the estimated percentage of surface light available at various depths using the median zonal light 
extinction coefficients (kj). Results indicated that the median kd provided for persistence of seagrass at 
depths within the two estuarine zones that were comparable to current depth distributions, and that 
were consistent with results from the seagrass modeling effort. 
The State of Oregon dissolved oxygen criterion (6.5 mg l' 1 ) is relatively high compared to other 
estuarine DO criteria (see values in U.S. EPA, 2003). The historical record of DO in Zone 2 
demonstrates that this portion of the estuary may be susceptible to DO degradation. Our analyses 
showed that DO levels fall below of the present State of Oregon DO criterion in both Zones 1 and 2, 
but more frequently in Zone 1. We believe these periods of low DO in Zone 1 are related to the import 
of hypoxic water from the coastal ocean into the estuary; however, the causes of the low DO in Zone 2 
are unknown. There are several potential causes of low DO in Zone 2 including import of hypoxic 
ocean water, in situ processes occurring within the estuary, as well as possible effects of WWTF 
effluent discharge. The current Oregon DO criterion should be adequately protective of estuarine 
resources, but is closer to the 25 th percentile value rather than the median value for DO data in Zone 2. 
Using the present numeric Oregon DO criterion, we estimate that between 20 and 30% of 
measurements would not meet the criterion in Zone 2. 
There are still uncertainties with respect to the development of nutrient criteria and the testing 
of potential criteria using the Seagrass Stress-Response Model (SRM). Uncertainties in defining the 
minimum light requirement of eelgrass include the effects of tidal action and temperature. The 
primary input used for the SRM was water clarity, which is affected by multiple factors, including 
turbidity that may be independent of nutrient loads. The SRM did incorporate water column DIN as a 
limiting nutrient, but it did not incorporate indirect nutrient effects, such as relationships between 
nutrient loading and water column chlorophyll a. The SRM does not yet include a term for epiphyte 
effects on light attenuation to seagrass, and thus there is still some uncertainty about precise levels of 
water column light required for maintenance of healthy seagrass. This uncertainty will be resolved in 
future versions of the SRM. 
13.1 Recommendations 
Based on the analyses presented in this report, we suggest that criteria be developed for the wet 
and dry seasons to address the extremely strong seasonal variation in nutrient loads and sources in the 
Yaquina Estuary. Establishment of dry season criteria (May-October) is of first priority since during 
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