5.3 Chlorophyll a 
Previous studies have demonstrated that chlorophyll a is advected into estuaries along the 
Oregon and Washington coasts from the coastal ocean during the dry season (Roegner and Shanks, 
2001; Roegner et ah, 2002). Brown and Ozretich (in review) found similar results for Yaquina 
Estuary. In Yaquina Estuary, peak chlorophyll a concentrations imported from the coastal ocean 
during the dry season reach 50 pg E 1 with a median value of 4 pg l' 1 (n=181). The input of 
phytoplankton to the estuary lags upwelling favorable winds by approximately 6 days, suggesting that 
it takes this amount of time for phytoplankton to utilize the recently upwelled nitrogen and be 
transported across the shelf into the estuary (Brown and Ozretich, in review). 
Figure 5.4 shows the import of chlorophyll a from the ocean, as indicated by the fact that high 
chlorophyll a occurs at high salinities. The oceanic signal attenuates more rapidly for chlorophyll a 
3 - 
compared to NO3 and PO4 . The statistically significant relationship between oceanic chlorophyll a 
concentrations and within estuary chlorophyll a is only evident up to about 11 km into the estuary 
(Brown and Ozretich, in review). The more rapid decline in the ocean signal in chlorophyll a is 
probably the result of benthic grazing on oceanic phytoplankton. Oyster aquaculture is present in 
Yaquina Estuary in the region 10-15 km from the mouth (Figure 2.1) and in the lower estuary there are 
tidal flats that have high densities of filter-feeding burrowing shrimp (DeWitt et al., 2004, see Section 
3.2.3). Data from an in situ fluorometer (located 3.7 km from the mouth of the estuary) indicate that 
there is an import of oceanic chlorophyll a to the estuary and that a 60% reduction in chlorophyll a 
occurs between successive flood and ebb tides. Flood tide chlorophyll a values (median = 14 pg 1"') 
1 • 
were significantly higher than ebb tide values (median = 9 pg f ; Mann Whitney Rank Sum, p<0.001, 
n = 53). The import of chlorophyll a to Zone 1 is consistent with the findings of Karentz and Mclntire 
(1977) that during the spring through fall seasons marine diatom genera dominated in the lower estuary 
(stations 3.4 and 6.7 km from the mouth of the estuary), while freshwater and brackish taxa dominated 
in the upper estuary (stations located 12.3 and 18.8 km from the mouth). Phytoplankton blooms occur 
in the tidal fresh portion of the estuary as indicated by the high chlorophyll a values at low salinities 
(Figure 5.4). 
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