5.5 Relationships between TN and TP and Chlorophyll a 
Relationships between causal and response variables are useful for demonstrating the 
relationship between nutrient loading and biological effects. Several studies have found relationships 
between nutrients (nitrogen and phosphorous) and chlorophyll a in estuaries (e.g., Monbet, 1992; 
Smith, 2006; Dettmann and Kurtz, 2006). During 2006, we conducted monthly cruises of the Yaquina 
Estuary to examine if similar relationships were present. The cruises included 12 sampling stations 
extending from the mouth of the estuary to the tidal fresh region. During the dry season, the Yaquina 
Estuary receives nitrogen from both the riverine and oceanic sources, resulting in a curvilinear 
relationship in total nitrogen (TN) versus distance, while the ocean is the main source of phosphorous 
(TP) to the estuary (Figure 5.5). There is also a curvilinear pattern in the chlorophyll a versus distance 
resulting from oceanic input of chlorophyll a from the ocean (Figure 5.6). Relationships between dry 
season nutrients (TN and TP) and chlorophyll a are driven by ocean input (rather than a response to 
watershed nutrient sources) as evident by the significant trends of increasing chlorophyll a with 
increasing nutrients (TN and TP) in Zone 1 but not in Zone 2 (Figure 5.7 and Figure 5.8). Based on 
these findings, we feel that these types of relationships would not be useful for developing nutrient 
criteria for the Yaquina Estuary. 
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