13. Conclusions and Recommendations 
The Yaquina Estuary is characterized by strong seasonal variation in the magnitude of natural 
nutrient loading and in the dominant nutrient sources. Response variables (particularly, chlorophyll a 
and dissolved oxygen) show similar patterns of seasonal variation. During the wet season, riverine 
nitrogen inputs dominate, while during the dry season oceanic nitrogen sources dominate. There are 
also strong zonal differences in nutrient levels, response variables, and dominant nutrient sources 
within the Yaquina Estuary. In the lower estuary (Zone 1), water quality conditions are strongly 
influenced by ocean conditions, while in the upper portions of the estuary (Zone 2), watershed and 
point source inputs increase in importance. 
3- 
The DIN and PO 4 levels in the Yaquina Estuary would represent medium levels using the 
criteria developed by Bricker et al. (2003) for eutrophication assessment. During the wet season, water 
column DIN levels within the estuary are relatively high. These high nitrogen levels are believed to be 
a naturally high background condition associated with the presence of red alder in the watershed. 
Some portion of the red alder related nitrogen inputs may be related to anthropogenic activities, since 
there may have been changes in red alder distribution related to logging activities in the watershed. 
However, we are presently unable to quantify the relative importance of natural and anthropogenic 
3- 
factors influencing watershed forest composition. During the dry season, PO 4 , NO 3 , chlorophyll a, 
and dissolved oxygen levels in Zone 1 are primarily determined by ocean conditions. There is 
considerable interannual variability in ocean conditions that results from Pacific - scale processes, 
such as El Nino/La Nina and the Pacific-Decadal Oscillation. The high degree of ocean-estuary 
coupling found for Zone 1 within the Yaquina Estuary suggests that monitoring for compliance with 
nutrient criteria in this region may be problematic. For example, hypoxic water and dense 
phytoplankton blooms at times are advected into Zone 1 from the coastal ocean during the dry season. 
Nutrient criteria developed for Zone 1, and any proposed monitoring process to determine compliance, 
would need to take into account this variability in ocean conditions. Distinguishing responses to 
anthropogenic nutrient inputs from those due to natural background variability for such indicators as 
chlorophyll a and dissolved oxygen may be difficult. At a minimum it may require acquisition of 
continuous monitoring data from multiparameter datasondes, an approach which is currently both 
expensive and labor intensive. WED is examining several rapid assessment approaches to allow 
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