5.2 Nutrients 
During the wet season, NO3 is the primary form of DIN in the estuary (median of 88% of DIN, 
n = 873). There is little utilization of dissolved inorganic nutrients by phytoplankton within the estuary 
during the wet season due to short residence time (high freshwater inflow) and low solar irradiance. 
The average incident photosynthetically active radiation (PAR) varies from 15 mol quanta m 2 d ’ 1 
during the wet season to 38 mol quanta m 2 d ’ 1 during the dry season. Mixing diagrams (property 
salinity plots) are often used to infer biogeochemical cycling occurring within estuaries (e.g., internal 
sources and sinks). Mixing diagrams of DIN for wet season cruises exhibit conservative mixing 
behavior, indicating river inputs are the primary nitrogen source and that there is little utilization 
within the estuary during this time. Minimal utilization of nutrients is also evident in the low 
chlorophyll a levels observed during the wet season (see Chapter 7). 
The dry season coincides with the growth season and with upwelling on the shelf. As 
discussed in Chapter 3, nutrient rich water associated with coastal upwelling is advected into Yaquina 
. 3- 
Estuary during flood tides. During the dry season, high levels of DIN and PO4 enter the estuary about 
two days after upwelling conditions (Brown and Ozretich, in review). Median concentrations of 
3- 
oceanic NO3 and PO4 entering the estuary during the dry season are 8.6 pM and 1.3 pM, respectively 
3- 
(n = 830). The maximal nutrient concentrations (NO3 =31.5 pM and PO4 = 2.9 pM) entering the 
Yaquina Estuary during upwelling periods are similar to those found in other upwelling regions 
(Dugdale, 1985) and elsewhere on the Oregon shelf (Corwith and Wheeler, 2002). 
During the dry season, NO 3 is the primary form of DIN (median of 75%, n =2028), while NO 2 
is a minor component only composing 2% of DIN. There is a mid-estuary minimum in mean dry 
season N0 3 ’+ N0 2 (with a mean of 7 pM, Figure 5.2) suggesting that the estuary receives N0 3 from 
both the ocean and the river. For mixing diagrams to be useful in identifying the importance of 
internal processes (e.g., biological uptake) steady state conditions need to apply. Due to the temporal 
variability of the ocean end member, it is not appropriate to use mixing diagrams to determine the role 
of internal estuarine processes (i.e., biological uptake) in the formation of this mid estuary minimum. 
3 - 3 - 
The primary source of PO 4 to the system is the ocean and there is a steady decline in P0 4 with 
distance into the estuary (Figure 5.3). The oceanic signal in N0 3 and P0 4 3 propagates approximately 
13 km up the estuary (Brown and Ozretich, in review). 
' t 
35 
