There was also a decline of log rafting in the Yaquina Estuary from 1962 through the 1980s 
(Seddell and Duval, 1985). One effect on the water column of bark debris associated with log rafts is 
increased biochemical oxygen demand (Seddell and Duval, 1985). Due to the multiple stressors on the 
Yaquina Estuary during this time period, there is no way to determine the cause of the observed trend 
in DO levels in Zone 2. Recent (2002-2006) DO levels in Zone 2 are similar to DO levels during the 
mid 1980's, suggesting that there has been no recent changes in DO levels. In contrast, there were no 
significant trends in dry or wet season DO in Zone 1 (Figure 8.2a), suggesting that the trend in historic 
DO levels in Zone 2 was not a result of differences in ocean conditions. 
Since 2002 there has been an increase in the incidence of hypoxic events on the Oregon shelf 
(Grantham et al., 2004), which have the potential to influence DO levels within the estuary 
(particularly Zone 1). DO data collected 3.7 km from mouth of the estuary (using a YSI datasonde 
deployed at a mean depth of 1 m below the surface; WED, unpublished data) demonstrate that there is 
import of hypoxic shelf water into Yaquina Estuary during flood tides. A time series of DO and 
salinity measured during July 9-19, 2002, coinciding with a documented hypoxic event on the Oregon 
shelf off of Newport, Oregon (Grantham et al., 2004), clearly shows import of hypoxic shelf water to 
the estuary (Figure 8.3a). Minimum DO levels occurred during maximum salinities, demonstrating 
that the hypoxic water was imported into the estuary during flood tides. In addition, minimum DO 
levels occur during minimum water temperatures (~ 9 deg C), which is indicative of recently upwelled 
water. This trend of increasing DO with increasing temperature is opposite the trends of solubility, 
suggesting that differences in solubility are not causing the observed variability in DO levels. During 
this 10-day interval, minimum DO levels were 0.42 mg T 1 . The intervals of low DO conditions were 
relatively short, with DO levels increasing to 6-8 mg T 1 during ebb tides. The DO versus salinity plot 
(Figure 8.3b) shows that low DO levels occurred at high salinities (> 33 psu). A plot of dissolved 
oxygen expressed as percentage of saturation versus salinity had a similar pattern to that presented in 
Figure 8.3b, demonstrating that differences in solubility of dissolved oxygen are not the cause of the 
variability. 
The import of hypoxic shelf water into Oregon estuaries is not a recent phenomena. Gibson 
(1974) found low dissolved oxygen (5 mg T 1 ) in the lower Yaquina Estuary during July 1968, which he 
attributed to coastal upwelling. Callaway observed the intrusion of low dissolved oxygen (< 2 mg T 1 ) 
into the Umpqua Estuary (as cited in Percy et al., 1974). The NCA data set is also suggestive of 
import of low dissolved oxygen at a regional scale as indicated by lower DO values in Zone 1 
56 
