80 
> 
Salinity, psu 
Figure 5.4 Dry season chlorophyll a versus salinity (all stations from 1974-2006) showing high 
chlorophyll a at high salinities, demonstrating the oceanic import of chlorophyll a from the 
coastal ocean into the Yaquina Estuary. Plot also shows the high chlorophyll a in the tidal 
fresh portion of the estuary. 
5 .4 Nutrient Limitation and Primary Productivity 
Potential for nutrient limitation of phytoplankton is often estimated by examining the ratio of 
dissolved inorganic nutrients relative to the Redfield ratio (16 mol N: 1 mol P) and comparing the 
ambient dissolved inorganic nutrient concentrations to phytoplankton half saturation constants for 
nutrient uptake (e.g., Eyre, 2000). Typically, if the N:P ratio of the water column falls below 10:1 then 
phytoplankton may be limited by nitrogen, and if the ratio is greater than 20:1 there is the potential for 
phosphorous limitation (Boynton et al., 1982). In addition, if the ambient water column concentrations 
are less than the half saturation constants for nutrient uptake then we assume that the phytoplankton 
may be nutrient limited. Typical half saturation constants for DIN and DIP are 1.0 - 2.0 pM and 0.1 - 
0.5 pM, respectively. 
The median N:P ratio during the dry season is approximately 12:1, suggesting that nitrogen will 
be depleted prior to phosphorous for the majority of the estuary. There is evidence of phosphorous 
limitation in the upper portions of the estuary (17- 27 km from mouth) with the N:P ratio reaching as 
high as 260:1. In only 12% of the estuarine sampling events was the N:P ratio greater than 20 and DIP 
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