3.3 Zonation Based Upon Nitrogen Sources 
We divided the estuary into two zones, one of which is dominated by ocean input 
(Zone 1) and the other which is more influenced by watershed and point source inputs (Zone 2, 
Figure 2.1). We used a transport model combined with natural abundance stable isotopes (5 I5 N) 
of green macroalgae to identify the dominant nitrogen sources within the estuary as a function of 
time and location for two years (2003 and 2004). The transport model was validated by 
comparing predicted isotope ratios (using the transport model to mix isotopic end members) to 
observed macroalgal isotope ratios at five locations. For more details on this analysis, see 
Chapter 5 of Lee et al. (2006). 
Model simulations combined with 8 I5 N of green macroalgae suggest that during the wet 
season, riverine nitrogen sources dominate throughout the estuary, which is consistent with our 
comparison of nutrient loadings presented in Section 3.2. During the dry season, ocean nitrogen 
sources dominate in Zone 1, comprising between 53 - 87% of DIN (depending upon location 
within the zone), whereas riverine and WWTF inputs contribute 12-40% and 2-8%, respectively. 
In Zone 2, riverine nitrogen sources dominate contributing between 56-92% of DIN (depending 
upon location). WWTF contribution to water column DIN is maximal during the month of 
August. 
During the dry season, oceanic input of nitrogen propagates up estuary as the freshwater 
inflow declines. This can be seen in simulation results from 2004 (Figure 3.1) which show that 
Station N1 is ocean dominated (fraction > 0.5) during the entire dry season (May - September), 
while Station N2 is river dominated during May and ocean dominated from June - September. 
At Stations N3, N4, and N5 ocean inputs increase in importance from May - August, but never 
dominate. There is interannual variability in the position of the line demarking the oceanic and 
riverine dominated zones. The exact location of this line varies with ocean conditions (e.g., El 
Nino, La Nina conditions) as well as freshwater inflow. To be conservative, we placed the line 
demarking the two zones at the most seaward location found in our analysis (see Figure 2.1 for 
location). Analysis of salinity data reveals that the demarcation of the two zones corresponds to 
a dry season median salinity of 26. 
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