The surface-water concentration of ammonium in waters of the West 
Coast shelf, exclusive of the waters of the SCB for which ammonium was not 
analyzed, ranged from 0 to 50 (jg/L at the 146 stations for which data were 
available. The 50 th percentile of area of the surface waters of the West Coast 
shelf sampled had an ammonium concentration of 2.2 pg/L, with the 90 th 
percentile of total area characterized by an ammonium concentration of 21.4 
pg/L. The mean value of ammonium in surface waters was highest in California 
and Oregon and lowest in Washington, with the lowest mean concentration of 
ammonium being found from stations sampled in the OCNMS (Figure 3.2.15). 
The surface-water concentration of total dissolved inorganic nitrogen (DIN: 
nitrogen as nitrate + nitrite + ammonium) in waters of the West Coast shelf, 
exclusive of the waters of the SCB for which ammonium was not analyzed, 
ranged from 0.1 to 596.7 pg/L for the 146 stations with data. The 50 th percentile 
of area of the surface waters of the West Coast shelf sampled had a DIN 
concentration of 47.4 pg/L, with the 90 th percentile of total area characterized by 
a DIN concentration of 367 pg/L. The mean value of DIN concentration in 
surface waters was highest in California as compared to Washington and Oregon 
(Figure 3.2.16 A). DIN concentration for the CA NMSs was slightly higher than 
for the CA non-NMS stations, but the difference was much smaller than was the 
case for nitrate + nitrite only (Figure 3.2.16 B). 
The surface-water concentration of orthophosphate in waters of the West 
Coast shelf ranged from 0 to 80.1 pg/L for the 188 stations with data. The 50 th 
percentile of area of the surface waters of the West Coast shelf sampled had an 
orthophosphate concentration of 11.4 pg/L, with the 90 th percentile of total 
estuarine area characterized by a concentration of 61 pg/L. The mean value of 
orthophosphate concentration in surface waters was higher in California than in 
Washington and Oregon, where values were similar (Figure 3.2.17 A). Mean 
orthophosphate concentration in surface waters of the CA NMSs was more than 
three times greater than the mean value for the OCNMS and the non-NMS areas 
of the shelf (Figure 3.2.17 B). The elevated orthophosphate values are again 
consistent with the occurrence of upwelling during sampling of the CA NMS 
stations. 
The ratio of total dissolved inorganic nitrogen (nitrogen as nitrate + nitrite + 
ammonium) concentration to total orthophosphate concentration was calculated 
as an indicator of which nutrient may be controlling primary production. A ratio 
above 16 is generally considered indicative of phosphorus limitation, and a ratio 
below 16 is considered indicative of nitrogen limitation (Geider and La Roche 
2002). The N/P ratio ranged from 7.9 to 24.0, across the 146 stations in waters 
of the West Coast shelf where sufficient measurements were collected to 
compute the ratio. Approximately 93% of area of the West Coast shelf had N/P 
values <16. The 50 th percentile of area of the waters of the West Coast shelf 
sampled had a ratio of 12.8, while the 90 th percentile of area had a ratio of 14.6. 
The mean N/P values were similar for the three states, while that for the OCNMS 
46 
