interactions were investigated in 

 manipulative experiments using Milorganite 

 (commercially available dried sewage sludge) 

 added to microcosms containing channel 

 macroalgae and phytoplankton (Rudnicki 

 1986 and Fong 1986; Section 4.1). 



4.4.1 Nitrogen Fluxes in 1977-1978 



Winfield (1980) set out to determine 1) 

 the net direction of inorganic nitrogen 

 movement of selected tidal cycles, 2) the 

 relative importance of ammonium, nitrate and 

 nitrite in channel waters, and 3) the seasonal 

 patterns in inorganic nitrogen flux. His field 

 data included both flood and nonflood years, so 

 that comparisons became possible and our 

 ability to extrapolate to longer periods of time 

 was improved. 



Concentrations of inorganic nitrogen were 

 sampled monthly in two tidal creeks during 

 the flood and ebb cycles of spring tides. Water 

 samples were analyzed for ammonium, 

 nitrate, and nitrite, using methods outlined in 

 Strickland and Parsons (1972). One of the 

 tidal creeks drained an area dominated by 

 mixed cordgrass and pickleweed, and the other 

 drained an area dominated by succulents. The 

 two stations did not differ in nitrogen 

 dynamics, despite their difference in vascular 

 plant dominance. Therefore, they were 

 averaged to calculate nitrogen fluxes. 



Ammonium was usually the dominant form 

 of nitrogen. Only after the January-February 

 1978 flooding of Tijuana River did nitrate 

 concentrations exceed those of ammonium. 

 Ammonium (as atomic N) ranged from to 

 16.8 u.g N/l (monthly means), with higher 

 concentrations in winter and spring, and 

 higher concentrations in flood, rather than 

 ebb tides (Figure 4.5). A net import was 

 calculated for the study period. 

 Concentrations of nitrate, averaged monthly, 

 ranged from 0.2-3.6 u,g N/l, except for the 

 March 1978 postflood sample (25 u,g N/l). 

 Nitrite was lower, at 0.7-1.4 Lig N/l, and was 

 usually highly correlated with nitrate 

 concentrations. Calculations for the study 

 period indicated that both nitrate and nitrite 

 were exported in small amounts. Overall, 



a Succulent 



16 



^12 



13 8 



~ 4 



Flood Tidal Flow 

 •Ebb Tidal Flow 



i fr*r 



, MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMARAPR 



I -1977 ' — 1978— ; 



b Mix 



16 

 w12 

 « 8 



a. 4 



MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMARAPR 



1977- ' — !— 1978—1 



Figure 4.5. Concentrations of ammonium 

 nitrogen in flood and ebb tidal waters for 



a) the succulent-dominated study site and 



b) the mixed cordgrass-succulent study site 

 of Winfield (1980). Vertical bar is ±_1 

 standard deviation. Reprinted from Winfield 

 (1980) with permission. 



however, there was a net import of inorganic 

 nitrogen (2.2 g N/m 2 /yr; Figure 4.6). 



Streamflows did not appear to be 

 important to the nitrogen cycle except during 

 flood years. The shift from dominance of 

 dissolved inorganic nitrogen by ammonium to 

 dominance by nitrate is a good indictor of 

 riverine influence. Sewage inputs, on the 

 other hand, are dominated by ammonium 

 (Covin 1984). The preponderance of 

 ammonium in 1977-78 channel waters and 

 also in 1985 data of Rudnicki (1986) and 

 Fong (1986) indicate long-term nitrogen 

 subsidies from urban and agricultural 

 wastewater. 



89 



