observation. 
Figure 11.6 Relationship between distance from mouth of the estuary and the lower depth limit (below 
Mean Sea Level) for Z. marina. The three regression lines are for Zone 1, Zone 2, and the 
entire estuary. 
The maximum depth to which a seagrass grows is dependant upon water clarity which is often 
presented in.the literature as the fraction of surface irradiance found at the maximum seagrass 
colonization depth (Duarte, 1991). This value is calculated from kd as 
where / is the irradiance at depth, I 0 is the surface irradiance, kd is light attenuation coefficient, and z is 
depth (m below MSL). Using this equation, the amount of surface irradiance reaching the observed 
lower depth limits for Z. marina within the Yaquina Estuary was estimated. These values ranged from 
7 to 68% with a mean ± standard deviation of 12.6 ± 1.9 % (n = 64). Standard deviation was 
determined using the propagation of error associated with estimating by linear regression from 
multiple kd values at a given distance from the mouth of the estuary (see ANOVA with regression in 
Sokal and Rohlf, 1981). 
t t 
79 
