represented in the data. Nighttime respiration can significantly reduce water column oxygen levels 
below daytime levels. 
Date 
Figure 8.1 Seasonal pattern in dissolved oxygen at all locations and all years in the Yaquina Estuary 
and River with squares and triangles representing samples from Zones 1 and 2, respectively. 
Solid line is nonlinear least-squares fit to data, which was modeled as a constant during wet 
season and a cosine function of date during the dry season. 
During the interval of 1960-1984, there was a significant trend of increasing DO in Zone 2 
during both the dry and wet seasons (Figure 8.2b; Mann Kendall, p<0.05). In addition, there was a 
significant seasonal trend in Zone 2 (Seasonal Kendall, p<0.05). Similar significant trends were found 
regardless of whether dissolved oxygen was expressed as non-transformed, residual, or percent 
saturation. A report by the Federal Water Pollution Control Administration (1966) stated that the 
water quality in the lower portion of the Yaquina basin was “adversely affected by existing and man¬ 
made conditions,” including “inadequately treated wastes from municipalities and industries” that 
placed “an excessive demand on oxygen resources of Yaquina Bay during annual periods of low 
streamflow.” In 1956, the City of Toledo upgraded their wastewater treatment facility to primary 
54 
