogma and claimed that 

 phosphorus is likely to limit estuarine 

 systems, even though small-scale 

 experiments might indicate that nitrogen 

 controls plankton growth. Howarth and Cole 



(1985) support the theory of nitrogen 



limitation and provide a convincing argument 



that anaerobic conditions favor nitrogen 

 limitation. 



Winfield (1980) studied nitrogen 

 dynamics in detail and determined that there 

 is a net flux of inorganic nitrogen from the 

 tide waters to the marsh. His estimate of the 

 amount of nitrogen imported by the marsh 

 (2.2 g N/m 2 /yr) was far less than the total 

 required for above-ground plant growth (only 

 28%), and even a smaller portion of the 

 nitrogen required for vascular-plant and algal 

 productivity combined (6%). While these 

 calculations do not rule out phosphorus or 

 other nutrients as limiting, they do show a 

 nitrogen deficit. 



Data from the San Diego Regional Water 

 Quality Control Board (G. Peters, WQCB, pers. 

 comm.) show high ratios of phosphorus to 

 nitrogen, so that phosphorus is less likely to 

 limit whole-system productivity than 

 nitrogen. Field sampling of soil nitrogen 

 (Covin 1984) and channel nitrogen 

 concentrations (Winfield 1980; Rudnicki 

 1986; Fong 1986) indicate that inorganic 

 nitrogen is often present in very low 

 quantities. 



Nitrogen fixation rates have not been 

 measured at Tijuana Estuary. A one-year 

 study of nitrogen fixation at Sweetwater 

 Marsh National Wildlife Refuge on San Diego 

 Bay (Zalejko 1989) showed that microbes 

 were fixing nitrogen both at the marsh 

 surface and in the rhizosphere. However, 

 absolute rates (as estimated by acetylene 

 reduction) appear to be very low. 



In experimental work at Tijuana Estuary, 

 nitrogen additions stimulated vascular plant 

 growth (Covin 1984; Covin and Zedler 

 1988). Not only was nitrogen found to limit 

 plant growth, it was shown to affect the 

 competitive interaction between cordgrass and 

 pickleweed (Section 4.4.2). 



In 1983, frequent sewage spills from 

 Mexico turned our attention to the more 

 practical problems of how wastewater affects 

 estuarine organisms. Nutrient-algae 



88 



