data). However, even during these periods of apparently inadequate irradiance, Z. marina in the 
Yaquina Estuary continued to grow (Boese et al., 2005). 
While it is tempting to directly apply the existing light criteria values to PNW estuaries like the 
Yaquina, there are several additional factors that need to be considered. The estuaries from which 
Duarte (1991) and Thom et al., (1998) derived their relationships are generally less turbid (mean kj~ 
0.5 nf 1 ) than the Yaquina (see Table 9.1). Z. marina has been shown to adapt to lower winter 
irradiance by increasing chlorophyll content (Zimmerman et al., 1995). Although we are not aware of 
any study that documents an analogous response to turbidity, a similar response to chronically more 
turbid water might allow for deeper colonization. 
Temperature is a possible confounding factor. The range of near-surface temperatures within 
Chesapeake Bay, Puget Sound, and in the estuaries used in Duarte’s (1991) review are likely greater 
than those observed within the Yaquina Estuary (Boese et al., 2005) due to the latter’s twice daily 
flushing with cold ocean water. Increased respiration rates due to higher summer temperatures would 
potentially need to be offset by increased irradiance for plants not only to maintain themselves but to 
store carbohydrates in rhizomes which could then be used to maintain the plant during the winter when 
irradiances may be less than optimal (Zimmerman et al., 1995; Burke et al., 1996; Zimmerman and 
Alberte, 1996). Therefore, it is possible that eelgrass in the Yaquina Estuary may require less spring 
and summer irradiance to perform the same function because of the generally cooler waters of these 
systems. 
Additionally, the Yaquina Estuary is mesotidal. Koch and Beer (1996) found that greater tidal 
'amplitude reduced the range of water depths that Z. marina colonized in Long Island Sound. Due to 
increased tidal amplitudes and turbidity, Z. marina growing in western Long Island Sound was limited 
to a 1 m depth range compared to the 4 m range observed in eastern Long Island Sound (Koch and 
Beer, 1996). Plants that are forced into a narrower depth range by these factors are likely to be more 
vulnerable to stressors such as storm events which may have contributed to the historic losses of Z. 
marina meadows. Thus, to assure seagrass survival in mesotidal and macrotidal estuaries, it may be 
prudent to establish more restrictive water clarity requirements in those estuaries. 
Our study of epiphytes growing on Z. marina leaves in the Yaquina Estuary revealed a 
reduction in the amount of epiphyte biomass in upriver, lower salinity areas. With the exception of 
2004 there appeared to be a seasonal pattern in epiphyte biomass such that the greatest biomass 
occurred in the summer and fall, when ambient light levels are highest. The accumulation of epiphytes 
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