seagrasses (Koch and Beer, 1996) and due to other differences including lower temperature ranges and 
faster estuarine flushing rates. Criteria for Chesapeake Bay Z marina were derived for a spring 
through fall growing season (Batiuk et al., 2000), when carbohydrates are accumulated and used to 
maintain plants during the winter when they cannot maintain a positive carbon balance (Zimmerman et 
al., 1989). In contrast, for Z. marina in the Yaquina Estuary, winter irradiance appears to be sufficient 
for the maintenance of a positive carbon balance and as a result plants continue to grow through the 
winter, albeit at a slower rate (Boese et al., 2005). 
11.4.2 Methods 
During the summers of 2004 and 2005 the lower depth limit of Z. marina was determined at 64 
randomly selected locations in Yaquina Estuary by underwater video and direct visual observation 
techniques (for methods see Appendix B). These data were then compared to calculated light 
attenuation coefficient (kj) values that were measured at or near the same locations during a series of 
sampling cruises (See Section 4.1.1). 
11.4.3 Relationship between Lower Margin and Water Clarity 
Figure 11.6 shows the relationship between the lower depth limits of Z. marina and the distance 
from the mouth of the Yaquina Estuary. Although this relationship shows a great deal of variability, 
the lower depth limit appears to be greater toward the estuary’s mouth and in the ocean dominated 
estuarine areas. There also appears to be a difference in this depth-distance relationship depending 
upon whether it is determined in the ocean or river dominated sections of the estuary, as illustrated by 
the lack of a significant linear relationship when the data from the ocean dominated section of the 
estuary are excluded (Figure 11.6). 
The reduction in the Z. marina lower depth limit is consistent with a reduction in mean water 
clarity, which was also linearly related to distance from the mouth (Figure 9.2). We derived an 
additional relationship between the lower limits for Z. marina and the estimated kd values as follows. 
Depth = 4.4 - (1.79*fo), r 2 = 0.32. (11.1) 
where Depth = m below Mean Sea Level (MSL) 2 and kd is the light attenuation coefficient (m 1 ) 
computed from Figure 9.2 (for wet and dry seasons combined) at the location of the lower limit 
2 For the Yaquina Estuary MSL = MLLW + 1.39 m 
78 
