6. Nitrogen and Phosphorus as Water Quality Criteria 
6.1 Seasonal, Zonal, and Long-term Trends in N and P 
There are seasonal differences in water column nutrients within the estuary. DIN levels are 
significantly higher during the wet season (median = 21.1 pM, n = 874) than during the dry season (dry 
season median =13.9 pM, n = 2028; calculated using data from 1998-2006 combining Zones 1 and 2; 
3 - 
Mann-Whitney Rank Sum Test, p< 0.05). In contrast, PO4 levels during the dry season (median = 
0.97 pM, n = 2029) are almost twice as high as those during the wet season (median = 0.52 pM, n = 
873; calculated using data from 1998-2006 combining Zones 1 and 2; Mann-Whitney Rank Sum Test, 
p<0.05). During both the dry and wet seasons, Zone 1 has significantly higher PO4 concentrations 
than Zone 2, reflecting the ocean input of phosphorous (Mann-Whitney Rank Sum Test, p<0.05). The 
3 - . 3 - 
ocean input of PO 4 dominates during the dry season with Zone 1 PO 4 levels (median = 1.25 pM, n = 
1114) twice that of those in Zone 2 (median = 0.64 pM, n = 915). During the dry season, there are no 
significant differences in DIN levels between Zones 1 and 2 (Mann-Whitney Rank Sum Test, p>0.05). 
In contrast, during the wet season, DIN levels in Zone 2 (median = 55.2 pM, n = 354) are significantly 
higher than those in Zone 1 (median = 11.6 pM, n = 520), reflecting the dominance of riverine inputs 
(Mann-Whitney Rank Sum Test, p< 0.05). 
■F 
Because there are limited historic NH4 data, we were unable to assess whether there are any 
long-term trends in DIN; however, we do have sufficient historical data to examine trends in NO 3 + 
NO 2 (Figures 6 .1-6.4), the major component of DIN, and PO4 (Figures 6 .5-6. 8 ). Recent (1998-2006) 
dry season NO 3 + NO 2 and PO 4 3 in Zone 1 are significantly higher than historical data (Table 6.1 and 
Table 6.2; Mann-Whitney Rank Sum, p<0.001). In contrast, historical dry season N0 3 + N0 2 and 
3. 
PO 4 in Zone 2 are significantly higher than recent data (Table 6 .1 and Table 6.2). Peak wet season 
N0 3 + N0 2 concentrations in Yaquina Estuary are similar to wet season N0 3 ' observed in Oregon 
Coast Range streams (peak N0 3 of 172 pM; Wigington et al., 1998). The relatively high N0 3 + N0 2 
concentrations that occur in the historic data from Zone 1 dry season (Figure 6.1) are related to an 
anomalous freshwater inflow event in June 1984 (peak flow of 634 cfs compared to long-term mean 
for June of 81 cfs; calculated using data from Chitwood gauge on Yaquina River). Historical wet 
season N0 3 + N0 2 levels are significantly higher than recent observations in Zones 1 and 2 (Table 6.1; 
' /■ 
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