less than 0.5 pM, suggesting the potential for phosphorous limitation. During the dry season, the 
median DIN concentration is 14 pM (n=2028), and 95% of the time the DIN concentration is greater 
than 2 pM (typical half saturation constant for phytoplankton). In only 5% of the estuarine sampling 
events was the N:P ratio less than 10 and DIN less than 2 pM. This suggests that although the N:P 
ratio often falls below 16:1, the estuary is not usually limited by either nitrogen or phosphorous. This 
is supported by assimilation ratio data (primary production : chlorophyll a) of Johnson (1980) that was 
collected during the dry season at a station about 16 km from the mouth of the estuary (Figure 2.1). 
Johnson’s data showed 77% of the time there were sufficient nutrients for planktonic primary 
production, while 15% of the time there was borderline nutrient deficiency and 8% of the time there 
was evidence of nutrient depletion. 
Specht (1975) conducted algal bioassays at six locations in Yaquina Estuary during 1972-1975 
to examine the potential for nitrogen and phosphorous limitation. These experiments suggested that 
the upper portion of the estuary (26 km from mouth to tidal fresh) was predominantly phosphorous 
limited, while in the lower estuary, the system is nitrogen limited during the dry season and 
phosphorous limited during the wet season. 
There is limited water column primary productivity data for Yaquina Estuary. Water column 
primary production (at a station 14 km from the mouth of the estuary) during the dry season ranged 
from 0.25-2.8 g C m' 2 d' 1 with mean of 0.9 g C m' 2 d' 1 (Johnson, 1980). For comparison, primary 
productivity associated with benthic microalgae in the lower portion of the estuary (Zone 1) ranged 
from 125-325 g C m' 2 y' 1 (depending upon the location and elevation; Riznyk and Phinney, 1972). 
Davis (1981) measured net primary production during the dry season in the lower portion of the 
estuary of 46 g C m' 2 d' 1 and 0.26 g C m' 2 d' 1 for green macroalgae and benthic microalgae, 
respectively. Net primary production for Zostera marina and Z. japonica in the lower portion of the 
estuary was 181 and 130 g C m' 2 y' 1 , respectively (Kaldy, 2006ab). 
Based on the existing primary productivity data, Yaquina Estuary can be characterized as 
mesotrophic. Water column planktonic primary production is a minor component of the total primary 
productivity, which is dominated by benthic primary producers (macroalgae, microalgae and 
seagrasses). This is consistent with the findings of Valiela et al. (2000b) that for systems with 
moderate and high nitrogen loading, macroalgae is the dominant primary producer in short residence 
time estuaries (<3 days), while phytoplankton dominate in systems with relatively long residence times 
(> 45 days). 
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39 
